Amino acids, on the basis of number of amino and carboxyl groups, are classified as follows:
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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.
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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:
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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:
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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.