Non-protein amino acids: a review of the biosynthesis and taxonomic significance

The non-protein amino acids, with which we are concerned here, are not incorporated into the proteins of the organisms that synthesize them nor are their residues formed by the post-translational modification of protein amino acid residues. Non-protein amino acids are of value in the study of relationships between species and higher taxa of organisms because most of them are of restricted distribution. If a particular non-protein amino acid is only known to occur in a limited group of species which are related in other respects then it is possible that these species have all arisen from a common ancestral form in which the biosynthetic pathway to that particular non-protein amino acid already existed.