Phosphoenolpyruvate Carboxylase: Activation by Nucleotides as a Possible Compensatory Feedback Effect

The activation of partially purified phosphoenolpyruvate carboxylase from Salmonella typhimurium by various pyrimidine and purine nucleotides is described. The most potent activators are found to be cytidine diphosphate, guanosine triphosphate, and cytidine monophosphate, in that order. It is shown that in the presence of acetyl coenzyme A the affinity of the nucleotides for their binding site is increased. Evidence is presented to demonstrate that the nucleotides do not participate in the enzyme reaction, but act only catalytically. From these studies the physiological concept of compensatory feedback loops in the control of branched pathways is presented.