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Chapter-05 Enzymology: General Concepts and Enzyme Kinetics

BOOK TITLE: Textbook of Biochemistry for Medical Students

Author
1. Vasudevan DM
2. S Sreekumari
3. Vaidyanathan Kannan
ISBN
9789350250167
DOI
10.5005/jp/books/11359_5
Edition
6/e
Publishing Year
2011
Pages
20
Author Affiliations
1. Faculty of Medicine, Amrita Vishwa Vidyapeetham, (Amrita University), Kochi, Kerala, Formerly Principal, College of Medicine, Amrita, Kerala; Formerly, Dean, Sikkim Manipal Institute of Medical Sciences, Gangtok, Sikkim, Amrita Vishwa Vidyapeetham (Deemed University), Cochin, Kerala, E-mail: dmvasudevan@aims.amrita.edu, PG Programs and Research College of Medicine, Amrita Institute of Medical Sciences, Kochi, Kerala, India, College of Medicine, Amrita Institute of Medical Sciences, Kochi, Kerala, India; Sikkim Manipal Institute of Medical Sciences, Gangtok, Sikkim, India
2. Jubilee Mission Medical College, Trissur, Kerala, Sree Gokulam Medical College and Research Foundation, Thiruvananthapuram, Kerala, India, Jubilee Mission Medical College and Research Institute, Thrissur, Kerala, India, Government Medical College, Thrissur and Thiruvananthapuram, Kerala, India; Sree Gokulam Medical College and Research Foundation, Thiruvananthapuram, Kerala, India; Jubilee Mission Medical College and Research Institute, Thrissur, Kerala, India
3. Amrita Institute of Medical Sciences, Kochi, Kerala, India, Pushpagiri Institute of Medical Sciences and Research Centre, Thiruvalla, Kerala, India, Believers Church Medical College and Hospital, Thiruvalla, Kerala, India
Chapter keywords

Abstract

Almost all enzymes are proteins. Enzymes can be classified into: (i) Oxidoreductases (e.g. alcohol dehydrogenase), (ii) Transferases (e.g. hexokinase), (iii) Hydrolases (e.g. acetyl cholinesterase), (iv) Lyases (e.g. Aldolase), (v) Isomerases (e.g. triose phosphate isomerase), and (vi) Ligases (e.g. acetyl CoA carboxylase). Enzymes may be simple or compound proteins. In case of compound proteins, the protein component is termed ‘apoenzyme’ and the prosthetic group is termed ‘co-enzyme’. The combination produces a functional ‘holoenzyme’. Co-enzymes may be involved in the transfer of hydrogen (e.g. NAD, FAD, and FMN) or groups other than hydrogen (e.g. amino group by PLP, hydroxyethyl group by TPP). Enzymes requiring the presence of a certain metal ion for their activity are called Metalloenzymes. Examples are Zinc in carbonic anhydrase, Iron in catalase and peroxidase, Calcium in lipase etc. Michaelis-Menten theory states that an enzyme (E) combines with a substrate (S) to form an enzyme-substrate (E-S) complex, which breaks down to give product (P). Theories proposed to explain the mechanism of enzyme action are Fischer’s template (lock and key) theory and Koshland’s induced fit theory. Area of an enzyme where the catalysis occurs is called the ‘active site’. Amino acid residues of the enzyme involved directly in the binding of the substrate are called ‘catalytic residues’. Enzyme activity is influenced by enzyme concentration, substrate concentration, pH, temperature and presence of inhibitors. Km value is characteristic of a given enzyme. No two enzymes can have the same Km value. It denotes the affinity of the enzyme to its substrate. Lesser the Km, greater the affinity and vice versa. Enzyme inhibition can be competitive, non-competitive or un-competitive. Competitive inhibition is usually reversible. Actions of drugs such as Sulfonamides, Methotrexate, Dicoumarol and Isoniazid are based on the principle of competitive inhibition. Suicide inhibition is an irreversible inhibition. The inhibitor makes use of the natural reaction of the enzyme for inhibition, e.g. ornithine decarboxylase. The allosteric site is different from the catalytic site and enzymes, which possess the site, are termed ‘allosteric enzymes’. Key enzyme or Rate limiting enzyme in a pathway has the lowest Km value for its substrate. Enzyme activity in the body is regulated by allosteric inhibition, covalent modification, induction, repression. Specificity of enzymes could be Absolute (Urease), Group (Trypsin) or Stereo (Lactate dehydrogenase) Iso-enzymes are physically distinct forms of the same enzyme activity. They may be products of the same gene or different genes. Iso-enzymes, which are products of different genes, are called ‘true iso-enzymes’, e.g. salivary and pancreatic amylase.

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