Amino acids (AAs)3 are organic compounds that contain amino and acid groups (1). Based on the configuration of glyceraldehyde (l- or d-isomers as introduced by Emil Fischer in 1908), AAs (except for Gly, taurine, β-alanine, and γ-aminobutyrate, which have no asymmetric carbon) exist as either l- or d-AAs. l-AAs are much more abundant than d-AAs in nature and are the physiologic isomers in animal and plant proteins. The AAs whose carbon skeletons are not synthesized de novo by animal cells were termed “nutritionally essential” AAs (EAAs) in ...
