Amino acids (AA) are defined as organic substances containing both amino and carboxylic acid groups. Except for glycine, all AA have an asymmetric carbon and exhibit optical activity. The absolute configuration of AA (L- or D-isomers) is defined with reference to glyceraldehydes. Except for proline, all protein AA have a primary amino group and a carboxyl group linked to the α-carbon atom (hence α-AA). In β-AA (e.g., taurine and β-alanine), an amino group links to the β-carbon atom. Post-translationally modified AA occur in some proteins. Because of variations in their side chains, AA have remarkably different biochemical properties and functions. AA are generally stable in aqueous solution at physiological pH, except for (1) glutamine which is slowly cyclized to pyroglutamate (<1% per day at 1 mM at 25°C) and (2) cysteine which undergoes rapid oxidation to cystine.
Among more than 300 AA in nature, only 20 of them (α-AA) serve as building blocks of protein. However, non-protein α-AA (e.g., ornithine, citrulline, and homocysteine) and non-α AA (e.g., taurine and β-alanine) also play important roles in cell metabolism. Because of its large mass (representing 40–45% of body weight), skeletal muscle is the largest reservoir of both peptide-bound and free AA in the body.
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