Nitrogen content of ordinary proteins is on the average 16% by weight. Protein structure can be defined and studied at four levels viz. primary, secondary, tertiary and quaternary. Quaternary structure is present in proteins with more than one polypeptide chain. Examples of oligomeric proteins with quaternary structure are hemoglobin, creatine kinase. Cysteine forms disulfide linkages between two polypeptide chains in oligomeric proteins. Primary structure determines the biological activity of the protein. Alterations lead to loss of functional capacity, e.g. sickle cell hemoglobin (HbS). Secondary structure of protein is stabilized by hydrogen bonds, ionic bonds, hydrophobic interactions and van der Waals forces. Secondary structure could be an alpha helix or a beta pleated sheet. A beta-pleated sheet may further be parallel or anti-parallel. Tertiary structure of a protein is the most thermodynamically stable configuration. Protein folding into its 3-D configuration is assisted by chaperones. Incorrect folding has been proposed as a mechanism in the origin of prion diseases. N-terminal amino acid can be determined by Dansyl Chloride, Edman’s reagent and Sanger\'s reagent (fluorodinitrobenzene-FDNB), while the C-terminal amino acid can be determined using Carboxypeptidase. Solubility of a protein is dependent on the ionic concentration of the medium. Hence, proteins may be ‘salted in’ or ‘salted out’. Denaturation of protein results in loss of biological activity but not the primary structure. Denaturation may be reversible. Proteins can be classified based on (i) Functions, (ii) Composition, (iii) Shape, and (iv) Nutritional value. Tyrosine and Tryptophan residues in proteins absorb strongly at 280 nm. This property is used for quantitation of proteins spectrophotometrically. Methods of protein estimation include colourimetry (Biuret and Lowry’s method), radioimmunoassay (RIA) and enzyme linked immunosorbent assay (ELISA).