Purification and Properties of a Bacterial Carbamyl Phosphate Synthetase

From the ¹ From the Department of Pharmacology, Stanford University School of Medicine, Palo Alto, California 94304

A glutamine-dependent carbamyl phosphate synthetase was purified from Escherichia coli B. The enzyme has a molecular weight of approximately 700,000. This enzyme can use either ammonia or glutamine as the amino group donor in carbamyl phosphate synthesis. Estimated values of the Michaelis constant for each of these substrates indicate that glutamine is probably the true amino group donor in vivo. The stoichiometry for the case where glutamine serves as a substrate indicates that 2 moles of adenosine triphosphate are required for the formation of 1 mole of carbamyl phosphate. The reaction is reversible, although the rate of the back reaction is about one-tenth as fast as that of the forward reaction. This reverse reaction, the formation of ATP from carbamyl phosphate and ADP, does not proceed in the absence of glutamic acid and orthophosphate. The single omission of glutamate or of orthophosphate reduces the synthesis of ATP to one-half the amount observed when both are present. We have suggested that these data indicate that the reaction may involve more than one step.

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