Biochemistry Made Easy—A Problem-based Approach N Haridas
INDEX
×
Chapter Notes

Save Clear


Statement and Comments1

1. The digestion and absorption of short chain and medium chain fatty acid containing triacylglycerol are different from the same of long chain fatty acids:
Most of the short and medium chain fatty acid containing triacylglycerols (TAGs) are hydrolyzed by gastric lipase. It does not require bile salts for digestion and absorption. In contrast to this, is the case of long chain fatty acid containing TAGs which are digested by pancreatic lipase after emulsification with bile salts. Their absorption also requires bile salts.
2. PUFA in general are anti-atherogenic:
Polyunsaturated fatty acids like linoleic acid, linolenic acid and arachidonic acid are present in the phospholipid lecithin, which is a component of HDL at the second carbon which is actually transferred to cholesterol to form ester cholesterol in the presence of lecithin cholesterol acyltransferase and high-density lipoprotein (HDL) transports this cholesterol ester to the liver for excretion through bile.
3. Aspirin is useful in heart patients:
Aspirin acts as an inhibitor of cyclo-oxygenase enzyme both in platelets and endothelial cells and hence both thromboxane and prostacyclin synthesis are inhibited. Platelets cannot regenerate cyclo-oxygenase and hence cannot form throm-boxane-A2 and so platelet aggregation is prevented. The endothelial cells on the contrary can form cyclo-oxygenase after sometime and so can form prostacyclins. So, aspirin inhibition of thromboxane-A2 formation is irreversible whereas prostacyclins can be regenerated.
4. In the case of unsaturated fatty acids, the energy yield is less by 2ATP moles per double bond:
In the case of unsaturated fatty acids, the intermediate step catalyzed by the enzyme hydroxyacyl-CoA dehydrogenase step is bypassed. Hence, there is no production of FADH2 which results in the production, two ATP moles less for 2every double bond. For example, when linolenic acid (with two double bonds) is oxidized, the number of ATP moles produced will be four less than stearic acid. Similarly, in a fatty acid with three double bonds (linolenic acid), the ATP production is reduced by six than a saturated fatty acid with the same number of carbon atoms like stearic acid.
5. PUFA are nutritionally essential:
Linoleic acid and linolenic acid are dietary essentials as these cannot be synthesized in our body. The third important PUFA, arachidonic acid can be formed from linoleic acid. Humans do not possess the necessary enzymes to add double bonds between the existing double bond and the omega carbon.
6. Trans fatty acids are injurious to health:
Trans fatty acids are formed during the hydrogenation of vegetable oils. They are also present in baked food items and are produced by deep frying practices.
They are found to be atherogenic because they increase the LDL “(bad cholesterol)” and decrease the “good cholesterol”, i.e. HDL.
7. Arachidonic acid is not a dietary essential if the essential fatty acids are consumed:
Arachidonic acid is an omega-6 fatty acid which can be synthesized from another omega-6 fatty acid, i.e. linoleic acid by increasing the carbon length by two carbons and introducing two more double bonds between the existing double bond and the carboxylic group. The enzymes necessary for this are present in the body.
8. Biomembranes containing PUFA are more prone to damage by oxygen reactive species:
The unsaturated fatty acids present in membranes may undergo lipid peroxidation caused by free radicals like hydroxyl, hydroperoxyl, superoxide, etc. to cause impairment in membrane structure and function.
9. Aspirin and indomethacin have anti-inflammatory properties:
Aspirin and indomethacin inhibit prostaglandin synthesis at the level of cyclo-oxygenase enzyme which convert 3arachidonic acid to prostaglandin G2 the precursor of other prostaglandins which are indicators of inflammatory response.
10. Eskimos have low incidence of coronary heart disease despite high fat intake:
Eskimos take a fish based diet which contains omega-3 fatty acids like docosahexaenoic acid and eicosapentaenoic acid. Omega-3 polyunsaturated fatty acids suppress cardiac arrhy-thmias and reduce serum triacylglycerol. It also reduces thrombosis and blood pressure. Hence, the risk of cardio-vascular disease is decreased.
11. Leukotrienes are chemotatic agents:
Leukotrienes are formed from arachidonic acid by lipoxygenase pathway in neutrophylls. It attracts cells to inflammatory sites. The slow reacting substance of anaphylaxis contains leukotrienes. They can cause vaso-dialation and bronchiolar constriction.
12. Niemann-Pick disease, Tay-Sachs disease and Gaucher's disease are examples lipid storage diseases:
Niemann-Pick disease is because of accumulation of sphingo-myelins due to the absence of sphingomyelinase. This leads to hepatosplenomegaly and neurodegenerative disease.
Tay-Sachs disease is due to the accumulation of gangliosides (GM2). This leads to neurodegeneration, blindness, muscular weakness and seizures. Here, the deficient enzyme is hexo-saminidase A.
Gaucher's disease is due to the deficiency of B-glucosidase due to which there is accumulation of glucoce rebrosides. This leads to hepatosplenomegaly, osteoporosis and nervous system involvements.
13. Brain tissue contains several very long chain fatty acids:
Brain tissue has fatty acids like timnodonic (eicosapentaenoic), clupanodonic (docosapentaenoic) and cervonic (doco-sahexaenoic) acid which are all ω3 fatty acids. They are also present in fish oils.
14. Peroxysmal fatty acid oxidation and mitochondrial fatty acid oxidation are different:
Very long chain fatty acids are oxidized in peroxysomes. A defect in this pathway can lead to Zellweger syndrome and 4x-linked adrenoleukodystrophy. Short, medium and long chain fatty acids are oxidized in the mitochondria.
15. Peroxysmal oxidation of fatty acids is necessary for bactericidal effect:
This produces H2O2 which has got a bactericidal effect.
16. The sites of synthesis and effects of thromboxanes and prostacyclins differ:
Thrboxanes are synthesized in platelets and they cause platelet aggregation and vasoconstriction. The prostacyclins are synthesized by endothelial cells and they cause was vasodilatation and prevents platelet aggregation.
17. Prostaglandin E series are used for relieving bron-chospasm:
Prostaglandin E series administered in aerosols cause bronchiolar dilation thereby relieving bronchospasm.
18. Cortisol acts as an anti-inflammatory agent:
Cortisol inhibits phospholipase which removes arachidonic acid from membrane phospholipid. This arachidonic acid is the precursor of prostaglandins which mediates an inflammatory response. As the prostaglandins are not synthesized, cortisol effects anti-inflammatory response.
19. Docohexaenoic acids present in fish liver oils and milk are required for the development of brain and retina:
Docohexaenoic acid (DHA) is synthesized from linolenic acid or obtained from fish oils. It is present in high concentrations in the brain and retina and hence required for their development. It is found that patients with retinitis pigmentosa have low levels of DHA.
20. Lipoprotein(a) excess is a health risk factor:
Lipoprotein(a) is structurally similar to tissue plasminogen required for fibrinolysis, a process of clot dissolution. For this, the plasminogen has to be converted into plasmin which brings about fibrinolysis. Lipoprotein(a) inhibits the activation of plasminogen to plasmin. Hence, lipoprotein(a) is atherogenic and so, is a health risk factor.
21. Hyperlipoproteinemia type II a carries great risk for coronary artery disease:
This is the result of a partial or a complete absence of LDL receptors in the patients. Normally, LDL receptors are located 5in the clathrin-coated pits on the cell membranes of various tissues. The LDL combines with the LDL receptors and this complex is internalized by endocytosis. The receptor is dissociated and they return to cell membrane to bring more LDL. The LDL now fuses with lysosomes and the apoprotein B100 is hydrolyzed to amino acids. The cholesterol may be esterified and stored or used for the formation of steroid hormones, vitamin, D, bile salts or new cell membrane.
In the event of absence or partial absence of LDL receptors, the LDL cannot be removed from the blood and the cholesterol accumulates in the arterial walls causing atherosclerosis. This is life-threatening and the patient may die of AMI or cerebral stroke.
22. Patients with hyperlipoproteinemia type-I can be benefitted by a diet containing more of short and medium chain fatty acids and less of long chain fatty acids:
This disorder is due to the absence of lipoprotein lipase which is required for the removal of chylomicrons by hydrolyzing its Triacylglycerol into fatty acids and glycerol. The same enzyme also hydrolyzes VLDL triglycerides. This can be diagnosed by keeping the plasma/serum overnight when one can note a creamy layer over a clear plasma/serum.
This condition can be improved by taking oils with short chain and medium chain fatty acid containing triacyl-glycerols which are relatively water soluble. Therefore, these can be directly absorbed into the blood without incorporating into chylomicrons, whereas, the long chain fatty acids have to be packaged into chylomicrons for their absorption. Hence, a deficiency of lipoprotein lipase can cause lipemia.
23. Vegetable oils or fish liver oils should be preferred in the diet in case of hypercholesterolemia:
Vegetable oils like cotton seed oil, ground nut oil and sunflower seed oil contain more of polyunsaturated fatty acids of omega-6 variety. The other vegetable oils like soyabean oil, canola oil and olive oil have both omega-6 and omega-3 fatty acids in the proper ratio. The fatty acid present on the 2nd carbon of glycerol part of phospholipids is invariably a PUFA and it is this fatty acid that is transferred from lecithin 6of HDL to cholesterol present in the HDL itself to form ester cholesterol catalyzed by the enzyme lecithin cholesterol acyl transferase. This is an important step in the transport of cholesterol from peripheral tissues to liver and its further excretion in bile thereby decreasing the cholesterol in the body.
24. Restricted intake of alcohol could be beneficial:
If alcohol is taken in moderate amounts, say, a couple of drinks per day can decrease the risk of coronary heart disease. This is brought about by increase in HDL cholesterol. Red wine contains certain phenolic compounds which are antioxidants that inhibit oxidation of LDL. Because of the risk involved in excess alcohol intake nutritionists are apprehensive in recommending alcohol in cholesterol lowering regimen.
25. Trans fatty acids pause health problems:
Trans fatty acids are formed during frying of food items, partial hydrogenation of vegetable oils and during the preparation of baked foods. Their metabolic effects include increasing LDL and decreasing the beneficial HDL. Because of this trans fatty acids increase the incidence of coronary heart disease.
26. Soyabean foods could be beneficial:
Soyabeans contain high amounts of proteins. This also is found to decrease LDL cholesterol which may reduce heart diseases.
27. Fatty acid synthase is a multi-enzyme complex consisting of seven enzymes and one acyl carrier protein:
This is an example of supraquaternary structure of proteins. They are present in the cytoplasm and all these proteins are grouped together so that the products do not detach from the complex. The product of the first enzyme becomes the substrate for the second enzyme and the product of the second enzyme serves as the substrate for the third enzyme and so on till the final product is generated. The individual enzymes of fatty acid synthase complex includes acetyl transacylase, malonyl transacylase, 3-ketoacyl synthase, ketoacyl reductase, hydratase, enoyl reductase and thioesterase. During this process the growing acyl group is carried by an acyl carrier protein. It is advantageous to group 7the enzymes of a particular metabolic pathway together as it stops interfering reactions without compartmentalization, thereby avoiding permeability and transport hindrances. The fatty acid synthase is a dimer consisting of two monomers each with all the seven enzymes and the ACP. The two monomers are arranged in a head to tail manner, with two fatty acids being formed at the same time, at the two ends of the dimer.
28. Bile salts are necessary for lipid digestion and absorption:
Bile salts are sodium taurocholate and sodium glycocholate. The cholic acid is obtained from cholesterol. Cholic acid is then combined with glycine and taurine to form glycocholic acid and taurocholic acid respectively. The bile salts (so called because these are secreted in the bile) have the property of lowering the surface tension and interfacial tension and helps in the preparation of an emulsion of fat droplets and water. The emulsification is a preliminary step in the digestion of lipid. This increases the surface area for the lipase to act on the fat droplets. Further the products of digestion of lipids viz. fatty acids, fat soluble vitamins, etc. are packaged into small spherical particles called micelles stabilized by bile salts. Then these micelles are presented to the microvilli of intestinal mucosal cells. The products of digestion of lipids are absorbed into the intestinal mucosal cell and the bile salts are left behind. If there is an absence of bile salts, the absorption of lipids is affected and split fat (digested fat) is excreted in the stool causing stetorrhea. The deficiency of lipase results in the excretion of unsplit fat in stool.
29. Leptin protein is important in obesity:
The adipocytes are meant for storing fat. They also produce certain regulatory molecules like adiponectin, resistin and leptin which could be playing an important role in obesity. The product of obesity (ob) gene is leptin protein which can be considered as a hormone. It is produced by adipocytes and messages the brain about the level of fat storage. It binds to hypothalamus and controls the appetite and feeding. Leptin secretion is suppressed in fasting (i.e. when the fat stores are less) and is increased in well fed stale (i.e. when fat storage is improved). Most of the studies conducted in rodents have 8implicated leptin in obesity but the work in humans has not been fully validated as it is found that the leptin level in obese individuals is found to be normal. Other regulatory molecules generated by adipocytes may also be involved.
30. Obesity may be of different types:
One is said to be obese when the body mass index (weight in kg/height in meters2) is more than 30. People are said to be overweight when the BMI is between 25 to 30. A BMI below 25 is normal. Another measure of obesity is waist to hip ratio. The fat distribution in males takes place in central portion (abdomen) of the body and this is denoted as android type. The fat is deposited in the females in the lower part of the body, i.e. the buttocks and gluteal regions. This is known as gynoid type. The abdominal fats are lipolyzed and metabolized faster after entering the portal circulation and subsequent entry into the liver. The VLDL production and its conversion to LDL can take place easily resulting in raised cholesterol levels. As the abdominal fat deposition is more prevalent in males, they are at higher risk of CAD. This is in comparison to the fat in the buttocks and gluteal regions as seen in females which has no preference for metabolism in liver as it enters the general circulation and hence less dangerous.
Another important point to be remembered is that when once the fat cells are formed, it is not possible to reduce their number and only their size can be reduced by dieting. When the dieting is discontinued the fat cells may again start to increase in size. This shows that the best way to control obesity is to take preventive measures.
31. Raised level of homocysteine is unhealthy. The raised levels of homocysteine above 15 micromol/liter have been proposed to increase the risk of coronary heart diseases. It has been found that homocysteine levels are inversely related to the serum levels of folate, B12 and B6. Hence, the treatment includes the administration of these vitamins. A deficiency of cystathionine beta synthase can also result in homocysteinuria.
Homocysteine replaces the lysine residues in the collagen fiber of capillary and arterial walls thereby interfering with the cross-linking in collagen making it immature. This may 9also cause the generation of free radicals which prevent the collagen to attain a quarter staggered triple helical structure. This in turn leads to the deposition of cholesterol in the arterial walls leading to coronary artery disease and AMI.
32. Fatty liver may be caused due to various factors.
The liver is not meant for storing fat. The adipose tissue is where the fat is to be deposited. If the fat (triglycerides and cholesterol ester) is deposited in liver due to certain reasons, it is called as fatty liver.
Chronic alcoholism can result in deposition of fat in liver. The hepatocyte oxidizes ethyl alcohol to acetaldehyde with the help of alcohol dehydrogenase. NAD serves as the electron acceptor and becomes reduced. In the next step, acetaldehyde is oxidized to acetic acid, NAD again serving as the hydrogen acceptor. The raised levels of NADH drives the reaction of malate dehydrogenase to the formation of malate from oxaloacetate. The TCA cycle becomes sluggish due to this and the acetyl CoA is routed for the formation of fatty acids which after conversion to TAG gets deposited in liver. The acetaldehyde also causes damage to the hepatocytes, thereby, affecting liver functions. This results in the formation of fibrous tissue made up of collagen culminating in cirrhosis of liver. Finally the chronic alcoholic patient may die of hepatic coma.
For the removal of triglycerides and cholesterol from the liver VLDL is synthesized by incorporating these lipids with apoprotein B100. Phospholipids containing choline and ethanolamine are also required for the assembly of VLDL. Hence, a deficiency of amino acids can affect the synthesis of apoproteins as is evident in protein malnutrition (Kwashiorkor). Methionine being a methyl donor is required for the syntheses of choline and hence lecithine. The joining of lipids with the apoprotein B100 and the secretion of VLDL by the liver also may be defective. In all the above states, fat can be deposited in hepatocytes which can be lyzed after accumulating fat. Substances that prevent fatty liver like choline, methionine, etc. are known as lipotropic factors. Fatty liver can be corrected by administering choline. The antioxidants like tocopherol and selenium are having beneficial effects.
10
Diabetes mellitus can also cause fat deposition in liver. The excessive lipolysis common in diabetes leads to the flooding of the plasma with fatty acids which are taken up by the liver. These excess fatty acids may be oxidized by beta oxidation, but the resultant acetyl-CoA cannot be oxidized in TCA cycle as the oxaloacetate is in short supply. The result could be the deposition of fat in the liver as the capacity of the liver to synthesis and excrete VLDL is relatively less than its capacity to take-up fatty acids.
33. Obesity could be multifactorial:
The often used criteria to identify obesity are based on body mess index which is the ratio of weight in kilograms and height in meter. A BMI from 19 to 25 is considered normal 25.1 to 29.9 is overweight and above 30 is said to be obese.
The obesity in general results from gluttonous eating with minimal physical activities. It also depends on the type of food eaten. For example, energy rich foods are the culprits. The food intake is directly linked to the palatability of the food. Variety of food available these days could be another contributing factor. Another important aspect could be the group in which you eat. In short obesity results when the energy intake is more than the energy expenditure. The modern technological advances has further contributed to a less active form of life.
There is also a genetic component for obesity. If one of the identical twins is obese the other one also would be obese even if they are brought up in different places with varying food habits.
The hormone leptin is secreted by adipocytes if the fat stores are saturated. This is the product of a gene known as “ob” gene denoting obesity. The leptin protein has receptors in the hypothalamus. The binding of leptin to the receptors stops the feeding. A mutated leptin or a leptin receptor leads to gluttonous eating and obesity. It is proved that the obese rats if injected with leptin protein could experience weight loss. However, this has not been replicated in the humans.
Another hormone involved in feeding is ghrelin secreted by gastric mucosa in response to hunger thereby increasing appetite. The neuropeptide y increases the craving for 11carbohydrate food. Galanin is implicated in the liking of fatty foods.
34. Several important substances are derived from cholesterol:
Cholesterol is known as the parent sterol because several important substances are synthesized from it like vitamin D, bile acids and steroid hormones. The cholesterol is essential for cell membrane formation as well.
The 7-dehydrocholesterol present in the epidermis of skin is converted to cholecalciferol which is hydroxylated at 25th position in the liver. 25-hydroxycholecalciferol is hydroxylated at first position in the kidneys which results in 1, 25-dihydroxycholecalciferol or calcitriol which is important in the homeostasis of calcium.
In the bile acid biosynthesis hydroxyl groups are added at specific positions in the steroid structure. The double bond of the ‘B’ ring of cholesterol is reduced, the hydrocarbon chain is reduced by 3 carbons and a carboxyl group is added at the end of the chain thereby generating the primary bile acids like cholic acid and chenodeoxy cholic acid. The rate limiting enzyme of bile acid synthesis is cholesterol 7-alpha-hydroxylase. This is present in the liver and is cytochrome P 450 dependent enzyme. The bile acids contain 24 carbons with two or three hydroxyl groups. They are amphipathic molecules as they have both polar (hydroxyl groups) and the nonpolar (methyl groups) and hence are able to act as emulsifying agents for preparing complex lipids for digestion and subsequent absorption from the intestine. The bile acids conjugate with either glycine or taurine to from glycocholic and taurocholic acid respectively. The bonding is through an amide bond between the amino group of either glycine and taurine and the carboxylic group of bile acid. The sodium salt of bile acids are known as bile salts which are more effective as they are more amphipathic. The bile salts secreted into the duodenum are reabsorbed after their function and are reused by an enterohepatic cycle.
Solubilization of cholesterol in the bile is achieved by bile salt and lecithin the absence of which can lead to cholelithiasis. The decrease in bile salts may be due to decreased synthesis 12of the same as seen in severe hepatic dysfunction or due to biliary obstruction.
The steroid hormones synthesized from cholesterol are cortisol, aldosterone and androgens. The synthesis consists of shortening of hydrocarbon chain and hydroxylation of the steroid ring. The product is a 21 carbon pregnenolone. There is a need for NADPH, molecular oxygen and the enzyme desmolase. Pregnenolone is the parent compound for all other steroid hormones. Pregnenolone is converted to progesterone by oxidation and isomerization which undergoes further modifications to other steroid hormones by hydroxylation. The steroid hormone cortisol is synthesized from progesterone. Progesterone is the precursor for aldosterone also. In another pathway progesterone is converted to testosterone which can be further forming the estrogen (estradiol).
Adrenal cortex synthesizes cortisol, aldosterone and androgens whereas ovaries and placenta form estrogens and progestins and testes is responsible for the production of testosterone.
The steroid hormone binds to a specific cytoplasmic or nuclear receptor of target cells. The hormone receptor complex binds to hormone response elements in DNA and causes increased transcription of targeted genes leading to biological effects.
35. Generally even chain fatty acids cannot give rise to glucose in the body:
The even chain fatty acids are sequentially oxidized to 2 carbon units known as acetyl-CoA which then becomes citrate in combination with oxaloacetic acid. Citrate then becomes cis-aconitate which is then converted to isocitrate. ICDH then converts isocitrate to alpha ketoglutarate liberating one molecule of carbon dioxide. The reaction needs NAD which becomes reduced NAD. The CO2 released comes from acetyl-CoA portion of isocitrate. The second carbon of acetyl-CoA is released in the next reaction involving alpha ketoglutarate, NAD and the corresponding dehydrogenase. Therefore, both the carbons derived from acetyl-CoA (and so indirectly from fatty acid) are liberated as CO2 and hence cannot be forming glucose. This is in contrast to the last three carbon unit (Propionyl-CoA) generated from odd chain fatty acids. This three carbon unit can 13be converted to D-methyl malonyl-CoA, L-methyl malonyl-CoA and then to succinyl-CoA which is glucogenic.
36. Various hypolipidemic drugs are available:
The statin group of drugs, the mevastatin and lovastatin inhibit HMG CoA reductase enzyme, the rate limiting enzyme of cholesterol biosynthetic pathway. The enzyme converts HMG CoA to mevalonate.
Bile acid building resins are employed to interrupt the enterohepatic cycle. Normally, the bile salts after their function in lipid digestion and absorption are reabsorbed and re-secreted constituting the enterohepatic cycle. Very little bile salts are lost from the body everyday. The bile acid binding resins like cholestyramine form a complex with bile salts which prevents the reabsorption of the latter. Hence, more and more cholesterol are converted to bile acids which when continued decreases the cholesterol level in the body.
Another hypolipidemic drug is clofibrate which activates lipoprotein lipase which helps in the clearance of chylomicrons and also the hydrolysis of VLDL and IDL to LDL. Probucol enhances the LDL breakdown and ezetimibe inhibits cholesterol absorption. Aspirin is widely used to prevent thrombus formation. Antioxidants prevent oxidation of LDL, thereby decreasing atherosclerosis. Nicotinic acid prevents lipolysis and so decreases VLDL and LDL. It is also known to lower lipoprotein ‘a’ level.
37. Different types of lipases are present in the body:
The lingual lipase present in the saliva has an optimum pH in the acidic medium obtained in the stomach. Gastric juice contains another lipase, the gastric lipase. Both lingual lipase, the gastric lipase are responsible for the digestion of short and medium chain fats which are relatively water soluble. The pancreatic and intestinal lipases have an optimum pH in the alkaline medium and they mainly clear the long chain fats in presence of bile salts. The adipose tissue contains a hormone sensitive lipase which is activated by glucagon through cyclic AMP and inhibited by insulin. Cyclic AMP further activates cyclic AMP dependent protein kinase which in turn phosphorylates lipase making it active. The lipase 14then mobilizes fatty acids from adipose tissue triacyl glycerol. Insulin is a lipogenic but antilipolytic hormone. Yet, another lipase is lipoprotein lipase anchored in the capillary wall of various tissues like skeletal muscle. The lipoprotein lipase acts on chylomicrons, chylomicron remnants and VLDL.
38. Osteocalcin and bone matrix proteins gla protein of osteo-beasts are subjected to post-translational modification:
Vitamin K is required for the gamma carboxylation of glutamic acid derivatives of osteocalcin and Glaprotein which are bone matrix proteins. This is an example of post-translational modification of protein. This helps in the calcification process.
39. Vitamin C is involved in collagen maturation:
Iron in the Fe2+ state is necessary for the action of lysyl hydroxylase and prolyl hydroxylase for the formation of mature collagen which is a prerequisite for cross-linking of collagen triple helix thereby increasing the tensile strength of collagen. To keep the iron in the reduced state ascorbic acid is required. This is also an example of post-translational modification of proteins and helps to explain the bleeding seen in scurvy, as a result of ascorbic acid deficiency leading to the formation of immature collagen in the capillary walls.
40. Trans-cobalamin and transcorrin are important in vitamin B12 function:
Methyl B12, the predominant form in blood is carried by trans-cobalamin. The storage form of B12 is adenosyl cobalamin which is stored after combining with transcorrin. Generally water soluble vitamins are not stored in the body but B12 is an exception. It is stored in the liver in small quantities.
41. Pure vegetarians (vegans) are likely to suffer from vitamin B12 deficiency:
Vitamin B12 is not present in vegetable sources. It is seen only in nonvegetarian food. Hence, if the diet does not contain enough milk or curd, then the person definitely suffers from vitamin B12 deficiency.
42. Vitamin B12 deficiency can lead to folate deficiency also:
The role of folic acid is in the one carbon metabolism. The active forms of folic acid include methenyl THFA, Methylene THFA, Formyl THFA, Methyl THFA, etc. All the forms except 15methyl THFA are inter convertible. The methyl group from methyl THFA can be removed only by vitamin B12 which becomes methyl cobalamine and THFA is regenerated. Hence, vitamin B12 deficiency can further cause folate lack in the body. This is called as methyl folate trap.
43. Very few enzymes are known to use vitamin B12 as coenzyme:
The one enzyme requiring B12 is methyl malonyl-CoA mutase which converts methyl malonyl-CoA to succunyl-CoA in the metabolism of propinyl-CoA produced by the oxidation of odd chain fatty acids. Either in the absence of this enzyme or in B12 deficiency methyl malonic aciduria may be caused which can also lead to methyl malonic acid accumulation in the brain leading to demyelination and other neurological changes.
The second enzyme requiring B12 is a transmethylase which transfers the methyl group of methyl THFA to cobalamine thereby forming methyl cobalamine and free THFA.
The third reaction involving B12 is catalyzed by homocysteine methyl transferase which transfers the methyl group from methyl cobalamine to homocysteine forming methionine and cobalamine.
The deficiency of this enzyme can cause homocystinuria which is one of the risk factors in coronary heart disease. Homocysteine can replace lysine in collagen of arterial walls so that collagen triple helical structure is disturbed resulting in thrombosis.
44. Neurological manifestations are seen in vitamin B12 deficiency but not in folate deficiency:
Folate deficiency produces macrocytic anemia. Here, the DNA synthesis is delayed. Hemoglobin synthesis is normal. In B12 deficiency megaloblastic anemia is seen. In B12 deficiency as active methionine is not available phosphatidyl ethanolamine cannot be methylated to phosphatidyl choline causing deficient myelination, neurological lesions and sub-acute combined degeneration.
45. Intrinsic factor is required for B12 absorption:
Intrinsic factor combines with vitamin B12 for the absorption of the latter. Due to the production of antibodies to the 16intrinsic factor, it no longer can combine with B12 and so this stops its absorption from the gut, causing pernicious anemia. It is an autoimmune disease.
46. Vitamin B12 deficiency is common in poor Indian vege-tarians:
Vitamin B12 is present only in nonvegetarian food. Poor vegetarians cannot afford milk or milk products and so they suffer from B12 deficiency.
47. Treatment of pregnant women with warfarin can lead to fetal bone abnormalities:
Two proteins of the bone osteocalcin and bone matrix protein gla protein require vitamin K for the gamma carboxylation of their glutamic acid residues. Warfarin is a vitamin K analoge and so can adversely affect bone development.
48. Both vitamin K and vitamin C are required for proper bone growth and maintenance:
Osteocalcin of the bone requires vitamin K for gamma carboxylation of glutamic acid residues. It also contains hydroxy proline which requires vitamin C for hydroxylation. Another protein present in bone, collagen also needs vitamin C for the hydroxylation of its lysine and proline residues, required for collagen maturation and hence healthy bones.
49. Vitamin B12 is protective against coronary heart disease:
Vitamin B12 accepts the methyl group from methyl THFA to form methyl cobalamine and THFA. This methyl cobalamine gives its methyl group to homocysteine to form methionine. A deficiency of vitamin B12 can therefore cause homocysteine excess in the body which replaces lysine from collagen of arterial walls. This can cause atherosclerosis, thrombosis and hypertension.
50. Atrophy of gastric epithelium can lead to deficiency B12:
When there is atrophy of gastric mucosa, the parietal cells are unable to secrete the intrinsic factor, a glycoprotein, which is necessary for the absorption of vitamin B12.
51. All macrocytic anemias in general are treated with folate and vitamin B12:
The second and eighth carbons of purine come from one carbon units carried by folic acid. Hence, for the synthesis and 17division of bone marrow cells DNA replication is required. DNA replication requires purine and pyrimidine formation. To make the active folate continuously available vitamins B12 is required as the methyl group of methyl tetrahydrofolate can be removed only by vitamin B12.
52. Curd is a good source of vitamin B12:
The lactobacilli present in curd synthesize vitamin B12 which can supply the daily requirement of the vitamin in vegetarians, as it is absent in vegetarian food.
53. Vitamin C is involved in the maturation of collagen:
Collagen is a structural protein present in the ground substance of connective tissues of bones, cartilages, tendons, skin, walls of blood vessels, lens, etc. It is the most abundant protein in the body. It is a fibrous protein with great tensile strength. It is triple helix in nature and is wound over each other to form a quarter staggered structure. There is cross-linkings to increase the tensile strength of the collagen fiber. Every third amino acid in collagen is glycine. Lysine, proline, hydroxy lysine and hydroxy proline are other predominant amino acids. Prolyl hydroxylase and lysyl hydroxylase are required to hydroxylate proline and lysine respectively. This requires iron in the ferrous state and vitamin C. Hence, vitamin C is very important in collagen maturation. Cross-linkages are formed by lysyl oxidase enzyme which requires copper.
54. Antioxidant enzymes and vitamins play an important role in balancing the action of free radicals:
A free radical is a molecule which contains unpaired electrons in the outer orbit. Fee radicals are written with a superscript dot, for example, OH.. They are highly reactive and can, if in excess damage different biomolecules and structures like DNA, proteins, phospholipid membranes and other structures. They are also implicated in the process of aging, atherosclerosis, cataract, cancer and diabetes. The products of partial reduction of oxygen are highly reactive and they are termed as reactive oxygen species. The examples include the following:
18
Superoxide anion
2)
Hydroperoxyl radical
(HOO˙)
Hydroxyl radical
(OH˙)
Lipid peroxide
(ROO˙)
Nitric oxide
(NO˙)
Peroxy nitrite
(ONOO˙)
Hydrogen peroxide and singlet oxygen are not considered as free radicals but they are also highly reactive compounds. The free radicals can initiate chain reactions and it has got a short lifespan.
The reactions that generate free radicals include oxidation of food stuffs, xanthine oxidase reaction, NADPH oxidase, super-oxide dismutase, myeloperoxidase, nitric oxide synthase and lipoxygenase. Myeloperoxidase is important in macrophages for bactericidal action.
There are enzymes that nullify the effect of free radicals. Examples include superoxide dismutase which detoxify superoxide anion and convert it to hydrogen peroxide. A defect in SOD can cause amyotrophic lateral sclerosis. Another enzyme is glutathione peroxidase which acts on hydrogen peroxide to form water. It is dependent on selenium. The third enzyme is glutathione reductase which helps in the reduction of oxidized glutathione, formed in the glutathione peroxidase reaction to reduced glutathione. This reaction requires NADPH which is generated by glucose-6-phosphate dehydrogenase in HMP shunt pathway. H2O2 can also be converted to water by catalase enzyme.
In addition to free radical scavenging enzymes there are other antioxidants like vitamin E, uric acid, vitamin C, beta-carotene, caffine and ceruloplasmin. Vitamin E is the lipid phase antioxidant whereas vitamin C is the aqueous phase antioxidant. Beta-carotene is a chain breaking antioxidant like alpha tocopherol but the latter is more efficient.
55. Various diseases can be the result of free radicals:
Inflammatory diseases thought to be due to the deleterious effects of free radicals are rheumatoid arthritis, ulcerative colitis and glomerulonephritis. Adult respiratory distress syndrome causes pulmonary edema. Free radicals can 19cause cataract and also oxidize LDL and this oxidized LDL is more atherogenic and the macrophages are converted to foam cells which accumulate more cholesterol and cause atherosclerosis. As the free radicals damage the DNA molecule, carcinogenic mutations can be started. The free radicals generated by chemotherapy and radiotherapy can cause cell death with medical benefit. It is believed that free radicals could be involved in Parkinsonism and Alzheimer's diseases and the process of aging.
56. The iron is said to be a one way metal:
The iron is present in almost all tissues; the maximum being in blood (75 %) and the remainder being in bone marrow, liver and muscles. The total content of iron is around 4 to 5 grams. The heme containing proteins hemoglobin and myoglobin and non-heme iron protein ferritin carry most of the iron in the body.
About 1 to 2 mg of iron is absorbed daily which is only one tenth of the dietary intake. The iron absorption is regulated well by the mucosal block mechanism so that very less iron is absorbed in the gastrointestinal tract. A vegetarian based diet contains phytates which actually inhibit iron absorption. Leafy vegetables are good sources of iron. Brown jaggery, liver and meat are good sources but milk contains very little iron.
Ascorbic acid is required for iron absorption to keep it in the reduced ferrous form. Only ferrous iron is absorbed in the duodenum. Oxalic acid and phytanic acid inhibit the absorption of iron as is the case with calcium and copper. The concentration of a particular constituent in the body is regulated (otherwise known as homeostasis) at the level of excretion. Iron is an exception to this because the iron homeostasis is achieved by controlling the entry of iron into the blood from the duodenum and ileum. This is explained by the mucosal block theory, which involves the increased iron absorption when the body iron stores are in deficit and suppression of absorption when, the body stores are sufficient. In the mucosal cell iron is oxidized to ferric state and combines with apoferritin to form ferritin from which the metal is absorbed into the blood stream if iron is required in the body, otherwise, the iron is lost by desquamation. Iron in 20the blood is in the ferric state and combines with transferrin for its transport. Total iron binding capacity is about 360 to 400 mg/dl which is due to transferrin. One-third of this capacity is saturated. Serum iron is reduced and total iron binding capacity (TIBC) is increased in iron deficiency anemia. The transferrin is absorbed into the cells by transferrin receptors. The mucosal block of iron absorption is essential as the iron absorbed once into blood can not be removed from the body through urine. The way to remove iron is through blood loss, for example, during menstruation in females. Iron excess in the body resulting from increased absorption of iron is known as hemosiderosis which can also be seen in patients receiving repeated blood transfusion. Hemosiderosis or hemochromatosis in pancreas can lead to cell death leading to diabetes. The treatment for hemosiderosis is repeated phlebotomy.
57. Calcium has several functions in the body. Calcium calmodulin complex activates various kinases which phos-phorylates several other enzymes like adenyl cyclase, glyco-gen synthase, phosphorylase kinase, pyruvate kinase, etc.
Another important function is in the muscle contraction. Calcium on release from endoplasmic reticulum acts on ATPase, actin and myosin thereby bringing about muscle contraction.
Calcium is required for blood coagulation.
Factors like prothrombin; factors VII, IX, X, XI and XII require gamma carboxylation of their glutamic acid residues mediated by vitamin K for chelation with Ca2+ for blood clotting.
The major share of calcium is present in the bones and teeth. Bones also serves as a depository for calcium.
Calcium also acts as a second messenger like cyclic AMP.
The calcium homeostasis is maintained by parathyroid hormone, calcitonin and calcitriol.
Calcium is important in nerve impulse transmission and hormone secretion also.
58. Calcium homeostasis involves PTH, calcitonin and calcitriol:
Parathyroid hormone, calcitriol and calcitonin are involved in the homeostasis of Ca2+. PTH is secreted by the parathyroid 21glands. It is first synthesized as prepro PTH with 115 amino acids. It is then transformed into PTH by removing 31 amino acids from the amino terminus. PTH activates adenyl cyclase. The site of action of PTH includes bone, kidneys and intestine. In bones PTH brings about lactic acid formation which solubilizes the bone calcium. Collagenase causes hydrolysis of matrix collagen leading to bone resorption. The calcium released increases serum Ca2+ level.
In kidneys, the function of PTH is to decrease calcium excretion by re-absorption of Ca2+ through tubules but PTH increases phosphate excretion.
The conversion of 25-hydroxycholecalciferol to 1, 25-di-hydroxycholecalciferol is carried out by alpha1 hydroxylase enzyme present in kidneys. This enzyme is stimulated by PTH. 1, 25-dihydroxycholecalciferol termed as calcitriol enhances the absorption of calcium from intestine by synthesizing a calcium binding protein.
Calcitonin is a peptide hormone having about 34 amino acids secreted by the thyroid parafollicular cells. It is secreted in response to raised serum calcium levels. Calcitonin incre-ases the osteoblastic activity. Calcitonin decreases the serum calcium level which is opposite to the action of PTH. Both PTH and calcitonin facilitate phosphorus excretion through urine.
Calcitriol is the active form of vitamin D. Its synthesis starts with seven-dehydrocholesterol present in the epidermis of the skin where the ultraviolet rays break the steroid ring between positions 9 and 10 to form cholecalciferol. We can consider vitamin D as a prohormone and calcitriol as a hormone. The cholecalciferol is hydroxylated at 25th position to form 25-(OH) cholecalciferol. The hydroxylation takes place in the liver. 25-hydroxycholecalciferol is transported to kidneys through plasma for which the vitamin is bound to a specific vitamin D binding protein. In the kidneys the alpha1 hydroxylase converts 25-hydroxycholecalciferol to 1, 25-dihydroxycholecalciferol otherwise called as calcitriol. Both the above hydroxylations require cytochrome P-450 and ferredoxin. The calcitriol is considered as a hormone as it is formed from an inactive prohormone, transported through 22blood and also has target organs like kidneys, intestinal cells and bone. Calcitriol acts as a steroid hormone and binds to cytoplasmic receptors. The complex further binds to specific regions on DNA and subsequently causes the transcription of calbindin gene to the corresponding m-RNA. The m-RNA is then translated into calbindin protein. This increases the Ca2+ absorption from the intestine. On the bones, the calcitriol has an osteoblastic effect and the bone mineralization is enhanced.
In the renal tissue, calcitriol increases the reabsorption of calcium and phosphorus by renal tubules. The serum levels of calcitriol depend on the levels of calcium, phosphorus and PTH.
59. Vitamin A has got three active forms:
This vitamin is present only in animals. The precursor of vitamin A, Beta-carotene is present in plants. All retinoids have polyisoprenoid units having beta-ionone ring. The three bioactive forms include retinol, retinal and retinoic acid. The retinal reductase reduces retinal to retinol. Retinal can also be oxidized to retinoic acid which cannot be converted back to other forms.
Vitamin A is absorbed along with other fats from the intestine as chylomicrons. Hence, a lack of bile salts as seen in biliary obstruction can hamper vitamin A absorption. The retinol binding protein transports vitamin A from liver to other tissues. The retinol is taken up by the tissues through specific receptors present on gonads, retina, skin, etc. and within the cells has to bind to cellular retinoic acid binding proteins. The vitamin then attaches to specific regions on DNA.
Rhodopsin present in the photoreceptor cells of retina. It is made up of opsin the protein and 11-cis retinal. The photons convert 11-cis retinal to all transretinal. The photo-excitation activates the G-protein leading to the generation of cyclic GMP which acts as cation channels. With the help of transducin, the nerve impulses generated are transmitted to the brain visual centers. The inactivation of rhodopsin is by the phosphorylation of serine residues by a rhodopsin kinase.
The photons produce confirmatory changes so that opsin and all trans-retinal are separated. All trans-retinal is then isomerized to 11-cisretinal in the retina and then reform 23rhodopsin in the dark. If a person suddenly shifts from a bright area to a dim lit zone, he finds it difficult to see. After spending a couple of minutes in the dark room, his vision improves. This time is known as dark light adaptation. This can be explained by the fact that in bright light the rhodopsin stores in the Rhodes are exhausted whereas the stores are replenished in the dark.
The rods are for vision in dim light and cons are for color vision and for vision in bright light. For color vision we have cyanopsin (blue), porphyropsin (red) and iodopsin (green). Less number of cons or defective chromophore may cause color blindness. 11-cisretinal is required for color vision also.
Retinol is required for normal reproductive function by acting like a steroid hormone. Another important function is in growth and differentiation of tissues. Both carotenoids and vitamin A have antioxidant properties and have effects against cancer. Vitamin A and beta-carotene protect heart and also preserves a normal epithelium and mucous membrane.
Deficiency of vitamin A leads to night blindness, xerop-hthalmia, Bitot's spots and keratomalacia. The vitamin A can ward off infections of respiratory tract by keeping the mucous membrane healthy and moist.
60. Vitamin E acts as an antioxidant:
Alpha-tocopherol is the most active form of vitamin E. It prevents the oxidation of cell constituents like polyunsaturated fatty acids. About 8 to 10 mg of vitamin E is required by humans daily. The vitamin E requirement increases with increase in PUFA intake. The vitamin E deficiency can be present in premature infants. It may be due to defective fat absorption and transport. Vitamin E deficiency can cause hemolysis due to increased level of peroxides. It is observed people taking diets containing fruits and vegetables suffer from decreased incidence of some chronic diseases. But the clinical trials have not been successful in confirming these observations.
It has been observed that vitamin E boosts the resistance to infections and also slows down the aging process. It prevents atherosclerosis by inhibiting oxidation of LDL which is more dangerous in causing atherosclerotic plaques. The selenium is a metal present in the enzyme glutathione peroxidase 24involved in the destruction of free radicals. Each decreases the requirement of the other meaning they act synergistically. Another selenium containing enzyme is thyroxine deiodinase converting T4 to T3. As its concentration is highest in tests, it is necessary for spermatogenesis. Selenocysteine acts as the 21st amino acid and its codon is UGA.
Vitamin E is the least toxic of all the fat soluble vitamins. Toxicity may be observed when the daily consumption exceeds 350 to 400 mg.
61. Folic acid is required for one carbon metabolism:
Certain anabolic steps need the addition of one carbon units remaining in different levels of oxidation states like formyl, formimino, methyl, hydroxymethyl, methylene and methyl groups. All the above one carbon units are carried by folic acid and are inter convertible except methyl tetrahydrofolate. The donors of one carbon units include formate and tryptophan (formyl THFA), histidine (formimino THFA and methenyl THFA) and choline, serine and glycine (hydroxymethyl THFA and methylene THFA).
Formyl THFA is needed for the synthesis of formyl methionyl transfer RNA and carbon-2 of purine nucleus. The conversion of uridine monophosphate to thymidine monophosphate requires the addition of methyl group from methylene THFA. Methylene THFA can be converted to methyl THFA which is necessary for transmethylation of serine to choline and norepinephrine to epinephrine and also for the synthesis of creatine. Methyl THFA can give the methyl group to vitamin B12 to form methyl transcobalamin which in turn gives its methyl group to homocysteine to make methionine.
62. The initiation step in translation requires several initiation factors:
The number of initiation factors required in prokaryotes is three whereas in eukaryotes the initiation is more complex and requires more initiation factors. The prokaryotic initiation factors include IF-1, IF-2 and IF-3. The initiation factors of eukaryotes are prefixed with “e”: eIF-1, eIF-2, eIF-3, eIF-4A, eIF-4B, eIF-4G, eIF-4F, eIF-4E and eIF-5.
25
63. The synthesis of one peptide bond utilizes four high energy phosphate bonds:
The activation of amino acid to amino acyl t-RNA uses one ATP hydrolyzed to AMP and pyrophosphate which is equal to the hydrolysis of two ATPs to two ADPs. This reaction requires a specific transfer RNA for a specific amino acid and is catalyzed by a specific amino acyl t-RNA synthetase.
Two more high energy phosphate bonds in the form of GTP are required for the binding of amino acyl RNA to the 4OS ribosome and another GTP for the elongation step specifically for the translocation of ribosome over m-RNA.
64. Peptide bond formation requires a ribozyme:
During protein synthesis, the peptidyl transferase which is an RNA enzyme, present in the large ribosomal submit is responsible for catalyzing the combination of the amino group of the amino acid on the amino acyl t-RNA occupying the “A” site and the esterified carboxyl group of the amino acid on the peptidyl t-RNA occupying the “P” site to form a peptide bond. The transferase is a ribozyme meaning it is an RNA and not a protein. It is 28S r-RNA.
65. Genetic code is a triplet:
It was a mystery that how a set of four nucleotides code for twenty amino acids in a protein. If we consider one nucleotide coding for one amino acid, only 4 amino acids could be coded and if we propose that two nucleotides code for an amino acid a maximum of only 16 amino acids could be indicated. Then, emerged the final hypothesis that the codons of the messenger RNA are actually triplets, i.e. made up of three nucleotides in a specific sequence for a particular amino acid. In this way there could be 64 (4 × 4 × 4 = 64) codons available for coding 20 amino acids. This was actually proved by the experiments carried out by Khorana and Nirenberg.
66. Genetic code is unambiguous:
As the genetic code is a triplet, there are 64 possible codes for 20 amino acids and out of these, three are noncoding meaning they do not code for any amino acid but rather they act as termination codons or stop codons. The remaining 61 26codons are responsible for indicating 20 amino acids. Therefore, one amino acid may be coded by more than one codon. At the same time given a codon one and only one specific amino acid is denoted. The transfer RNA contains the anticodon to a specific codon of m-RNA and hence carries a specific amino acid. This ensures the fidelity and accuracy of translation.
67. Genetic code is degenerate:
During protein synthesis, the anticodon present on transfer RNA (amino acyl t-RNA) shall bind to the codon of m-RNA as they are complementary. The number of codons are 61 and the number of t-RNA is only around 31. This means there is less stringency in the binding of third nucleotide of the codon and the corresponding nucleotide of the anticodon on the transfer RNA. Hence in a codon, the first-two nucleotides are more important and third-one is flexible to some extent meaning it can ‘wobble’ or we can say the code is degenerate. This degeneracy of the code can ward off the bad effects of some mutations. For example, AGA and AGG both code for arginine.
68. Genetic code is universal:
All the living organisms, i.e. both prokaryotes and eukaryotes use the same genetic code except mitochondria. These exceptions include the four codons read differently in mitochondria and cytoplasm. The codon AUA is read as methionine and UGA codes for tryptophan in mammalian mitochondria. Similarly, the codons AGA and AGG are read as chain terminator codons in mitochondria rather than arginine. As a result of this mitochondrial translation uses only 22 t-RNAs whereas the cytoplasmic system uses 31 t-RNAs. With these exceptions, we can conclude that the genetic code is universal.
69. The genetic code is non-overlapping and non-punctuated:
During protein synthesis the genetic code is read as separate entities and there is no overlapping of the codons. This takes care of the maintenance of the proper primary structure of proteins (sequence of amino acids). Similarly, the codons are read without any stoppages or punctuations in between codons and when once the translation process is initiated and it goes on till a terminator condon is encountered.
27
70. Post-transcriptional modifications are necessary for the formation of m-RNA.
m-RNA is derived from heteronuclear RNA (Hn RNA), heteronuclear RNA is the primary transcript formed during transcription. In comparison to m-RNA, it is much larger and also contains both intron and exon sequences. Hn RNA contain both 7-methyl guanosine triphosphate cap and poly adenosyl tail. To convert Hn RNA into mature RNA the first step is intron-exon splicing which is done by ribozymes (RNA enzymes like U1, U2, U4, U5 and U6 associated with more than 60 proteins to form small nucleoprotein molecules called as SnRNAs (or Snurps). It is of clinical interest that autoantibody production against snurps may lead to systemic lupus erythematosus. Defect in intron-exon splicing may also result in certain thalassemias. During the process of splicing the introns which are the noncoding sequences are removed and the exons which are the coding sequences are ligated.
The second step in the conversion of the HnRNA to m-RNA is the addition of a cap at 5'end of RNA in the form of 7-methyl guanosine triphosphate. The third step involves extension 3’ end of RNA by the addition poly adenosyl tail.
71. Elongation and releasing factors are also essential for protein synthesis:
After the completion of the initiation step in translation, the elongation of the protein chain has to take place. For this, the codon on the aminoacyl site of whole ribosome has to be recognized by the specific aminoacyl transfer RNA with a complementary anticodon carrying the specific amino acid. For the binding of this specific aminoacyl to RNA to the ‘A’ site need an elongation factor (EF-1) and a GTP molecule. Then, the amino group of the amino acyl t-RNA reacts with the esterified carboxyl group of the peptidyl t-RNA to form a peptide bond.
After the formation of the first peptide bond, dipeptidyl t-RNA on the aminoacyl or ‘A’ site of the whole ribosome has to be shifted to the peptidyl of ‘P’ site.
This step is known as translocation and requires one GTP and elongation factor-2.
28
Releasing factors are responsible for the release of protein chain from ribosomes after the appearance of a terminator codon on the ‘A’ site. No amino acyl t-RNA can recognize the codon present on ‘A’ site. The releasing factors are able to recognize the stop codon and so hydrolyze the ester bond between the carboxyl group of amino acid and the hydroxyl group of the adenosine of t-TNA, thereby releasing the nascent polypeptide.
72. The differences in the ribosomes of prokaryotes and eukaryotes have been instrumental in the development of antibiotics:
Eukaryotic and prokaryotic ribosomes are different and prokaryotic ribosomes are smaller with a 30s smaller subunit and a 50s larger subunit. The eukaryotes have larger ribosomes with subunits of 40s and 60s. The number and quantity of proteins are also different. This difference is exploited to develop newer drugs/antibiotics to combat infections caused by prokaryotes. This has helped the humanity to increase the lifespan of individuals.
73. Post-translational modification is necessary for insulin synthesis:
Insulin is first synthesized as preproinsulin with a signal peptide at its amino terminus. It has got 109 amino acids. As it is a secreted protein it has to go through the endoplasmic reticulum. The signal peptide is necessary for this after which the signal peptide is cleaved off thereby converting pre-proinsulin to proinsulin. The proinsulin consists of about 86 amino acids in a linear chain. For the activation of proinsulin to insulin a connecting peptide of length 35 amino acids is removed so that the insulin with two polypeptide chains (A chain with 30 amino acids and B chain 21 amino acids) is produced. The two chains are linked by cysteine amino acids at specific positions. The 7th cysteine of A chain and the 7th cysteine of B chain are linked. Further the 20th cysteine of A chain is linked to the 19th cysteine of B chain. There is an intrachain disulfide bond between the cysteines on 6th, 11th position of B chain.
74. Chaperons have an important role in protein folding:
It is true that proper protein folding depends on the primary structure of proteins. Having agreed on this, there are a group 29of specialized group of proteins named as chaperones for overseeing the proper folding of many species of proteins. These protein molecules are called as stress proteins as they are synthesized when the cells are subjected to stressfull states like heavy metals, heat, free radicals, etc. They try to maintain a quality control of proper folding and edit the wrongly folded proteins.
75. Proper protein targeting after synthesis essential:
Defective proteins are attached with ubiquitin and are degraded by proteasome, a proteolytic system in cytosol.
The correctly folded proteins have to be delivered to proper organelles in the cell. Each protein will have a specific sequence of amino acids denoting the place where it should head for. Secreted proteins are usually associated with endoplasmic reticulum.
Nuclear proteins will have nuclear entry signals and the mitochondrial proteins will have mitochondrial import signals. Improper delivery of proteins can cause diseases like Zellweger syndrome. This is due to the absence of enzymes in the peroxisomes needed to oxidize very long chain fatty acids. The enzymes are synthesized but they are not delivered to peroxisomes. Hyperoxaluria and cystic fibrosis are other examples of diseases resultant of improper delivery of proteins.
76. Prions can be harmful:
The prions are proteinacious infective molecules which are not either bacteria or virus. They are implicated in trans-missible spongiform en normal protein with correct structure whereas prpsc (sc stands for scrapie) is an abnormal protein with a different secondary and tertiary structure but with a normal primary structure. Therefore, a change in protein conformation makes the protein infective. The normal prp has more α-helices whereas the infectious prpsc has more beta pleated sheet. Because of its altered conformation, the infective proteins become more insoluble and become resistant to proteolytic degradation. This then starts a chain reaction and an abnormal protein making other normal proteins also abnormal.
30
77. In Alzheimer's disease the structure, properties and functions of certain proteins are altered:
Misfolding of proteins can be seen in Alzheimer's disease. The dominant protein of α-amyloid plaque which is seen in this disease is amyloid peptide which contains more beta pleated sheets. This is a neurotoxin and causes cognitive impairment seen in this disease. Most cases of Alzheimer's disease is not genetic but about 10 percent is familial. Another factor involved is the accumulation of neurofibrillary tangles in the brain. These neurofibrillary tangles contains an altered tau protein. In the normal state it helps in the normal assembly of the microtubular structure. Therefore, the pathologic event in Alzheimer's disease seems to be the misfolding of two proteins: α-amyloid and Tau, which undergo a conformational transformation from a soluble alpha-helix rich to a state of beta pleated sheet which becomes resistant to degradation and hence promotes aggregation. Apolipoprotein E is shown to influence this process. This disease is associated with a gradual memory loss, confusion, agitation and hallucinations. The fact that this is a disease of old age and as the number of elderly is increasing; this could become a complex social problem.
78. Point mutations include transition and transversion:
Mutation is a change in the nucleotide sequence in the gene. It may be a point mutation or a frame shift mutation. Frame shift mutation is due to deletion or addition of one or more than one nucleotide.
Point mutation is replacing of one nucleotide with another. It is called as transition if one purine is replaced by another purine or a pyrimidine is replaced by another pyrimidine. Transversion is when a purine is replaced by a pyrimidine or when a pyrimidine is placed by a purine.
79. Mitochondrial DNA is different from nuclear DNA:
Compared to nuclear DNA mitochondrial DNA is maternally transmitted. Human mitochondrial DNA contain 2 to ten copies of a small circular double stranded DNA molecule that makes up 1 percent of total cellular DNA. The majority of the peptides in mitochondria (about 54 out of 67) are coded by nuclear DNA. A number of diseases are associated with mutations in mt. DNA like myopathies, neurological disorders 31and other oxphos diseases. Tissues requiring more ATP like CNS, kidneys, liver and skeletal muscles are maximum affected by defects in oxidative phosphorylation.
80. Certain viruses possess reverse transcriptase activity:
The retroviruses like HIV contain RNA as the genetic material in place of DNA. After entering into the host cell the viral RNA is copied into a single stranded DNA with the help of RNA dependent DNA polymerase (reverse transcriptase). After this, a double strand DNA is synthesized from the single stranded by DNA polymerase. This double stranded DNA is then incorporated into the host DNA. The viral proteins are synthesized from the viral DNA segment using the host protein synthesizing machinery.
Further the reverse transcriptase has use in DNA recombinant technology for preparing complimentary DNA from messenger RNA of the protein to be synthesized by genetic engineering.
81. Frame shift mutations lead to the formation of abnormal proteins:
Frame mutations are the result of either deletion or addition. The deletion may lead to the removal of one nucleotide, two nucleotides, three nucleotides or multiples of triplets. Similarly, one, two, three or multiples of three nucleotides may be added. In any case both these can result in a garbled amino acid sequence. As the genetic code is read as triplets, the deletion of the third nucleotide or any of the other two nucleotides of a code, will be read after incorporating the first nucleotide of the next code, which naturally results in the incorporation of a different amino acid. The addition of a nucleotide can also lead to abnormal primary structure of proteins. If a triplet of nucleotides is added the protein formed shall contain one extra amino acid coded by the added codon. On the contrary the deletion of a triplet can result in a protein with one amino acid (the specific amino acid coded by the deleted codon) less. If the addition or deletion is in multiples of triplets, the protein translated shall have that many amino acids extra as the number of triplets or less respectively for addition and deletion.
32
If a deletion is followed by an addition, the amino acid sequence before the deletion and after the addition will be normal but sequence between the deletion and addition shall be garbled.
82. Elongation factors and GTP are required for the elongation step in protein synthesis:
The step preceding elongation in translation is initiation because of which the initiation complex with peptidyl t-RNA attached to the ‘P’ site and a free aminoacyl site to which the aminoacyl t-RNA carrying the second amino acid to be incorporated will be attached in the elongation step. The initiation complex consists of full ribosome, m-RNA and the peptidyl t-RNA.
In the elongation step, the aminoacyl t-RNA carrying the specific amino acid coded by the second codon reacts with a GTP molecule and an elongation factor-1 and this ternary complex then attaches to the aminoacyl site with the release of elongation factor and hydrolysis of GTP. This is followed by the formation of peptide bond between the two amino acids. Next step is the shifting of ribosome over the m-RNA by a codon distance. This requires elongation factor 2 and another GTP molecule which is hydrolized. This brings the peptidyle t-RNA from ‘A’ site to ‘P’ site so that the ‘A’ site becomes free to receive new aminoacyl t-RNA.
83. t-RNAs are important for protein synthesis:
There are 31 t-RNAs in the cytoplasm and 22 t-RNAs in the mitochondria. A specific amino acid is carried by a specific t-RNA. This attachment of the amino acid to its specific t-RNA is done by a specific enzyme, aminoacyl t-RNA synthetase. Therefore, there are 20 such enzymes as there are 20 aminoacids. These aminoacyl t-RNAs get attached one at a time on the ‘A’ site of ribosome depending on the codon present on the m-RNA that is being translated into a protein. Each t-RNA contains a specific anticodon that is complimentary to a specific codon present on the m-RNA. So, the function of t-RNA is to carry amino acids to the ribosomes for the fabrication of proteins. The t-RNAs have a characteristic structure with an acceptor arm (to attach amino acid), anti-codon arm, dihydrouracil arm and a thymidine pseudouridine arm. They are relatively smaller RNAs and have got the shape of a clover leaf.
33
84. RNA polymerase is an important enzyme:
DNA dependent RNA polymerase catalyzes the formation of different types of RNAs from the template strand of DNA.
The E. coli RNA polymerase consists of two alpha subunits and two beta subunits. The holoenzyme is formed by the attachment of a sigma factor. The RNA polymerase binds at the promoter site on the template strand of DNA. Then, the RNA formation is initiated at the start site. The RNA is synthesized in the 5’ to 3’ direction. The transcription includes the following steps: Initiation, elongation and termination. The sigma factor is required for the proper attachment of the RNA polymerase at the promoter site and the initiation step. Mammalian cells have three nuclear DNA dependent RNA polymerases and they transcribe different types of RNAs. RNA polymerase I is for r-RNA synthesis, RNA polymerase II for m-RNA and RNA polymerase III is for t-RNA and 5s rRNA. As the elongation process continues the RNA polymerase progresses along the DNA molecule, preceded by the unwin-ding of the DNA, which is to be transcribed, to get proper access for nucleotide base pairing to the template strand. This produces transcription bubbles. The RNA polymerase has an associated unwindase activity to open the DNA helix and a rewindase activity which rewinds DNA after the process of transcription. Topoisomerases help in the process by removing super coiling of DNA. The addition of new nucleotides in the RNA depends on the nucleotide present on the DNA and follows the base complementarity, i.e. A to U and G to C. The termination of transcription is signalled by the presence of Rho protein factor.
85. Transcription start signals in prokaryotes and eukaryotes are different:
Bacterial promoters are about 40 nucleotides long. Approxi-mately 35 base pairs upstream of the transcription start site-there is a sequence of 8 nucleotide pairs (TGTTGACA) to which RNA polymerase binds. About 10 nucleotides upstream is a six nucleotide pair (TATAAT) which is known as TATA box. These are known as transcription start signals.
In the eukaryotes, these signals are more complex which contains both “where” and “how frequently” signals 34whereas the prokaryotes have only “where” signals. The transcription start in eukaryotes depends upon a sequence located 32 nucleotides upstream from the start site. This region has the following sequence. TATAAAAG (TATA Box). TATA box is bound by TATA box binding protein which in turn attaches to several other proteins. There is another box known as CAT box. Eukaryotes also have enhancers and repressors (silencers). These genes also possess hormone response elements and metal binding sites for modulating the gene expression.
86. Post-translational modifications are required for certain proteins:
Some proteins when formed are inactive. For example, pro-collagen has to be activated to collagen. For this, the lysine and proline have to be hydroxylated with the help of hydroxylase enzymes which requires iron in the ferrous state. This in turn needs ascorbic acid. Other examples include activation of proinsulin to insulin, pepsinogen to pepsin, activation of blood clotting factors, etc.
87. Taq polymerase is used in PCR:
Thermus aquaticus is a bacterium which lives in hot water and so its enzymes can withstand high temperatures. In the first step of PCR, the double helical DNA is separated into single strands by heating at 90°C. Then, each strand is amplified by Taq Polymerase at 50°C. This is followed by another cycle of strand separation at 90°C and then amplification of the newly formed strands. As the reaction shifts between 50°C and 90°C, the enzyme used has to be resistant to temperature fluctuations and Taq polymerase is the ideal candidate for this.
88. DNA finger printing has applied value:
The nucleotide sequences of any two individuals are not alike. This can be used to identify the individuals. Because of this, it is important in medicolegal cases. The DNA from a hair follicle, mucous cell, blood or semen can be amplified and can be compared with the DNA of suspects.
This technique is useful in detecting genetic diseases like hemoglobinopathies, cystic fibrosis, hemophilia etc. Another use is to diagnose viral and bacterial infections.
35
The pattern obtained after restriction enzyme action on DNA and the variable number of tandem pepeats (VNTR) are also usually employed.
89. Polymerase chain reaction is an important diagnostic tool:
PCR helps us to amplify one DNA molecule into several copies in a very short time. The DNA then could be digested with restriction endonucleases to have several fragments of DNA. A few of these fragments could contain the defective DNA. This can be subjected to electrophoresis and then identified with specific probes which are complementary to the mutated (abnormal) DNA. Both genetic and infectious diseases could be diagnosed by this method. Probes are small single stranded DNA molecules with a complementary sequence to a defective/viral DNA. This also has applications in Medicolegal cases. The DNA from samples obtained from the crime scene can be amplified by PCR and compared with the DNA of suspects.
90. There are several RNAs with specific functions:
The different types of ribonucleic acids include messenger RNA, transfer RNA, ribosomal RNA, heteronuclear RNA and small nuclear RNAs.
Messenger RNA, having a sequence complementary to the template strand of DNA and a sequence same as the antitemplate strand of DNA with uracil replacing thymine in RNA. The enzyme required for RNA synthesis is type II RNA polymerase and the substrates are ATP, GTP, CTP and UTP. The messenger RNA carries the message (genetic code) from the specific gene (DNA) to the ribosomes for translating this coded message into a specific sequence of amino acids for the synthesis of a specific protein. The m-RNA has a 7-methyl guanosine triphosphate ‘cap’ at 5 end and a polyadenosyl tail at the 3’ end. The 5’ end of m-RNA corresponds to the amino terminus of the protein and it proceeds to the 3’ end of m-RNA representing the carboxy terminus of the protein.
The messenger RNA is derived from heteronuclear RNA by removing introns and joining of exons. It also involves the 5’-capping and 3’-poly-A tailing. The Hn RNA is much larger than m-RNA and it contains both coding and non-coding sequences. Improper processing of Hn RNA can lead 36to various diseases like systemic lupus erythematosus and thalassemias.
The transfer RNAs are meant for the transfer of amino acids from cytoplasm to ribosomes for protein synthesis: A specific t-RNA containing a specific anticodon carries a specific amino acid so as to prevent any defect in the primary structure of proteins. There is a minimum of one specific t-RNA for every amino acid. This energy requiring binding of amino acid to its specific t-RNA is done by amino acyl t-RNA synthases and GTP. The t-RNA on one hand recognizes a specific codon of m-RNA and on the other hand binds to a specific amino acid.
The ribosomes contain r-RNAs in the larger subunit (5S, 5.8S and 18S r-RNA) and the smaller subunit contain 28S r-RNA. The ribosomes are the factories of protein synthesis.
The spliceosome contains five small nuclear RNAs (U1, U2, U5, U4 and U6) and more than 60 proteins to form a small nucleoprotein complex called as “snurp”. These snurps are involved in the splicing which removes the introns and joins the exons. These small nuclear RNAs act as enzymes termed as ribozymes.
91. Mutations may be point or frame shift type:
Mutations are due to alterations in the sequence of nucleotides in the gene. Point mutation results when one base is replaced by another. For example, a purine may be replaced by another purine or by a pyrimidine. It may also occur when a pyrimidine is replaced by another pyrimidine or by another purine. Several of the hemoglobinopathies (Sickle cell disease, HbC, HbD and HbE) belong to this category. Frame shift mutations occur due to addition of nucleotides or by deletion. These changes may be concerning one or more than one nucleotides in the gene and so can have major changes in the structure and function of proteins, coded.
92. Blot techniques are useful in molecular biology:
There are three types of blot techniques:
  1. Southern blot
  2. Northern blot
  3. Western blot.
In southern blot, the target DNA is identified. This can be used to identify a genetic abnormality, viral infection or a 37bacterial infection. This can also be used in medicolegal cases and dispute over parenthood. The whole of the DNA from cells is extracted and then subjected to digestion by restriction endonucleases. This will produce several fragments of DNA depending on the type of restriction endonuclease employed because REs have different site specificities. Each RE recognize a specific nucleotide sequence of about four bases long. These are palindrome sequences that is; they read the same both forward and backward on the two strands of DNA. The fragments are then subjected to electrophoresis on agarose gel. It is then treated with sodium hydroxide so that the DNA becomes single stranded. Only a few fragments could be containing the target sequence. For identifying this, it is then blotted over to a nitrocellulose membrane. This membrane is then baked at 80°C for fixing the strands on the membrane. For the target DNA identification, radioactive DNA probe is placed over the membrane. The probe will bind and form a duplex with its complementary sequence which would be the target sequence. The probes are small single stranded DNA (about 20 nucleotides in length) which are complementary to a target DNA. The probes are now available for several diseases like cystic fibrosis, sickle cell anemia, Duchenne muscular dystrophy, hemophilia, etc.
In northern blot, the target RNA is identified after electro-phoresis. The probe used may be a complementary DNA or RNA. So, here the total RNA from the cell has to be isolated.
In western blot analysis the proteins are isolated from the tissue and then subjected to electrophoresis, transferred to nitrocellulose membrane. Here, the probe employed is the specific antibody against the target protein.
93. Nucleoside analogs are useful as anticancer agents:
Analogs are structurally same but have different functions. Nucleoside means a purine or a pyrimidine base attached to a ribose or a deoxyribose. Analogs can be prepared either by making changes in structure of the base or the pentose moiety. Examples include 5-fluorouracil and 5-iodouracil which are analogs of thymidine. During replication of DNA, these analogs may be incorporated in place of thymine in the growing strand of DNA which is to be synthsized complementary to the parent strand. But with the incorporation of the defective 38nucleoside in DNA, the further growth of DNA is not possible and hence cell division is stopped. This is the basis of treatment of cancer.
Other useful analogs include thioguanine, azacytidine, cytarabine, vidarabine, etc. The last two have arabinose in place of ribose.
94. VNTR and RFLP are used as a DNA profile of a particular person:
VNTR means variable number of tandem repeats. RFLP means restriction fragment length polymorphism. The human genome has sufficient DNA to code for as much as 15 lakh genes. But our body has only less than 25000 proteins. Therefore, only less than 2 percent of the DNA is coding. The remaining portion although apparently appears non-functional is thought to be involved in the regulation of gene expression and in development of new characteristics and genes. Large portions of DNA are made up of repeated sequences. Such sequences could be of thousands of nucleotides in length repeated several times. These sequences are unique to a particular individual. In other words no two individuals can have the same sequence making it an ideal identifying marker for a person.
There are hundreds of different types of restriction endo-nucleases with specific cleaving site properties. They recognize specific sequences (palindrome) and cleave the molecule at these sites. As no two individuals possess the same sequence, the restriction enzymes produce different patterns. These restriction fragments could serve as a genetic marker of the individual.
95. Von Gierk's disease can result in hyperuricemia:
This is a glycogen storage disease in which the enzyme glucose-6 phosphatase, the last enzyme in glycogenolysis and gluconeogenesis is deficient. Because of this, the accumulated glycogen in liver cannot be catabolized. This leads to hypoglycemia. As glucose-6 phosphate accumulates, it is diverted to the pentose phosphate pathway which generates ribose-5 phosphate which gets converted to purines via phosphoribosyl pyrophosphate. These excess purines (Adenine and guanine) are catabolized to uric acid causing 39hyperuricemia. This in turn causes deposition of uric acid in and around joints causing gouty arthritis.
96. HG-PRTase deficiency leads to Leish Nyhan syndrome:
Hypoxanthine guanine phosphoribosyl transferase is a purine salvage pathway enzyme which catalyzes the reaction between hypoxanthine/guanine and phosphoribosyl pyrophosphate to form IMP/GMP and pyrophosphate. This is specially important in the brain for synthesizing purine nucleotides from purine bases as there is no de novo synthetic pathway in the brain. As the HG-PRTase is deficient, there is accumulation of PRPP and hence the brain do not get nucleotides. The excess PRPP in other tissues goes for the formation of purines. This further is broken down to uric acid causing gout.
Here, the brain is maximally affected due to the lack of nucleotides. This leads to neurological symptoms like self-mutilation including biting lips and nails.
97. Allopurinol is an effective drug for gout:
Allopurinol is an analog of hypoxanthine and xanthine. Allopurinol utilizes xanthine oxidase to become alloxanthine which is a more potential inhibitor of xanthine oxidase. This is known as suicidal inhibition. This drug is useful as it inhibits the further formation of uric acid and reduces hyperuricemia and gout. This is an example of competitive inhibition.
98. Several competitive inhibitors have medical uses:
Competitive inhibitors act as antibiotics like sulfa drugs which are similar in structure to para-aminobenzoic acid (PABA). The bacteria form folic acid by combining PABA with pteroyl glutamic acid to form folic acid. So, in presence of sulfa drugs folic acid cannot be formed as the above reaction is inhibited and the bacteria is not able to grow and the infection is controlled. Sulfa is not toxic to humans because humans cannot synthesize folic acid.
Dicumarol is an analog of vitamin ‘K’ and so it is used to prevent internal clotting. Physostigmine is used in myasthenia gravis due to its inhibitory action on acetylcholine esterase which catalyzes the hydrolysis of acetylcholine to acetic acid and choline. This is responsible for the nerve impulse transmission. Presence of physostigmine ensures continued 40availability of acetylcholine for the treatment of myasthenia gravis.
Nucleoside analogs like 5-fluorouracil, 5-iodouracil, thiog-uanine, azacytidine, etc. are used to inhibit DNA replication in treating cancer. Ethyl alcohol is used in the treatment of methanol poisoning. This is based on the fact that alcohol dehydrogenase can act both on ethanol and methanol although the former is the preferred substrate. Alcohol produces acetaldehyde whereas methanol produces more toxic formaldehyde. Alcohol reduces the synthesis of form-aldehyde, thereby helping the patient to recover.
99. Catalase is the marker enzyme for peroxisomes:
Peroxisomes are found in many tissues including liver and are rich in catalase and oxidases. Oxidase enzyme produce H2O2 and catalases convert this H2O2 into water and O2. Hence, catalase is considered to be the marker of peroxisomes.
100. Several antibiotics inhibit prokaryotic protein synthesis:
The prokaryotes have smaller ribosomes (60s). There are only three initiation factors in prokaryotes in comparison to eukaryotes which have nine. This shows that the translation process in prokaryotes and eukaryotes are different. This difference is exploited in the development of antibiotics which are safe for humans but cause harm to bacteria. Bactericidal antibiotics include tetracyclins, chloramphenicol, streptomycin and penicillin. Cycloheximide inhits both prokaryotic and eukaryotic protein synthesis. Puromycin is not suitable for use in humans. Diphtheria toxin produced from Corynebacterium diphtheriae inhibits human protein synthesis.
101. Doctors would like to know the levels of glyco Hb in diabetic patients:
We can know about the long-term control of blood glucose level in a diabetic patient by measuring the glycol hemoglobin values. When glucose is added to hemoglobin by enzymes, it is called as glycosylation and if the addition is nonenzymatic it is called as glycation. In hyperglycemia the glycation rate is increased by a nonenzymatic process. This is an irreversible process and the glycated Hb remains throughout the lifespan of RRC.
41
The glucose forms a Schiff base with the amino terminal of hemoglobin and other proteins like albumin. The glycated hemoglobin is termed as HbA1 out of which about 75 to 80 percent molecules are HbA1C. Here, glucose is attached to the beta chain of hemoglobin on its amino terminal valine.
Monitoring of diabetic patient can be achieved by glyco Hb estimation. It cannot be used for diagnosis. The normal subjects can have a glyco Hb value of 4 to 7 percent. The level of glyco Hb can be elevated to 8 to 15 percent in diabetic patients depending on the control of blood glucose values.
It reflects the blood glucose level over a period of 3 to 4 months. It does not reflect the recent concentrations in glucose levels. If the glyco Hb values are elevated, it indicates a poor control of diabetes mellitus. This is of great importance that the danger of retinopathy, renal complications bear a positive correlation with elevated glyco Hb levels. Therefore, decreasing the glyco Hb level is key to avoiding long-term complications. The renal complication in diabetes is due to the glycation of basement membrane proteins of kidneys. In the eyes the glycation of lens, proteins can lead to cataract and retinopathy.
102. Glycolysis is regulated at three steps:
These three steps are catalyzed by glucokinase/hexokinase, phosphofructokinase and pyruvate kinase. All the three reactions are unidirectional reactions and they are key enzymes or rate limiting enzymes. The glucokinase/hexokinase catalyzes the conversion of glucose to glucose-6 phosphate in presence of ATP and Mg2+. The phosphofructokinase is required for the conversion of fructose–6 phosphate to fructose-1, 6-bisphosphate in presence of ATP and Mg2+. For the conversion of phosphoenol pyruvate to pyruvate the enzyme required is pyruvate kinase. Here, one ADP is actually converted to ATP an example of substrate level phosphorylation. All these three enzymes are stimulated by insulin and inhibited by glucagon. Additionally phosphofructokinase is an allosteric enzyme, ATP being an allosteric negative (inhibitory) modulator and ADP and AMP being positive (stimulatory) modulators. Allosteric enzymes are those enzymes having an allosteric 42site (different from substrate binding/catalytic site). The positive or negative modulators bind to this site initiating some conformational alterations at the catalytic site. The enzyme becomes active or inactive depending upon whether a positive or a negative effect or is present respectively at the allosteric site.
103. Metabolic syndrome is a combination of several disorders:
The term metabolic syndrome is used to describe a state of insulin resistance, obesity, dyslipidemias, atherosclerosis, hypertension and glucose intolerance. It is also known as insulin resistance syndrome or syndrome X. Individuals with this syndrome are at greater risk for developing cardiovascular complications. Insulin resistance in adipose tissue causes increased action of the hormone sensitive lipase which causes increased lipolysis of stored TAG to free fatty acids which after reaching liver could be converted to TAG and cholesterol associated with a decrease in beneficial HDL. Obesity is linked to various chronic conditions like maturity onset diabetes mellitus, hypercholesterolemia, hypertension, heart ailment, cancers, gout and arthritis.
104. Reversible phosphorylation is a method of enzyme acti-vation and inactivation and hence metabolic regulation:
Phosphorylation of serine and threonine residues of certain enzymes can lead to the regulation of their activities and hence metabolic pathways. An example of such an event can be encountered in glycogen metabolism: Both in glycogen synthesis (glycogenesis) and glycogenolysis. Addition of phosphate to glycogen phosphorylase at its serine residues with the help of ATP and phosphorylase kinase results in its activation which in turn brings about glycogen breakdown. The enzyme phosphorylase kinase in its turn is activated by cyclic AMP dependent protein kinase. The CAMP dependent protein kinase is activated by CAMP. CAMP is formed from ATP by adenylate cyclase which is activated by epinephrine and norepinephrine which binds to cell membrane receptors. The role of cyclic AMP is that of an intracellular signal of extracellular events. An example of an enzyme becoming inactive when phosphorylated is glycogen synthase needed for glycogen synthesis. Glycogen synthase is the rate limiting 43enzyme of glycogenesis and adds glucose to a glycogen primer from UDP glucose. This enzyme also remains in active and inactive states. The active form is the dephosphorylated form and the inactive one is the phosphorylated. The dephosphorylation of glycogen synthase is done by protein phosphatase-1.
The cyclic AMP is the one which brings about activation of glycogen phosphorylase on one hand (by phosphorylation) and the inactivation of glycogen synthase on the other hand (also by phosphorylation). Cyclic AMP is degraded by phosphodiesterase which is activated by insulin. Protein phosphatase-1 is inhibited by inhibitor-1 which is stimulated by CAMP.
105. Glycogen storage diseases:
Glycogen storage disease is defined as a state where abnormal amount or abnormal type of glycogen is stored in the liver and muscle but these are not available to the body as a source of glucose. The major types of the disease include: Type-I (Von Gierks’ disease): This is due to the deficiency of the enzyme glucose-6 phosphatase enzyme, the last common enzyme of glycogenolysis and gluconeogenesis. Due to this, the free glucose cannot be formed and so glucose cannot enter the blood. This causes hypoglycemia. As the glucose is not available as a source of energy, the use of fat as an alternative energy source leads to ketosis.
This can also lead to hyperuricemia.
The other diseases include McArdle's syndrome (deficiency of muscle phosphorylase), amylopectinosis (deficiency of branching enzyme), limit dextrinosis (lack of debranching enzyme) and another type with a deficiency of lysosomal maltase. Minor types of glycogen storage diseases comprise of deficiency of liver phosphorylase muscle phosphofructo-kinase, liver phosphorylase kinase and glycogen synthase.
106. TCA cycle is an amphibolic pathway and is also common to amino acid, glucose and fatty acids.
The TCA cycle starts with acetyl-CoA combining with oxalo-acetate to form citric acid. This is then sequentially converted to isocitrate, alpha ketoglutaric acid, succinyl-CoA, succinate, fumerate, malate and regenerating oxaloacetate. Three NADH 44and two FADH2 molecules are formed in these reactions. One GTP is generated when succinyl-CoA becomes succinate. Hence, a total of 12 ATP molecules are formed in one turn of the cycle. This aspect represents the catabolic side of the TCA cycle. This is common for glucose (through pyruvate and acetyl-CoA), amino acids (alanine through pyruvate, aspartic acid through oxaloacetate, glutamic acid, histidine, arginine and proline through alpha ketoglutarate, valine, isoleucine and methionine through succinyl-CoA and phenylalanine and tyrosine through fumerate) and odd chain fatty acids (through succinyl-CoA). Fatty acids are oxidized to acetyl-CoA. Therefore, all the major fuels are cataboilized by TCA cycle.
Talking about the anabolic side of TCA cycle various amino acids like alanine, aspartic acid and glutamic acid can be synthesized from intermediates of the cycle by transamination (glutamic acid from alpha ketoglutarate, aspartic acid from oxaloacetate and alanine from pyruvate). Further fatty acids could be synthesized from acetyl CoA. Gluconeogenesis is possible from the intermediates of TCA cycle from alpha keto-glutarate onwards. Therefore, glucogenic amino acids can be converted to glucose with the help of TCA cycle and then the designated steps of gluconeogenesis.
107. In electron transport chain or respiratory chain, oxidative phosphorylation takes place:
The electrons flow from more electronegative components to electropositive components. The ultimate acceptor of electrons is molecular oxygen which forms water. Molecules like glucose, fatty acids and amino acids, when oxidized generate energy rich reduced coenzymes like NADH and FADH2. They are also called as reducing equivalents. These reduced coenzymes give out a pair of electrons to a series of electron carriers known as electron transport chain. The passage of electrons from an electronegative to electropositive components liberates free energy. Part of this energy can trapped as ATP formed from ADP and Pi. This process is known as oxidative phosphorylation and takes place in the inner membrane of mitochondria. The untrapped energy is used to generate heat.
The outer membrane of mitochondria is freely permeable to most ions and molecules but the inner membrane 45impermeable to most of the small ions thereby necessitating the role of transport systems. The membrane contains convolutions called as cristae. The inner membrane can be divided into different complexes. Each complex can recieve electrons from an electron donor and can donate electrons to the next complex. With the exception of coenzyme Q, all the carriers of this chain are proteins and function as dehydrogenases containing porphyrin ring and iron. The components of the chain include NAD, coenzyme Q, and then a series of cytochromes b, c1, c and a-a3 and finally molecular oxygen. There are three sites at which necessary energy can be liberated to produce ATP from ADP and inorganic phosphate in presence of ATP synthase. The first site is between NAD and coenzyme Q. The second site is between cytochrome c1 and cytochrome c and third site is between cytochrome a-a3 and oxygen. ATPs are produced one each at these two sites also. Therefore, from NADH three ATPs can be formed. FADH2 donates electrons to coenzyme Q directly pypassing the first site and hence generates only two ATPs. But as per the new calculations, the energy produced is less. Hence NADH generates 2.5 ATPs and FADH2 produces 1.5 ATPs. The coupling of oxidation with phosphorylation can be explained by chemiosmotic theory which explains that the transport of electrons from inside to outside leads to a proton gradient across the membrane. Protons which accumulate outside the membrane results in an electrochemical potential difference. This force is responsible for driving ATP production.
There are several inhibitors of ATP synthesis like atractyloside, oligomycin, valinomycin and gramicidin. Cyanide stops oxidative phosphorylation by blocking cytochrome a-a3 and results in complete cessation of respiration. 2, 4-dinitro phenol allows oxidation to go on but prevents the trapping of energy in ATP and the energy is released as heat. This is as a result of removing the electrochemical potential. In newborn infants, this uncoupling of oxidation and phosphorylation is useful to adjust to cold temperatures soon after birth. The brown adipose tissue specialized in heat generation is an example. Thermogenin present in brown adipose tissue removes the proton gradiant and helps in thermogenesis. Another example of physiological 46un-coupler is the thyroid hormone. This could be the reason for heat intolerance in hyperthyroid patients and cold intolerance in hypothyroid patients.
There are almost 120 polypeptides necessary for oxidative phosphorylation out of which 13 are of mitochondrial origin. There may be defects in this process due to mutations in mitochondrial DNA. Tissues like skeletal and heart muscle, central nervous system, liver and kidneys are found to be more affected due to defects in oxidative phosphorylation. The above tissues have a higher requirement of energy-rich molecules like ATP. The related abnormalities comprise of mitochondrial myopathies and leber's hereditary optic neuropathy leading to loss of vision as a result of neuroretinal distruction. The mtDNA is maternally inherited. Another desease is known as mitochondrial encephalopathy lactic acidosis stroke like episodes (MELAS). The process of apoptosis may be initiated by damage to the outer mitochondrial membrane with the leaking of cytochromes into the cytosol.
108. The glycolysis in RBC is different:
As the RBC does not possess mitochondria, the glycolysis always ends in lactate. This is known as anaerobic glycolysis. As the RBC requires very less energy there is a provision for wasting energy in erythrocytes. This is achieved by the Rappoport luebering cycle which involves the isomerization of 1,3-BPG to 2, 3-BPG by a mutase enzyme. The phosphate present at the first position is a high energy one. When it becomes 2, 3-BPG it becomes low energy. Hence, there is a wasting of energy here. This is necessary for the continuance of glycolysis in RBC. In glycolysis in other tissues the conversion of 1,3 – BPG to 3 phosphoglycerate by phosphoglycerate kinase produces one ATP by substrate level phosporylation. In RBC, the conversion of 2, 3-BPG to 3-phosphoglycerate fails to generate any ATP. Further 2, 3-BPG is important in unloading of oxygen from hemoglobin in the tissues. It shifts the oxygen dissociation curve to the right. Another important point to be remembered is that HMP stunt pathway is very active in RBC.
109. There are several long-term complications in diabetes mellitus:
Angiopathy, nephropathy, peripheral neuropathy and retinopathy are the long-term complications of diabetes 47mellitus. The enhancement in lipolysis in diabetes due to lack of insulin and elevated levels of glucagon lead to elevated levels of NEFA in the blood. These fatty acids are taken up by tissues and are oxidised to acetyl-CoA. Due to the relative lack of oxalacetate, acetyl-CoA is prevented from entering the TCA cycle. Hence, the excess of acetyl-CoA either goes for the formation of ketone bodies or for cholesterologenesis. This cholesterol could be deposited in the subintimal layer of arterial walls and cause atherosclerosis. The macrophages accumulate cholesterol becoming foam cells which further take up lipids. The growth factors liberated by these cells cause proliferation of smooth muscle and forms collagen resulting in the formation of atherosclerotic plagues leading to vascular diseases. Microangiopathy takes place in small vessels.
Retinopathy and cataract can be the fall out of hyperglycemia which leads to the formation sorbitol. Sorbitol being an osmotically active substance can cause swelling of the lense. Peripheral neuropathy also is due to sorbitol resulting from decreased utilization of glucose. Neuropathy and paresthesis can lead to diabetic foot with ulcers.
Nephropathy is due to the effect of glycosylation of basement membrane proteins of renal tissues. Microalbuminuria is a useful test to detect the extent of the kidney damage and also can be employed as a predictor of renal complications.
110. Adenyl cyclase and phosphodiesterase are involved in the metabolism of cyclic AMP:
The adenyl cyclase present in the cell membrane is activated in presence of hormones like glucagon, norepinephrine, etc. which acts on ATP to form cyclic 3’ 5’ AMP. This acts as a second messenger and mediates hormone action. The cyclic AMP further activates CAMP dependent protein kinase which activates several other proteins and enzymes.
After the function is over, the cyclic AMP is degraded by phosphodiesterase which transforms cyclic AMP to AMP. The phosphodiesterase is activated by insulin.
111. IgG is a major immunoglobulin:
The IgG has gamma heavy chains. The concentration of IgG is maximum out of all the immunoglobulin in plasma. It can go across the placenta from mother to fetus. So, in fetal life only IgG is present. It is produced during the secondary immune 48response. It has the property of agglutination; compliment fixation and can bind to macrophages.
As it can cross the placenta, this can explain the ery-throblastosis fetalis occurring when an Rh-ve mother bears an Rh+ve fetus. During the delivery the fetal blood enters the mother and the mother starts producing anti Rh-antibodies. Although the first delivery may be saved, the subsequent deliveries can lead to a hemolytic crisis in the fetus as the antibodies generated against the Rh positive fetal blood can hemolyze fetal RBCs. Anti Rh antibodies if administered to the mother prior to delivery and soon after delivery can take care of the subsequent fetuses also.
112. IgE mediates allergic response:
When people are allergic to dust, pollen grains, food materials, drugs or insects, the body's immune system responds to this by hypersensitivity reaction. For example, when a drug like penicillin is administered the IgE get attached to mast cells. When another doze of the drug is given more molecules of IgE gets attached and this brings about degranulation and release of histamine and a slow reacting substance which brings about hypersensitivity reaction including vasodilation, low blood pressure and constriction of bronchioles. This is termed as anaphylactic shock or over reaction of the immune system.
113. Immunoglobulin ‘M’ is very effective in immune reaction:
IgM are pentamers with 10 heavy chains and 10 light chains. They are joined together by a J-chain polypeptide. As it is a pentamer it can combine with five antigens simultaneously and so it can agglutinate bacteria. It cannot come out of blood vessels being very large. IgM is produced during primary immune response. This means that the first antibodies to be produced after exposure to an antigen are of IgM class. IgM antibodies are also natural antibodies, meaning that they are produced without any antigenic stimulation. The ABO blood group system antibodies belong to this class. As the IgM cannot cross the placenta, the fetus, in spite of possessing an incompatible antigen, from that of mother, will not be affected.
114. In addition to the IgG and IgM, the third major immuno-globulin is IgA:
IgA are present in the mucous secretions of GI tract, nasopharyngeal tract and urogenital tract. It is also present 49in tears, saliva, etc. IgA are dimers having four heavy chains and four light chains. The joining piece, J, connects the monomers. The hydrolysis of IgA is prevented by the secretory piece.
115. Our body can produce many varieties of antibodies:
Although there are only five major classes of immunoglobulins our body can produce several types of antibodies. This is made possible due to the presence of constant sequences, variable sequences and joining sequences on the heavy and light chains. In other words, both heavy and light chains contain variable (V) and constant (C) regions. The variable region of light chain is written as VL and the constant region is written as CL. In the case of heavy chains the designation will be VH and CH respectively for variable and constant regions. In the case of ‘L’ chain, there are a few hundreds of VL segments and around 15 to 25 CL segments. The joining segment numbers about 5. When the ‘B’ lymphocytes get differentiated one of the 500 VL segments is joined to a JL (Joining Light) and CL unit. For example, the hundredth VL, the third JL and the seventh CL can be combined to form a light chain. Similarly, the heavy chains also could be formed by combining genes of constant, variable, diversity and joining regions. In another B lymphocyte a different recombination event is underway and in a third yet another recombination is on. Thus, we can produce a library of antibodies. So, the number of antibodies that could be produced is quite high.
116. Although there is very little molecular difference between ‘A’ and ‘B’ blood group antigens, the immunological system can differentiate this:
It is necessary to know the structure of the H substance because it is the precursor of both the A and B blood group substances and also H substance is the blood group substance present in individuals with type O. H substance is formed by the action of fucosyltransferase which catalyzes the addition of a terminal fucose to the galactose of precursor (Galactose-N-acetylgalactosamine-protein) to form fucose-galactose-N-acetylgalactosamine-protein. Persons having H antigen are having blood group O. The A antigen and the B antigen have only slight differences from each other and the H antigen.
50
Therefore, the difference is only an additional acetyl group in A antigen which is absent in B antigen. It is found that this small difference also can be identified by the immunological system.
117. Serum creatine kinase isoenzyme estimation is clinically useful:
The creatine kinase or creatine phosphokinase is required for hydrolyzing creatine phosphate and this liberates energy for muscle contraction. This enzyme exists in three forms. The isoenzymes of CK are: CKMM, CKMB and CKBB. The isoenzymes are multiple molecular forms of the same enzyme activity. The CK is made up of two types of polypeptide chains, ‘M’ and ‘B’. It is a dimer. CKMM is seen in skeletal muscle, CKMB is seen in cardiac muscle and CKBB is seen in the brain. Therefore, cardiac disease like acute myocardial infarction results in the elevation of CKMB enzyme in serum within 4 to 6 hours after starting chest pain. Hence, CKMB is an early cardiac marker. In normal subjects CKMB concentration is only 9 percent of total CK whereas CKMM is 90 percent and CKBB is only 1 percent. The total CK in females is 10 to 80 U/L and in males it is 15 to 100 U/L. The CKMM increase indicates skeletal muscle damage like muscular dystrophy. An increase in CKBB denotes a disease of the brain.
118. Troponin-I and troponin-T estimations help in the early diagnosis of AMI:
The cardiac troponins although not enzymes are early markers of myocardial infarction. The troponin complex consists of three components: The calcium binding troponin ‘C’, the actomyosin ATPase inhibitory element troponin I and troponin ‘T’ Troponin I is present in three isoforms, the skeletal muscle variety (two) and the cardiac isoform. Troponin I is 51released into the blood before four hours after the symptoms of ischemic heart attack. The peak is at 18 to 24 hours and remains elevated for the next 4 to 6 days. Troponin-I is not increased in skeletal muscle injury. Hence, it has an advantage over CKMB which may be elevated slightly in certain muscle injuries. Similarly, troponin T is found to increase in serum after AMI within six hours. It remains elevated for over a week and the peak is attained in three to four days.
119. Aspartate aminotransferase (AST), lactate dehydrogenase (LDH) and brain natriuretic peptide (BNP) are also used as cardiac markers:
AST is found to show significantly higher levels in AMI. AST starts rising around 20 hours after the onset of chest pain and reaches peak soon after to gradually decline over the next 7 days. The normal level of AST is 10 to 20 U/L. Incidentally, there is an increase of AST in liver disease also but this increase is relatively less in comparison to heart disease.
The LDH starts rising around one day after AMI peaking at 4 to 5 days and then declining over a period of two weeks. The normal serum value of LDH is 100 to 200 U/L. Exercise can increase the value. Hemolyzed serum should not be used for LDH estimation because RBC contains much higher levels of LDH than serum. In AMI the LDH-1 isoenzyme is increased. The size of the infarct can be roughly gauged by the area under the graph of LDH. As the LDH elevation can be seen in cancers, leukemias, muscular dystrophy, hepatocellular damage and hemolytic diseases, for the differential diagnosis, we have to study its isoenzyme pattern. In AMI, the LDH-1 is increased. Normally LDH-2 is higher. The presence of elevated LDH-1 in AMI is known as flipped pattern. LDH-1 is a tetramer of ‘H’ chains. The skeletal muscle form is LDH-5 with four ‘M’ chains. In hepatocellular jaundice, the LDH-4 (HMMM) is raised. The brain natriuretic peptide is found to be elevated in congestive cardiac failure.
120. Alkaptonuria is an in born error of metabolism:
In this disease, the urine turns black on exposure to air and it becomes alkaline. It was reported by Gaarod in 1902 when he got complaints of blackening of the patients’ clothes. In 52these patients, the enzyme homogentisate oxidase is absent and so the further metabolism of homogentisic acid fails to occur. Alkaptonuria, albinism, pentosuria and cysteinuria are included in Gaarod's tetrad. It is due to an autosomal condition which is recessive and of rare occurrence (1 in 2 .5 lac births). The absence of homogentisate oxidase results in excretion of homogentisic acid in urine of these patients. These patients are able to lead a normal life. The homogentisic acid is converted to black colored alkaptones. Another abnormality is the deposition of alkaptones in nose and ear. This may later produce arthritis as accumulation also takes place in joints. Dihydroxyphenyl acetic acid (homogentisic acid) is derived from phenylalanine via tyrosine and parahydroxylphenyl-pyruvic acid. In the normal subjects, the homogentisic acid is converted to maleyl acetoacetic acid which is further made into fumaryl acetoacetic acid. This is further hydrolized into acetoacetic acid and fumeric acid.
121. Phenylketonuria is an abnormality in the metabolism of aromatic amino acids:
Phenylketonuria is due to the deficiency or abnormality of the enzyme phenylalanine hydroxylase which converts phenylalanine to tyrosine. Several important compounds are derived from tyrosine like dihydroxyphenylalanine (DOPA), dopamine, norepinephrine and epinephrine.
Phenylalanine is an essential amino acid. Tyrosine has a sparing action on phenylalanine. The phenylalanine hydroxylase which forms tyrosine from phenylalanine needs tetra hydrobiopterine which gets converted to dihydrobiopterine. The regeneration of tetra hydrobiopterine requires NADPH and a reductase enzyme also. Being a mono-oxygenase, the phenylalanine hydroxylase needs molecular O2. The occurrence of PKU is found to be quite high (as high as 1 in 1500 births). In India the incidence is less. It is a recessive state. The absence of the enzyme phenylalanine hydroxylase results in the accumulation of phenylalanine. Further the alternate pathway becomes predominant and phenylpyruvate, phenyllactate and phenylacetate are increased in blood and hence excreted in urine. Clinically the surviving child develops mental retardation with a low IQ. Convulsions and agitation can be manifested in the patient. The PKU could hamper the neurotransmitter function. A deficiency of 53dihydrobriopterine reductase or the decrease in the formation of tetrahydrobiopterine, required also for serotonin synthesis, can lead to neurological symptoms. The affected children are fair due to the lack of melanin pigments and the sweat gives off a mousy smell due to phenyllactic acid.
122. Albinism is a type of inborn disease.
It is because of the absence of the enzyme tyrosinase which converts tyrosine to DOPA in melanocytes. Dopa is then converted to melanin. In the absence of tyrosinase the melanin is not formed and hence the affected individual becomes an albino having a white skin, hair, eyelashes and eyebrows. Oculocutaneous albinism is the most severe form. Their ocular fundus are hypopigmented and vision becomes affected. There is photophobia and the skin becomes sensitive to sun light. Melanomas and cancers can be seen in the skin.
123. Serotonin and melatonin are formed from tryptophan and each has an important role in the body:
Tryptophan is hydroxylated to 5-hydroxytryptophan by trypto-phan hydroxylase which like phenylalanine hydroxylase requires tetrahydrobiopterine and NADPH. In the next step requiring pyridoxal phosphate as a coenzyme and decarloxylase as enzyme, 5-hydroxytryptophan is converted to 5-hydroxytryptamine otherwise termed as serotonin. It is produced in the brain and mast cells. Serotonin serves as a neurotransmitter which brings about a relaxed calming effect. It also induces sleep. Serotonin is important in mood, sleep and gastrointestinal motility.
Monoamine oxidases convert serotonin to 5-hydroxyindole-acetic acid. The inhibitors of this enzyme are used as mood elevators. Serotonin secretion is increased when carcinoid tumors are developed in small intentine. Bananas and vegetables like tomatoes contain serotonin.
The serotonin can be further acetylated and methylated in pineal gland to produce melatonin which also serves as a neurotransmitter.
124. Nitric oxide has important functions in the body:
Nitric oxide (NO) is the endothelium derived relaxing factor causing vasodilatation. NO can act as a neurotransmitter, 54prevents platelet aggregation and aids in macrophage action. Nitric oxide should not be confused with N2O, nitrous oxide. NO has a very short half-life of 3 to 10 seconds.
Nitric oxide synthase enzyme reacts on arginine, O2 and NADPH in presence of FMN, FAD, Heme and tetrahydrobiopterin to form citrulline and nitric oxide. Three types of NO synthases have been identified. Two are produced by endothelium and neural tissues. The third type is seen in several other tissues like liver.
Nitric oxide activates guanylate cyclase and results in an increase in cyclic GMP. The cyclic GMP phosphorylates and hence activates protein kinase G which causes decreased entry of calcium into smooth muscle cells. This in turn de-creases the contraction and favors relaxation. Drugs such as nitroglycerine and nitropruside are converted to nitric oxide which lowers blood pressure.
Macrophages start to produce nitricoxide synthase when exposed to bacteria. Subsequently, nitric oxide is synthesized by the macrophages which also produce superoxide radicals which react together to produce hydroxyl radical (OH˙) which kills the microorganisms like bacteria, viruses, fungi and protozoa.
In addition NO acts as an inhibitor of platelet aggregation and also as a neurotransmitter.
125. Maple syrup urine disease is an inborn error of amino acid metabolism:
It is a very rare disease and is an autosomal recessive disorder. It involves branched chain amino acids like leucine, isoleucine and valine. The defective enzyme is branched chain alpha-ketoacid dehydrogenase. Because of the deficiency of this enzyme, the branched chain amino acids and their alpha ketoacids accumulate in the blood and brain causing brain dysfunction. The affected child suffers from vomitting, dehydration and acidosis. The urine has got the smell of maple syrup. This may further lead to mental retardation. The symptoms may become evident in the first month of life. Thiamine dependent condition is also seen as thiamine pyrophosphate is a coenzyme for the dehydrogenase enzyme. If the condition is diagnosed antenatally, the disease could 55be treated with a synthetic diet containing limited amounts of leucine, isoleucine and valine for the protein synthesis. Early diagnosis and life-long dietary management shall be benefitial.
126. In-born errors in the metabolism of homocysteine are also seen:
This disease results in elevated levels of methionine and homocysteine in plasma and urine. The cysteine level is found to be low. To understand this abnormality we should have an idea about the metabolism methionine. The active form of methionine, the S-adenosly methionine is formed from methionine and ATP. Hence, the S-adenosyl group is transferred to methionine by the enzyme methionine adenosyl transferase. In the next step, the adenosyl met-hionine (SAM) donates its methyl group to an acceptor to form S-adenosyl homocysteine with the help of the enzyme methyl transferase. In the next step, adenosine is removed by adenosine homocysteinase to form homocysteine which may be converted either to methionine or may join with serine to form cystathionine in presence of cystathionine beta synthase. The cystathioninase further acts on cystathionine to form cysteine and homoserine. Cysteine is further converted to propionyl COA.
The cause for homocystinuria is the deficiency of cystathionine synthase. These patients have abnormalities like osteoporosis skeletal abnormalities, ectopia lentis and mental retardation. The patient management includes less methionine in the diet and supplementation with vitamins B6, B12 and folic acid. Cystathionine synthase require B6 as the coenzyme. Cystathioninuria can be precipitated by the deficiency of cys-tathioninase enzyme. Homocystinuria also may be because of the deficiency of vitamins B12, B6 and folate.
Drug induced homocysteinemias can also be present. Raised level of homocysteine interferes with collagen and may cause coronary artery disease.
127. Carbamoyl phosphate synthelase-1 and carbamoyl phos-phate-2 have different roles:
Carbamoyl phosphate synthase-1 is the enzyme involved in formation of urea. The ammonia produced by the oxidative 56deamination of glutamate by the mitochondrial glutamate dehydrogenase in presence of two ATP moles and one carbon dioxide is converted to carbamoyl phosphate in presence of the enzyme carbamoyl phosphate synthase-1. The enzyme also needs n-acetyl glutamate as a positive allosteric effector. This is the first step in the biosynthesis of urea. In the next step carbamoyl phosphate is converted to citrulline in the presence of ornithine and the enzyme is ornithine transcarbamylase.
The carbamoyl phosphate synthase-2 participates in the biosynthesis of pyrimidines and does not require N-acetyl glutamate and occurs in the cytoplasm. The carbamoyl phosphate is formed from glutamine (as the donor of ammonia) and CO2. It is inhibited by UTP, the end product of the pathway and is activated by ATP and PRPP. Another important point is that none of the above two enzymes require biotin as a coenzyme unlike other carboxylases like pyruvate carboxylase and propionyl CoA carboxylase.
128. Plasma albumin has important functions:
The most important functions of albumin is in the develop-ment of oncotic pressure. The normal value of serum albumin is from 3.5–5 g/dl. Albumin is a relatively small sized protein but high in concentration and so exerts greater oncotic pressure. At the arterial end of capillary the oncotic pressure of 25 mm Hg opposes the blood pressure which is around 35 mm Hg and so there is a net filtration pressure of 10 mm Hg. Therefore, the fluid goes out of the vascular compartment at the arterial end. At the venous end, the blood pressure is around 15 mm Hg and the oncotic pressure remains at 25 mm Hg. There, the net absorption pressure is 10 mm Hg and the fluid is absorbed into the blood vessels. In case the serum albumin level is decreased, the oncotic pressure is also decreased and this leads to more fluid going into the tissues and less fluid returning to the vessels. This results in the accumulation fluid in the tissue spaces leading to edema.
Another important function of albumin is to serve as a transporter of unconjugated bilirubin, calcium free fatty acids and several drugs and steroids. The free bilirubin is insoluble in water and hence combines with albumin for its transport. 57In the newborn infants the bilirubin concentration is high in the first week of life especially in the preterm infants. When these infants are to be given some medicines like aspirin which also requires albumin for its transport, the drug may replace bilirubin from albumin. The free bilirubin can cross the blood-brain barrier and can cause kernicterus, resulting in mental retardation.
Albumin contains all the essential of amino acids and hence is a good quality protein and so serves a nutritional function. Decreased albumin levels can be seen in the following conditions:
  1. Cirrhosis of the liver where the synthesis of albumin by liver is reduced.
  2. Nephrotic syndrome where considerable quantities of albumin are lost in the urine.
  3. Protein energy malnutrition like kwashiorkor, where protein intake in the diet is deficient.
  4. Protein losing enteropathy.
129. Amino acids form several important compounds in the body:
The important products formed from the amino acids include dopamine, norepinephrine epinephrine melanin, tyramine (all from tyrosine), niacin, NAD, NADP, serotonin and melatonin (all from tryptophan). Further histamine is synthesized from histidine which is a heterocyclic ring containing amino acids. Creatine is synthesized from glycine and the guanidino group of arginine and the methyl group from the active form of methionin, S-adenosyl methionine. Glycine is required for the formation of heme, glutathione and purines glutamine and aspartate are also required for purine formation.
Histamine is a chemical messenger for the mediation of several processes. Histamine regulates gastric secretion, it is a vasodilator and it is also responsible for allergic and inflammatory responses. It is formed by decarboxylation of histidine amino acid. This reaction requires pyridoxal phosphate as a coenzyme. Histamine is secreted by platelets 58and mast cells. Agents which interfere with the action of histamine have important therapeutic applications. Drugs like penicillin can lead to the binding of IgE to mast cells. When penicillin is administered again degranulation of mast cells results in the release of histamine and slow reacting substance of anaphylaxis. Histamine causes a fall in blood pressure and smooth muscle contraction.
Creatine phosphate is a high energy compound that can donate a phosphate group to adenosine diphosphate to make ATP. At the same time creatine can accept a phosphate from ATP to produce creatine phosphate and ADP. The creatine phosphate helps to maintain the level of intracellular ATP during the initial time of muscle contraction. The concentration of creatine phosphate is proportional to the muscle mass. The guanidino group of arginine reacts with glycine to form guanido acetic acid catalyzed by amide transferase. Guanido acetic acid is then methylated by S-alenosyl methionine by methyl transferase to form creatine. The former reaction takes place in kidney and the latter in the liver. In the third step creatine is phosphorylated to creatine phosphate in presence of creatine kinase and ATP. This reaction takes place in muscle, brain and liver. The creatine phosphate and creatine may be transformed into creatinine the anhydride form, and is excreted in the urine. The serum level of creatinine and urinary excretion of creatinine remains a constant provided the muscle mass does not alter. The normal level of serum creatinine is 0.7–1.4 mg/dl. The serum creatinine level is measured to assess the renal function because creatinine is excreted rapidly from the body. About 12 to 15 mmol of creatinine is excreted everyday by a male. The creatinine is neither reabsorbed nor secreted by the renal tubules whereas about 40 percent of urea is reabsorbed by proximal convoluted tubules. The urinary concentration of both creatinine and urea are 70 times that of their respective concentrations in plasma.
The first step in heme synthesis is the condensation of succinyl CoA with glycine in presence of ATP, aminolevulinic acid synthase and pyridoxal phosphate to form alpha-amino beta-ketoadipic acid which then becomes Delta-aminolevulinic acid catalyzed by the same enzyme, ALA 59synthase and in presence of pyridoxal phosphate co-enzyme. The aminolevulinic acid is converted sequentially to prophobilinogen, uroporphyrinogen, coproprophyrinogen, protoporphyrinogen, protoporphyrin and finally heme. The purine synthesis requires glycine which forms the C4, C5 and N7 of purine ring. Amide nitrogen of glutamine is required for N3 & N9 of purine. Amino group of aspartic acid donates the N1 of purine. For pyrimidine synthesis, the first step involves the formation of carbamyl phosphate from bicarbonate and the nitrogen of glutamine. The enzyme required is CPT-II·N3 and C2 of pyrimidine are obtained from carbamoyl phosphate. C4, C5, C6, and N1 of pyrimidine ring is donated by aspartic acid.
Glutathione is a tripeptide formed by gamma glutamyl cysteinyl glycine. The glutathione is very important in various tissues including erythrocytes and lense. In the erythrocytes, the H2O2 produced is detoxified by glutathione peroxidase in presence of reduced glutathione (2G-SH) which becomes oxidized glutathione and H2O2 becomes nontoxic water. The accumulation H2O2 can oxidize hemoglobin to methemoglobin and cause hemolysis. The oxidized glutathione is reduced with NADPH from HMP shunt pathway in presence of glutathione reductase. A deficient HMP shunt pathway especially deficiency of glucose-6-phosphate dehydrogenase can cause less NADPH production and hence hemolytic anemia.
Tryptophan can be entering the anthranilic acid pathway and can synthesize niacin vitamin. This pathway of tryptophan metabolism accounts for a very minor portion. Maize which is deficient in the essential amino acid tryptophan can cause pellagra like symptoms, if consumed as a staple diet. Further typtophan can be converted to alanine which is glucogenic and it is also involved in one carbon metabolism as a donor of formyl group. Other important substances produced from tryptophan are serotonin and melatonin. Tryptophan is first hydroxylated by tryptophan hydroxylase to 5-hydroxy tryptophan which is then decarboxylated to 5-hydroxytryptamin or serotonin. The enzyme catalyzing this reaction is decarboxylase which requires pyriodoxal phosphate 60as a coenzyme. Serotonin is an important neurotransmitter. The release of serotonin has a calming effect and it induces sleep. Serotonin levels are found to be less in patients of depression. Serotonin is catabolized by monoamine oxidase to hydroxyindoleacetic acid. MAO inhibitors are used to counter depression. Another important compound produced from tryptophan is melatonin. For the synthesis of melatonin, the serotonin is acetylated and then methylated with S.adenosyl melatonin. It is synthesized in the pineal gland and is involved in biological rhythms and it also functions as a neurotransmitter.
The important compounds synthesized from phenylalanine and tyrosine are dihydroxyphenylalanine, dihydroxyphen-ylamine (Dopamine), norepinephrine, epinephrine, thy-roxine and melanin.
130 Uric acid can exist in different forms depending on the pH of the medium:
It is found that in an alkaline medium the uric acid exists in its salt form, i.e. sodium urate whereas in an acidic medium, the predominant species will be uric acid. The sodium urate is several folds more soluble than the acid form. The urine, being acidic the insoluble acid form prevails and hence might develop nephrolithiasis. Hence, alkalinizing the urine may alleviate symptoms of gout and hyperuricemia. Intake of citrus fruits can help in this matter.
131. Certain porphyrias lead to neuropsychiatric symptoms and urticaria:
Porphyrias are a group of diseases which may be hereditary or acquired. The hereditary porphyrias may be erythropoietic hepatic or erythrohepatic.
The hereditary porphyrias consist of acute intermittent porphyria, congenital erythropoietic porphyria, Porphyria cutanea tarda, hereditary coproporphyria and hereditary protoporphyria.
Acute intermittent porphyria: This is due to the deficiency of porphobilinogen deaminase (uroporphyrinogen-1-synt-hase). Because of this enzyme deficiency, the end product 61of this pathway, i.e. heme is not formed. This results in the excessive production of aminolevulnic acid (ALA) and porphobilinogen. This is due to the depression of ALA synthase gene. The first step in heme synthesis is the reaction between glycine and succinyl-CoA to form ALA in presence of ALA synthase which is under the feed back control of heme and also by repression depression mechanism at the transcriptional level as explained above.
Here, the elevated ALA and porphobilinogen are excreted in urine which when voided is light in color but after sometime becomes colored on oxidation.
Other symptoms in the patient include abdominal pain and neuropsychiatric manifestations like hallucinations, confusion and agitation which may lead to maniac behavior. Here, there is no cutaneous photosensitivity as the porphyrins are not in excess.
In the second type known as congenital erythropoietic porphyria, the UPG cosynthase enzyme is defective due to which the type I isomer of uroporphyrinogen is produced in excess in place of the normal type III isomer.
Due to the fact that it is the type III that is required for heme synthesis the feedback inhibition is ineffective which in turn results in over production of ALA, porphobilinogen and type I uroporphyrinogen. The increased presence of uroporphyrins causes cutaneous photosensitivity (urticaria) and dark urine. Porphyrins in the skin get converted to free radicals which cause dermatitis and skin scaring on exposure to sun light. The patient may have the appearance of a leprosy patient. erythrodontia also can be seen in UV light.
All the other porphyrias also have a biochemical basis like: porphyria cutanea tarda (due to the deficiency of uro-porphyrinogen decarboxylase), coproporhyria (coproporphy-rinogen oxidase lack) and hereditary protoporphyria (deficiency of ferrochelatase). Yet, another porphyria is due to the absence of protporphyrinogen oxidase enzyme which is termed as variegate porphyria.
62
132. Heme synthesis is regulated at the level of ALA synthase:
The heme synthetic pathway is as follows:
Hematin is an allosteric inhibitor of ALA synthase and hence hematin is used in the treatment of porphyrias. A diet consisting of glucose alleviates the symptoms of porphyrias.
In the regulation of heme synthesis the heme itself acts as a corepressor which when sufficiently present binds to an aporepressor to form a holorepressor. The holorepressor binds at the operator locus of the ALA synthase gene and represses the transcription of ALA synthase gene by preventing RNA polymerase action into the corresponding m-RNA, thereby, decreasing the synthesis of ALA synthase. This is known as repression. When the heme concentration is less, there is not sufficient heme to combine with the aporepressor and the holoreppressor cannot be formed. Now derepression results and the ALA synthase gene is transcribed and the corresponding enzyme is synthesized.
133. Figlu excretion test can be use to detect folate deficiency:
One of the intermediates of histidine catabolism is formimino-glutamate which is further converted to L-glutamate and formimino THFA. This reaction requires tetrahydrofolate and is catalyzed by glutamate formimino transferase. Hence, in the absence of THFA, formimino glutamate is excreted in urine and figlu test after a test dose of histidine can be used to detect folate deficit.
63
134. Vitamin B6 deficiency can indirectly lead to pellagra:
Tryptophan is an essential amino acid. It has an indole ring. In its metabolism it is first oxidized by tryptophan pyrrolase which is a hemoprotein to formyl kynurenine which is converted to kynurenine which further becomes 3-hydroxy kynurenine by kynureninase enzyme. This enzyme has the requirement for vitamin B6. The product of kynureninase is hydroxy anthranilic acid which can be converted to NAD. Hence, vitamin B6 deficiency leads to less niacin and NAD in the body causing pellagra. This can be proved by showing that a diet of maize lacking in tryptophan can lead to pellagra.
135. Important specialized products formed from tyrosine include dopamine, catecholamines and thyroxine:
Catecholamine synthesis starts with the hydroxylation of tyrosine to dihydroxy-phenylalanine (DOPA) with the help of the enzyme tyrosine hydroxylase which requires tetra-hydrobiopterin and NADPH. This is analogous to phenyl-alanine hydroxylase.
In the next step DOPA is decarboxylated to dihydroxy phenyl amine (DOPAMINE) by DOPA decarboxylase. This reaction requires pyridoxal phosphate. Dopamine is an important nerve transmitter. The deficiency of dopamine can cause Parkinson's disease.
The dopamine is further hydroxylated to norepinephrine (noradrenaline) which also serves as a neurotransmitter. The enzyme involved is dopamine hydroxylase. The next step involves the transfer of a methyl group from S-adenosyl methionine to norepinephrine to generate epinephrine or adrenaline with the help of a methyl transferase enzyme. These products are formed in adrenergic nerve endings and adrenal medulla. The major functions of catecholamines are to increase blood pressure and myocardial contraction. Adrenaline is known as the hormone of flight, fright and fight.
The catecholamines are degraded sequentially by methyl transferase and monoamine oxidase to the major end product vanilyl mandelic acid (VMA). The VMA estimation in urine is of significance in pheochromocytoma and neuroblastoma. Intake of vanilline containing foods can give erroneous results.
64
Melanin pigment is also formed from DOPA quinone and indolequinone which polymerizes to form melanin. Copper deficiency may affect melanin of skin and hair. Defects in melanin formation can result in leukoderma and premature graying of hair.
The thyroid hormones are formed by iodination tyrosine. The thyroid gland can take up iodine which is stimulated by thyroid stimulating hormone secreted by anterior pituitary. TSH is a glycoprotein having two polypeptide chains (alpha with 89 amino acids and beta with 114 amino acids. TSH acts through cyclic AMP and stimulates synthesis of thyroxine. TSH in its town is under the control of TRH (Thyrotropin releasing hormone, a tripeptide) secreted by hypothalamus. High levels of TSH are seen in hypothyroidism and low levels are encountered in hyperthyroidism.
The thyroperoxidase enzyme oxidizes the iodine taken up by the thyroid gland. This requires the NADPH generated in pentose phosphate pathway. Both the iodine uptake and its oxidation are stimulated by TSH but inhibited by anti-thyroid drugs like thiourea. Thyroglobulin is a large glycoprotein with 115 tyrosine residues of which 30 to 35 are iodinated. The iodination of tyrosine produces monoiodo and diiodo-tyrosine. Two diiodotyrosine residues which are opposite to each other are coupled to tetraiodothyronine (T4). Tetraiodothyronine is deiodonated to triiodothyronine (T3) by a deiodinase enzyme.
In some patients, there may be defect in the coupling of iodotyrosine residues. Thyroid hormone is special in that it can be stored in the gland unlike other endocrines. The stored T4 from the thyroglobulin can be hydrolyzed by proteases. This step is also stimulated by TSH. Iodide (potassium iodide) inhibits the hydrolysis and may be used in treating hyperthyroidism.
The T4 formed and secreted from the thyroid gland can be deiodonated to T3 by a selenium containing enzyme in the peripheral tissues. The unutilized MIT and DIT also can be deiodinated and the iodine and amino acids can be reutilized. A defect in deiodination may result in iodine deficiency as MIT and DIT may be lost in urine.
The thyroid hormones are transported by albumin and prealbumin. The T3 is more active than T4. The half-life of T3 65is only 24 hours where T4 has a longer half-life. The T3 and T4 may be conjugated with glucuronic acid and excreted.
The thyroid hormones after binding to the nuclear receptors gets transported to DNA and binds with specific regions on the DNA, enhancing transcription producing the biologic effect of the hormone which include uncoupling of oxidative phosphorylation and hence thermogenesis and increasing BMR. Hyperthyroidism can result in decreased level of cholesterol in serum and negative nitrogen balance.
136. Thyroid function tests are important to know about the thyroid status:
The thyroid hormones bind to specific nuclear receptors and then bind to specific regions of the DNA. This in turn enhances the transcription rate thereby eliciting metabolic and other responses like increasing protein synthesis (early effect) and causing protein catabolism, later culminating in negative nitrogen balance. Carbohydrate oxidation is increased initially and later the gluconeogenesis is also increased. GTT becomes abnormal. Beta oxidation and cholesterol degradation are enhanced as well resulting in low serum cholesterol values. In the past BMR ad serum cholesterol were being used for the diagnosis of thyroid disorders. The basal metabolic rate is found to be increased in hyperthyroidism.
The T3 and T4 can be estimated by radioimmunoassay, ELISA or by chemiluminiscent method. In hyperthyroidism, both the T3 and T4 levels are increased. The concentration of TSH is decreased due to feed back inhibition by T3 and T4 on anterior pituitary. In hypothyroidism, the reverse occurs, i.e. the T3 and T4 are low and the TSH is elevated as there is very less T3 and T4 to inhibit the TSH release by anterior pituitary. But in secondary hypothyroidism as a result of hypothalamic or pituitary defect, T3, T4 and TSH are all decreased. Here, either the hypothalamus is unable to secrete thyrotropin releasing hormone (TRH) or the pituitary is incapable of responding to the stimulatory signal from hypothalamus mediated through TRH.
As it is the free hormones that all are the active ones, techniques have been standardized to measure free T3 and free T4.
In primary hyperthyroidism both T3 and T4 are elevated with low levels of TSH; but in secondary hyperthyroidism resulting 66from pituitary or a hypothalamic derangement, T3, T4 and TSH are all elevated.
The serum cholesterol is not of diagnostic value in hypo-thyroidism as it can also be elevated in many other states like Diabetes mellitus, nephrotic syndrome, biliary obstruction and hypertension but could be useful in gauging the therapeutic response in thyroid ailments.
Out of the various endocrine estimations done in a clinical lab, the TFT is most common showing a high prevalence of thyroid disorders in comparison to other endocrine disorders.
Hyperthyroidism is otherwise called as thyrotoxicosis. These patients have increased metabolic rate, weight loss, tachycardia and anxiety. They also may exhibit sensitivity to heat. In Graves’ disease, one type of hyperthyroidism, the thyroid stimulating antibodies bind to TSH receptors on the thyroid gland and stimulates the production of Thyroid hormones. This cannot be controlled by feedback mechanism.
Hypothyroidism may be due to an autoimmune disorder which leads to myxedema. The symptoms include cold intolerance, weight gain, dry skin, sluggishness and low metabolic rate. This is more seen in female population. In children, this can lead to cretinism.
In Hashimoto's thyroiditis, hypothyroidism can result due to cell destruction by anti-thyroglobulin antibodies.