Phillips Robert S.
Biochimica et Biophysica Acta (BBA)-Proteins and Proteomics, 2015
Abstract
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, β- and γ-eliminations and substitutions, retro-aldol and Claisen reactions. These reactions depend on the ability of PLP to stabilize, to a varying degree, α-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 γ-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 Cβ-Cγ bond cleavage is accelerated by ground state strain from the bending of the substrate ring out of the plane with the Cβ-Cγ bond. This article is part of a Special Issue entitled: Cofactor-dependent proteins: evolution, chemical diversity and bio-applications.
Keywords
Acid–base catalysis; Pyridoxal-5′-phosphate; Reaction mechanism; Steady-state kinetics; Stopped-flow kinetics; Substrate strain.
Copyright © 2015 Elsevier B.V. All rights reserved.
| PMID: | 25615531 |
|---|---|
| DOI: | 10.1016/j.bbapap.2014.12.028 |
| Category: | Amino Acids |
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