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