4 - Metabolism of the Aromatic Amino Acids in Micro-organisms and Higher Organisms

Abstract

Protein ingested by a living organism generally undergoes hydrolysis, and the liberated amino acids join the various pools of free amino acids in the organism. These pools may be supplemented in appropriate situations by amino acids biosynthesized by the organism itself, and from them, amino acids are withdrawn for the synthesis of new protein and for various biosynthetic operations. The aromatic amino acids are unique among amino acids for the range of biosynthetic processes in which they engage. They are the parent substances in many organisms for several physiologically important metabolites that are derived by oxidative pathways from the amino acids. Under appropriate conditions, all the amino acids may be metabolized to yield simpler metabolites such as succinate, fumarate, acetoacetate, and acetate that can be further metabolized by entering the Krebs cycle or by conversion to fatty acids. In the case of L-phenylalanine and L-tyrosine, the recognition of this property gave rise historically to the definition that these two amino acids were ketogenic, that is, that they gave rise to acetoacetic acid and other ketonic fragments in mammalian systems.