The Linkage of Catalysis and Regulation in Enzyme Action. Fluoropyruvate as a Probe of Regulation in Pyruvate Decarboxylases

Abstract

Pyruvate decarboxylase, a thiamin-diphosphate (ThDP) dependent enzyme from the yeast Saccharomyces cerevisiae (SCPDC), catalyzes the conversion of fluoropyruvate to carbon dioxide, fluoride ion and acetate ion, as earlier reported for the wheat-germ enzyme (Gish, G.; Smyth, T.; Kluger, R. J. Am. Chem. Soc. 1988, 110, 6230−6234). While the fluoropyruvate reaction with the wheat-germ enzyme does not display the sigmoidal kinetics associated with hysteretic regulation by substrate that pyruvate as substrate produces with the wheat-germ enzyme, this feature and the induction period characteristic of hysteretic regulation are preserved when fluoropyruvate acts as substrate and regulator for SCPDC. The C1−13C kinetic isotope effect for fluoropyruvate with SCPDC is the same as that for pyruvate (1.008−1.009), indicating the decarboxylation step to be about 15% rate limiting in the substrate-binding/decarboxylation part of the reaction. Solvent isotope effects and proton inventories are consistent with the view that fluoropyruvate regulation in SCPDC is not associated with an intracatalytic opening/closing of the active site coupled to sulfhydryl addition of cysteine-221 to the regulator keto group, as is thought to occur in pyruvate regulation of SCPDC. Product-release events in the first few catalytic cycles of the fluoropyruvate reaction, involving enzyme-catalyzed hydrolysis of acetyl-ThDP and release of acetate ion, may bring the wheat-germ enzyme into an ordinarily kinetically inaccessible enzyme-conformational regime analogous to the active form of the bacterial enzyme from Zymomonas mobilis, where the active site is permanently open without occupation of the regulatory site. Binding of fluoropyruvate at the regulatory site of SCPDC, in contrast, may remain necessary in the steady state for active-enzyme function but the active site may be permanently opened, thus not requiring the intracatalytic addition−elimination reactions at the regulatory site.