On the monomeric structure and proposed regulatory properties of phosphoenolpyruvate carboxykinase of Escherichia coli

Abstract

Phosphoenolpyruvate carboxykinase (ATP:oxaloacetate carboxy-lyase (transphosphorylating)) (EC 4.1.1.49) was purified to homogeneity from E. coli. The enzyme shows the same molecular weight (.apprxeq. 65,000) by sedimentation equilibrium under nondenaturing conditions or by polyacrylamide gel electrophoresis in the presence of detergent, indicating that the enzyme has a monomeric structure. The previous observation that NADH is an inhibitor of this enzyme was confirmed, but the previously reported appearance of homotropic cooperativity with respect to substrate binding in the presence of this inhibitor could not be detected. Lack of such homotropic interactions is in harmony with the conclusion that the enzyme is a monomer. Replacement of Mg2+ by Mn2+ in the assay medium lowers the Km for phosphoenolpyruvate by an order of magnitude, but does not affect the characteristics of inhibition by NADH.