Based on R-groups: Amino acids are classified based on the nature and polarity of their R groups. R can be aliphatic or aromatic. Their polarity may vary widely, from nonpolar and hydrophobic (water-insoluble) to highly polar and hydrophilic (water-soluble). Each amino acid is represented by a three-letter abbreviation and one-letter symbol.
Based on the ability of synthesis by organisms:
- Essential amino acid or indispensable amino acid: These are amino acids that cannot be synthesized by the organism from its food source using its metabolic processes. So, these amino acid are to be supplemented as such through its diet. Nine such amino acids that humans cannot synthesize are- Phenylalanine, Valine, Threonine, Tryptophan, Methionine, Leucine, Isoleucine, Lysine, Histidine.
- Conditionally essential amino acids: These amino acids can be synthesized by the organism but may be affected by the pathophysiological conditions of the organism. So they also become conditionally essential (added) in the human diet. They are six in number- Arginine, Cysteine, Glycine, Glutamine, Proline, and Tyrosine.
- Non-essential (dispensable) amino acid: These are amino acids, that are synthesized in sufficient amounts in the body and do not need any additional supplementation. They are Alanine, Aspartic acid, Asparagine, Glutamic acid, Serine.