Chemistry and diversity of pyridoxal-5′-phosphate dependent enzymes

Pyridoxal-5'-phosphate (PLP) is a versatile cofactor that enzymes use to catalyze a wide variety of reactions of amino acids, including transamination, decarboxylation, racemization, beta- and gamma-eliminations and substitutions, retro-aldol and Claisen reactions. These reactions depend on the ability of PLP to stabilize, to a varying degree, alpha-carbanionic intermediates. Furthermore, oxidative decarboxylations and rearrangements suggest that PLP can stabilize radical intermediates as well. The reaction mechanisms of two PLP-dependent enzymes are discussed, kynureninase and tyrosine phenol-lyase (TPL). Kynureninase catalyzes a retro-Claisen reaction of kynurenine to give anthranilate and alanine. The key step, hydration of the gamma-carbonyl, is assisted by acid-base catalysis with the phosphate of the PLP, mediated by a conserved tyrosine, and an oxyanion hole. TPL catalyzes the reversible elimination of phenol, a poor leaving group, from L-tyrosine. In TPL, the C-beta-C-gamma bond cleavage is accelerated by ground state strain from the bending of the substrate ring out of the plane with the C-beta-C-gamma bond. This article is part of a Special Issue entitled: Cofactor-dependent proteins: evolution, chemical diversity and bio-applications. (C) 2015 Elsevier B.V. All rights reserved.