Regulation of the Catabolic Ornithine Carbamoyltransferase of Pseudomonas fluorescens

The catabolic ornithine carbamoyltransferase from Pseudomonas fluorescens catalyzes the phosphorolysis of citrulline leading to the formation of ornithine and carbamoylphosphate. The initial rates have been measured in the experimentally most accessible direction of the reaction, the carbamoylation of ornithine. The plots of reaction rates against carbamoylphosphate (substrate), phosphate (activator), and putrescine (inhibitor) are sigmoid shaped. The homotropic interactions between carbamoylphosphate molecules are decreased in the presence of phosphate and increased in the presence of putrescine. The allosteric properties of the enzyme are accounted for by the concerted transition theory proposed by Monod, Wyman and Changeux. The treatment of the kinetic data according to the method of Blangy, Buc and Monod suggests that the enzyme is an octamer made up of eight identical protomers which can exist in equilibrium in two conformational states which differ in their dissociation constant for the substrate, activator and inhibitor. The values of these constants for the binding of carbamoylphosphate, phosphate and putrescine, and the allosteric transition constant have been determined for the wild‐type enzyme and for the enzyme of a mutant which can use the catabolic enzyme for anabolic purpose.