1. Neutral amino acids:
The neutral amino acids are mono- amino-mono-carboxylic acids and are characterized by the nature of their side chains. Such type of amino-acids contain one amino (—NH3) and one carboxyl group (—COOH), i.e., mono-amino mono- carboxyl group in their composition. They constitute most of the part of all proteins except protamines and histones.
These amino acids are further classified on the basis of group present at the position of’R’ of the empirical formula:
(i) Aliphatic amino acids:
Such amino-acids contain aliphatic group at the position of ‘R Examples—glycine, alanine, valine, leucine and isoleucine.
(ii) Aromatic amino acids:
These amino-acids contain aromatic group at the position of R. Examples—phenylalanine, tyrosine and tryptophan.
(iii) Sulphur containing amino acids:
In these amino-acids R is represented by sulphur containing group. Examples—cysteine,cystine, methionine.
(iv) Hydroxy amino acids:
These amino-acids contain hydroxyl group at the position of R. Examples —serine, threonine.
(v) Heterocyclic amino acids:
In these amino-acids such as histidine, proline and hydroxyproline, R is represented by heterocyclic group.
Proline differs from other amino acids in the basic set of twenty in containing a secondary rather than a primary amino group.
Strictly speaking, proline is an unino-acid rather than an amino-acid. The side chain of proline is bonded to both the amino group and the carbon, thereby forming a cyclic structure.
2. Acidic amino acids:
Such type of amino-acids contain one amino group (— NH,) and two carboxyl groups (—COOH) in their composition. Examples—asparatic acid, glutamic acid.
3. Basic amino acids:
These amino-acids contain one carboxyl group and two amino groups in their composition. Examples—lysine, arginine, glutamine and aspargine.