(1) Amino acids are normal components of cell proteins (called amino acid).

(2) They are 20 in number specified in genetic code and universal in viruses, prokaryotes and eukaryotes.

(3) Structure and Composition : Amino acids are basic units of protein and made up of C, H, O, N and sometimes S. Amino acids are organic acids with a carboxyl group (–COOH) and one amino group(-NH2) on the a -carbon atom. Carboxyl group attributes acidic properties and amino group gives basic ones. In solution, they serve as buffers and help to maintain pH. General formula is R-CHNH2.COOH.

(4) Classification

Based on R-group of amino acids

(a) Simple amino acids: These have no functional group in the side chain. e.g. glycine, alanine , leucine, valine etc.

(b) Hydroxy amino acids: They have alcohol group in side chain. e.g. threonine, serine, etc.

(c) Sulphur containing amino acids: They have sulphur atom in side chain. e.g. methionine, cystenine.

(d) Basic amino acids: They have basic group (-NH2) in side chain. e.g. lysine, arginine.

(e) Acidic amino acids: They have carboxyl group in side chain. e.g. aspartic acid, glutamic acid.

(f) Acid amide amino acids: These are the derivatives of acidic amino acids. In this group, one of the carboxyl group has been converted to amide (-CONH2). e.g. asparagine, glutamine.

(g) Heterocyclic amino acids: These are the amino acids in which the side chain includes a ring involving at least one atom other than carbon. e.g. tryptophan, histidine.

(h) Aromatic amino acids: They have aromatic group (benzene ring) in the side chain. e.g. phenylalanine, tyrosine, etc.