Yeast flavocytochrome b2 catalyzes the oxidation of lactate to pyruvate; because of the wealth of structural and mechanistic information available, this enzyme has served as the model for the family of flavoproteins catalyzing oxidation of α-hydroxy acids. Primary deuterium and solvent isotope effects have now been used to analyze the effects of mutating the active site residue Tyr254 to phenylalanine. Both the Vmax and the V/Klactate values decrease about 40-fold in the mutant enzyme. The primary deuterium isotope effects on the Vmax and the V/K lactate values increase to 5.0, equivalent to the intrinsic isotope effect for the wild-type enzyme. In addition, both the Vmax and the V/Klactate values exhibit solvent isotope effects of 1.5. Measurement of the solvent isotope effect with deuterated lactate establishes that the primary and solvent isotope effects arise from the same chemical step, consistent with concerted cleavage of the lactate OH and CH bonds. The pH dependence of the mutant enzyme is not significantly different from that of the wild-type enzyme; this is most consistent with a requirement that the side chain of Tyr254 be uncharged for catalysis. The results support a hydride transfer mechanism for the mutant protein and, by extension, wild-type flavocytochrome b2 and the other flavoproteins catalyzing oxidation of α-hydroxy acids.
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