ALLOSTERIC INTERACTIONS IN ASPARTATE TRANSCARBAMYLASE .3. INTERPRETATION OF EXPERIMENTAL DATA IN TERMS OF MODEL OF MONOD WYMAN AND CHANGEUX

A detailed analysis according to the two-state model of Monod, Wyman, and Changeux (Monod, J., Wyman, J., and Changeux, J.-P. (1965), J. Mol. Biol. 12, 88) has been made of data obtained for the allosteric interactions of aspartate transcarbamylase from Escherichia coli. The data presented in papers I and II of this series are quantitatively consistent with the equations of the two-state model and are fit by a single set of values for the parameters. These values suggest that (1) the substrate analog succinate is bound exclusively (at least 103-fold more strongly) by one of the putative states of the enzyme, while (2) the feedback inhibitor cytidine triphosphate is bound only preferentially (1.7-fold more strongly) by the other state of the enzyme. (3) In the absence of effectors, the dis tribution of enzyme molecules between these states favors in a 4:1 ratio the state to which the inhibitor binds preferentially and the substrate does not bind. A comparison is made between the extent of ligand binding and the extent of alterations in the protein conformation of the enzyme over a wide range of concentrations of substrate and feedback inhibitor analogs (data presented in papers I and II of this series). The conformational alterations are found to approach completion at concentrations of substrate analog which only partially saturate the specific binding sites of the enzyme (e.g., 50% completion when only 15% of the sites is filled). This observation is compatible with the two-state model but not with certain alternative models. © 1968, American Chemical Society. All rights reserved.