Isotope Studies on Citrate-Condensing Enzyme

Abstract

Citrate-condensing enzyme catalyzes the reaction Acetyl-CoA + Oxaloacetate⁻² + H₂O = Citrate⁻³ + CoASH + H This enzyme was first crystallized from pig heart by Ochoa and coworkers [¹], and since that time a number of different workers have studied the mechanism of the reaction it catalyzes. Two different chemical reactions occur during the course of the reaction. The first is an aldol condensation between the methyl carbon of acetyl-CoA and the ketocarbon of oxaloacetate, preceded by removal of hydrogen from the acetyl-CoA. The second is a hydrolysis of the thioester bond in the acetyl-CoA portion of the molecule to yield free CoA. As a result of this last reaction, the equilibrium of the reaction is far toward citrate formation. For this, as for most other enzyme-catalyzed reactions, no complete mechanistic picture of the enzyme-catalyzed reaction can yet be drawn. This enzyme has been studied with a wide variety of isotopic techniques and serves as a good example for showing how information relating to enzyme mechanism can be obtained using isotopic methods.