Role of coupling entropy in establishing the nature and magnitude of allosteric response.

The coupling free energy between an allosteric ligand and a substrate, delta Gax, is an explicit measure of the nature as well as the magnitude of impact that an allosteric ligand has on the binding of the substrate ligand to the enzyme, with positive values indicating inhibition and negative values indicating activation. By measuring the variation with temperature of the coupling free energy between the allosteric ligand and the substrate, it is possible to determine the enthalpic and entropic components that give rise to the coupling free energy. We have performed this analysis on two different K-type allosteric systems: the allosteric inhibition of rat liver phosphofructokinase by MgATP, and the allosteric activation of beef heart NAD+-dependent isocitrate dehydrogenase by ADP. In both cases the coupling free energy arises as the net result of opposing enthalpic and entropic components, with the coupling enthalpy (delta Hax) favoring activation and the coupling entropy (delta Sax) favoring inhibition. For phosphofructokinase at 25 degrees C, the absolute value of T delta Sax is greater than the absolute value of delta Hax, and net inhibition of rat liver phosphofructokinase by MgATP is realized. For isocitrate dehydrogenase, delta Hax dominates; however, the net activation is substantially mitigated by the magnitude of T delta Sax. Hence, the coupling entropy plays an important role in establishing both the nature and magnitude of the allosteric response. We hypothesize that the negative coupling entropy arises from the particular constraint placed upon the internal dynamical properties of the enzyme by the simultaneous binding of both allosteric and substrate ligands.