Cooperative binding of the bisubstrate analog N-(phosphonacetyl)-L-aspartate to aspartate transcarbamoylase and the heterotropic effects of ATP and CTP.

Most investigations of the allosteric properties of the regulatory enzyme aspartate transcarbamoylase (ATCase) from Escherichia coli are based on the sigmoidal dependence of enzyme activity on substrate concentration and the effects of the inhibitor, CTP, and the activator, ATP, on the saturation curves. Interpretations of these effects in terms of molecular models are complicated by the inability to distinguish between changes in substrate binding and catalytic turnover accompanying the allosteric transition. In an effort to eliminate this ambiguity, the binding of the 3H-labeled bisubstrate analog N-(phosphonacetyl)-L-aspartate (PALA) to aspartate transcarbamoylase in the absence and presence of the allosteric effectors ATP and CTP has been measured directly by equilibrium dialysis at pH 7 in phosphate buffer. PALA binds with marked cooperativity to the holoenzyme with an average dissociation constant of 110 nM. ATP and CTP alter both the average affinity of ATCase for PALA and the degree of cooperativity in the binding process in a manner analogous to their effects on the kinetic properties of the enzyme; the average dissociation constant of PALA decreases to 65 nM in the presence of ATP and increases to 266 nM in the presence of CTP while the Hill coefficient, which is 1.95 in the absence of effectors, becomes 1.35 and 2.27 in the presence of ATP and CTP, respectively. The isolated catalytic subunit of ATCase, which lacks the cooperative kinetic properties of the holoenzyme, exhibits only a very slight degree of cooperativity in binding PALA. The dissociation constant of PALA from the catalytic subunit is 95 nM. Interpretation of these results in terms of a thermodynamic scheme linking PALA binding to the assembly of ATCase from catalytic and regulatory subunits demonstrates that saturation of the enzyme with PALA shifts the equilibrium between holoenzyme and subunits slightly toward dissociation. Ligation of the regulatory subunits by either of the allosteric effectors leads to a change in the effect of PALA on the association-dissociation equilibrium.