Intramolecular Model for the Reductive Acyl Transfer Catalyzed by α-Keto Acid Dehydrogenases

Abstract

An intramolecular model was synthesized for the oxidative acyl transfer between thiamin diphosphate on the E1 and lipoic acid on the E2 subunit of the pyruvate dehydrogenase multienzyme complex. The model incorporates a 2-α-methoxybenzylthiazolium salt as a precursor of the enamine/2-α-carbanion, and lipoic acid. Upon addition of the base, the enamine/2-α-carbanion is generated (detected at 380 nm) and is oxidized by the lipoic acid. The oxidation is very significantly enhanced by the addition of PhHgCl. It is likely that the Hg(II) shifts the equilibrium toward the reductive acylation. This appears to be the first successful model for the reductive acylation for which all intermolecular models (including control experiments in this laboratory) have failed to date. The reaction requires both a high local concentration of reactants and the trapping of the reduced thiolate by an electrophile. It is also evident from the data that oxidation of the enamine/2-α-carbanion intermediate on the 2-oxoacid dehydrogenase multienzyme complexes requires very significant assistance by the protein (at least 105-fold rate acceleration as compared to the model here presented), unlike its oxidation by flavin (a model for the enzyme pyruvate oxidase) that requires no significant assistance once the coenzymes are bound to the enzyme (Chiu, C. C.; Pan, K.; Jordan, F. J. Am. Chem. Soc. 1995, 117, 7027−7028).