Fig 1.5 shows the structure of the protein insulin which helps in carbohydrate metabolism and stimulates protein synthesis. A protein consists of a sequence of polypeptide chains. The building blocks are amino acids which contain an amino group (NH2, NH) and a carboxylic acid group (-COOH). There are 20 amino acids commonly found in proteins. The amino acids can from polypeptide chains when an NH2 group of one amino acid combines with the COOH group of an adjacent amino acid by forming an amide linkage -CO-NH by eliminating a water molecule. Proteins constitute a complex class of macromolecules exhibiting versatile structures and they are fundamental to biology since they are the instruments through which genetic information is expressed. The sequence (of the covalent linkages through amide bonds) of amino acids forms the primary structure of protein. The geometrical arrangements of polypeptide chains along their axis constitute their secondary structure. (e.g., helical structures)
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