The Carboxylation of Phosphoenolpyruvate and Pyruvate I. THE ACTIVE SPECIES OF “COz” UTILIZED BY PHOSPHOENOLPYRUVATE

Previous studies with propionyl coenzyme A carboxylase and with phosphoenolpyruvate carboxylase using ‘*O-labeled bicarbonate have indicated that bicarbonate is the reactive species in these fixations of “C02.” We have investigated the species of CO:! in reactions catalyzed by pyruvate carboxylase, P-enolpyruvate carboxykinase, and P-enolpyruvate carboxytransphosphorylase. Since propionyl-CoA carboxylase and pyruvate carboxylase are biotin enzymes, they would be expected to have similar mechanisms. Likewise, the reactions catalyzed by P-enolpyruvate carboxykinase and carboxy transphosphorylase are, in some respects, similar to that of P-enolpyruvate carboxylase, and it has been suggested that bicarbonate might be the reactant in each case. By means of radiochemical and spectrophotometric techniques, we have obtained evidence that the active species in the carboxykinase and carboxytransphosphorylase reactions is CO2 and not bicarbonate. Bicarbonate appears to be the active species in the pyruvate carboxylase reaction, in conformity with the results obtained with propionyl-CoA carboxylase.