INTRAMOLECULAR MODEL FOR THE REDUCTIVE ACYL TRANSFER CATALYZED BY ALPHA-KETO ACID DEHYDROGENASES

An intramolecular model was synthesized for the oxidative acyl transfe r between thiamin diphosphate on the El and lipoic acid on the E2 subu nit of the pyruvate dehydrogenase multienzyme complex. The model incor porates a 2-alpha-methoxybenzylthiazolium salt as a precursor of the e namine/2-alpha-carbanion, and lipoic acid. Upon addition of the base, the enamine/2-alpha-carbanion is generated (detected at 380 nm) and is oxidized by the lipoic acid. The oxidation is very significantly enha nced by the addition of PhHgCl. It is likely that the Hg(LI) shifts th e equilibrium toward the reductive acylation. This appears to be the f irst successful model for the reductive acylation for which all interm olecular models (including control experiments in this laboratory) hav e failed to date. The reaction requires both a high local concentratio n of reactants and the trapping of the reduced thiolate by an electrop hile. It is also evident from the data that oxidation of the enamine/2 -alpha-carbanion intermediate on the 2-oxoacid dehydrogenase multienzy me complexes requires very significant assistance by the protein (at l east 10(5)-fold rate acceleration as compared to the model here presen ted), unlike its oxidation by flavin (a model for the enzyme pyruvate oxidase) that requires no significant assistance once the coenzymes ar e bound to the enzyme (Chiu, C. C.; Pan, K.; Jordan, F. J. Am. Chem. S ec. 1995, 117, 7027-7028).