Communication between dissimilar subunits in aspartate transcarbamoylase: effect of inhibitor and activator on the conformation of the catalytic polypeptide chains

Abstract

Although local, direct effects of ligand binding to proteins are readily differentiated conceptually from gross, indirect conformational changes in regions of the protein remote from the site of binding, it has been difficult experimentally to distinguish between them. In oligomeric proteins, for example, the binding of ligands to one chain may cause a conformational change in the unliganded chains, but many physical chemical probes are not sufficiently discriminating to demonstrate where the change occurred. Evidence has been lacking as to whether the inhibitor, CTP, or the activator, ATP, in binding to the regulatory chains of the allosteric enzyme aspartate transcarbamoylase (aspartate carbamoyltransferase; carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) causes a conformational change that is propagated throughout the enzyme to the catalytic chains. To demonstrate this "communication" of effects of binding at one site on the conformation of other polypeptide chains, we constructed molecules containing native regulatory subunits and enzymically active nitrated catalytic subunits having one sensitive nitrotyrosyl chromophore per polypeptide chain. These hybrid molecules exhibited the characteristic regulatory properties of the native enzyme. Upon the addition of CTP the population of molecules was shifted toward the constrained or T state, as shown by the change in the sedimentation coefficient and the altered enzyme kinetics. Moreover, there was a decrease in absorbance at 430 nm due to the altered environment of the nitrated catalytic polypeptide chains. In contrast, ATP caused a shift toward the relaxed or R conformation, and the absorbance due to the nitrotyrosyl residues was increased. Different types of experiments indicated that the modified enzyme molecules are in a preexisting equilibrium that is perturbed by CTP or ATP; the resulting conformational changes in the nitrated catalytic subunits are detected by opposite alterations in their absorbance spectrum.