The amino acids are the building blocks of proteins- the macromolecules of prime importance. These micomolecules have the following fea­tures in general:

(i) An alpha (α) carbon atom (C*) to which all other constituents (as indicated below) are attached.

(ii) An amino (NH-2) group with a poten­tial positive charge (NH3+). Amino group gives it a basic reaction. Pro­line is the only atypical member, in which the a-amino group is not free, but part of a ring structure.

(iii) A carboxyl (-COOH) group with a potential negative charge (-COO-). Carboxylic group pro­vides it an acidic property.


(iv) A side group (R) that varies in its structure in different amino acids: Of course, it is a hydro­gen atom (H) incase of the sim­plest amino acid, glycine. In others it may be straight or branched hydrocarbon chain or a cyclic group. The hydrocarbon may further be polar or non-polar.

(v) A hydrogen atom (H).

Further, the amino acids are col­ourless, crystalline solids and gener­ally soluble in water and insoluble in organic solvents.

There are more than 170 amino acids known to occur in living organisms. However, only 20 different types of amino acids and amides are known to occur in proteins of all organisms. Certain proteins may contain fewer of them and certain others may con­tain secondary modifications of the 20 principal ones, e.g., hydroxyproline (from proline) and hydroxylysine (from lysine), both found in collagen. These 20 amino acids in proteins ap­pear to have been so selected during evolution that they provide consider­able chemical versatility.


The amino acids found in proteins are grouped according to their structure, reaction and side chains into seven types.

The non-protein amino acids are many more in number and occur in free state as well as in combined state (but not with proteins). Many of these take part in some important biosynthetic pathways.

Functions of amino acids

1. They are building blocks (mono­mers) of proteins, both struc­tural and functional, i.e., en­zymes an d other specific ones like haemoglobin.


2. They serve as storage of nitro­gen in the form of amides.

3. Glycine, cysteine and glutamate form the co-enzymes, Glu­tathione.

4. Surplus amino acids are deami­nated in liver to organic acids, which in turn are changed to glucose (gluconeogenesis) or used in metabolism