If the side chain group (R) of an amino acid is aliphatic, it is called an aliphatic amino acid. Aliphatic amino acids are divided into five groups.
(i) Monoamino monocarboxylic acid: An amino acid having one amino group and one carboxyl group is called monoamino monocarboxylic acid. For example, Alanine, Leucine, Isoleucine, Glycine, Valine etc
(ii) Monoamino dicarboxylic acid: An amino acid having one amino group and two carboxyl groups is called monoamino dicarboxylic acid. For example – glutamic acid, aspartic acid etc.
(iii) Diamino monocarboxylic acid: The amino acid containing two amino groups and one carboxyl group is called diamino monocarboxylic acid. For example, lysine, arginine etc.
(iv) Monoamino monohydroxylic acid: An amino acid having one amino group and one hydroxyl group is called monoamino monohydroxylic acid. For example, serine, threonine etc.
(v) Sulphur-containing amino acids: Amino acids that contain amino groups, hydroxyl groups and sulfur are called sulfur-containing amino acids. For example, methionine, cystine etc.