Covalent catalysis by pyridoxal: Evaluation of the effect of the cofactor on the carbon acidity of glycine
Journal of the American Chemical Society, ISSN: 0002-7863, Vol: 129, Issue: 10, Page: 3013-3021
2007
- 50Citations
- 25Captures
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Metrics Details
- Citations50
- Citation Indexes50
- 50
- CrossRef43
- Captures25
- Readers25
- 25
Article Description
First-order rate constants for deprotonation of the α-imino carbon of the adduct between 5′-deoxypyridoxal (1) and glycine were determined as the rate constants for Claisen-type addition of glycine to 1 where deprotonation is rate determining for product formation. There is no significant deprotonation at pH 7.1 of the form of the 1-glycine iminium ion with the pyridine nitrogen in the basic form. The value of k for hydroxide ion-catalyzed deprotonation of the α-imino carbon increases from 7.5 × 10 to 3.8 × 10 to 3.0 × 10 M s, respectively, with protonation of the pyridine nitrogen, the phenoxide oxyanion, and the carboxylate anion of the 1-glycine iminium ion. There is a corresponding decrease in the pKs for deprotonation of the α-imino carbon from 17 to 11 to 6. It is proposed that enzymes selectively bind and catalyze the reaction of the iminium ion with pK = 17. A comparison of k = 1.7 × 10 s for deprotonation of the α-imino carbon of this cofactor-glycine adduct (pK = 17 by HPO with k/K = 4 × 10 M s for catalysis of amino-acid racemization by alanine racemase shows that the enzyme causes a ca 2 × 10-fold acceleration of the rate of deprotonation the α-imino carbon. This corresponds to about one-half of the burden borne by alanine racemase in catalysis of deprotonation of alanine. © 2007 American Chemical Society.
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