PLP‐Dependent Enzymes, Chemistry of

Abstract

AbstractPyridoxal phosphate‐dependent enzymes catalyze a wide variety of reaction types on amines and amino acids, generally by stabilizing carbanionic intermediates, which makes them very common in both primary and secondary metabolism. A fundamental mechanism by which reaction specificity is enforced is stereoelectronic effects; the bond to Cα that gets broken in the universal external aldimine intermediate is that which is aligned parallel to theporbitals of the conjugated, electron‐deficient π system. When a bond to Cα has been broken, enzymes must also carefully control the fate of the resulting carbanionic intermediate. Structurally, all known pyridoxal 5′‐phosphate (PLP)‐dependent enzymes are separated into five groups. The largest of these, Type 1, is the aminotransferase group, of which aspartate aminotransferase is the prototype. The prototypes for the other groups are as follows: Type 2, tryptophan synthetase; Type 3, alanine racemase; Type 4, D‐amino acid aminotransferase; and Type 5, glycogen phosphorylase.