An effect of enzyme and ligand concentration on the state of aggregation of aspartate transcarbamylase of E. coli: I. The binding of CTP and ATP to the enzyme.

Detailed binding studies of the inhibitor, cytidine triphosphate (CTP), to native Escherichia coli aspartate transcarbamylase (EC 2.1.3.2) reveal significant changes in subunit interaction when enzyme concentration is altered. In contrast, similar binding studies of the activator, adenosine triphosphate (ATP), do not reveal such changes, but do indicate more complex subunit interactions than previously reported. Equilibrium dialysis studies of 4 degrees C are consistent with six binding sites for CTP and ATP per enzyme molecule of molecular weight 310 000, at all enzyme concentrations. CTP binding studies reveal a progressive change from apparent positive to negative cooperativity as the enzyme concentration is decreased. ATP binding studies reveal complex subunit interactions involving a mixture of apparent negative and positive cooperativity. Sucrose gradient studies indicate the presence of at least three enzymatically active polymeric forms of the enzyme. The preliminary sedimentation studies indicate possible ligand and enzyme concentration perturbations of a preexisting association equilibrium in the aspartate transcarbamylase system. The binding data are therefore interpreted in terms of an association model.