Selectivity in the metal-complex-catalyzed decarboxylation of oxaloacetic acid and a role of metal ion in an enzyme system.

Abstract

Abstract The catalytic action of metal ions in the presence of various coordinating agents and a role of metal ions for the activations of an enzyme in a decarboxylation reaction were investigated. Many of the ligands studied decreased the catalytic activity of metal ions. However, α,α′-dipyridyl selectively enhanced the catalytic activities of divalent metal ions, except Cu2+. The activity of Cu2+ was specifically enhanced only with histidine and its derivatives. Histidine destroyed Ni2+ and Co2+ catalysis. The specific enhancement may be due to the increase in the electronegativities of the metal ions with dπ (metal)-pπ(ligand) back donation and to the stereo-configuration of the metal complexes. The catalytic behavior in these metal ion coordinating agent systems was different from that in the metal-enzyme systems. It was assumed that the activation of enzyme resulted from the change in the configuration of enzyme by the coordination of metal ions, and not from the catalytic action of metal ions enhanced by the coordination of enzyme. This was supported by the fact that the catalysis of compounds containing amino groups, active sites of enzyme, remarkably depended on the stereo-configuration. In the enzyme-model system, Mn2+ was generally preferable to Cu2+ and Ni2+. The catalytic actions of Cu2+ and Ni2+ were lowered by the complex formation with the active sites. This may solve the question why, in enzymatic reactions, Mn2+ having small complex-forming ability is more selective and effective than Cu2+ and Ni2+ with large complex-forming abilities.