Page "Amino acid" Paragraph 25

Some nonstandard amino acids are not found in proteins.

Examples include lanthionine, 2-aminoisobutyric acid, dehydroalanine, and the neurotransmitter gamma-aminobutyric acid.

Nonstandard amino acids often occur as intermediates in the metabolic pathways for standard amino acids — for example, ornithine and citrulline occur in the urea cycle, part of amino acid catabolism ( see below ).

A rare exception to the dominance of α-amino acids in biology is the β-amino acid beta alanine ( 3-aminopropanoic acid ), which is used in plants and microorganisms in the synthesis of pantothenic acid ( vitamin B < sub > 5 </ sub >), a component of coenzyme A.