A New Reaction Surface for Concerted Reactions: The Metal Ion Catalyzed Addition of Enolpyruvate to Pyruvate

Article abstract of DOI:10.1021/ja00189a028

The effects of Mg(2+) and Zn(2+) on rates of disappearance of enzymically generated enolpyruvate along parallel ketonization and pyruvate addition paths have been measured spectrophotometrically in acetate buffers.Zn(2+) -dependent rates of ketonization show acetate- and solvent-catalyzed pathways with rate constants of (3.2 +/- 0.6)E4 M^-2 s^-1 and (1.3 +/- 0.4)E2 M^-1 s^-1.The former value is consistent with rate constants earlier determined for Mg(2+)-, Mn((2+)-, and Cu(2+)-dependent ketonization, and, in accordance with the Marcus function and as found withthese earlier systems, the rate constant is in close agreement with the value predicted from the rate of decarboxylation of the corresponding metal ion complex of oxalacetate.Enolpyruvate adds to pyruvate in metal ion dependent step to yield pyruvate dimer (1), according to the rate law rate_{dimer formation} = kwith k = (1.2 +/- 0.1)E4 M^-2 s^-1 (Zn(2+)) and (4.3 +/- 0.6)E2 M^-2 s^-1 (Mg(2+)).Reaction surfaces obtained by averaging Alberty and modified Guthrie surfaces were found to provide calculated rate constants that are in good agreement with the observed values using an intrinsic barrier for carbon-carbon bond formation obtained from OH(1-)-dependent rates reported for aldol condensation and retroaldol condensation of 3-penten-2-one.In the reaction model it is assumed that carbon-carbon bond formation is concerted with proton transfer and occurs within the coordination sphere of a metal ion-mixed ligand complex.The calculations indicate that in the transition state carbon-carbon bond formation leads proton transfer and occurs to the same extent with both metal ions.The metal ions aid inin stabilizing negative charge that appears on the pyruvate keto oxygen atom as carbon-carbon formation proceeds.Proton transfer from the enol oxygen atom also aids in this process, with a greater portion of the stabilization being assumed by the proton in the case of the more weakly binding Mg(2+) ion