Role of Coupling Entropy in Establishing the Nature and Magnitude of Allosteric Response

• Published in: Proceedings of the National Academy of Sciences - PNAS 1989-06, Vol.86 (11), p.4032-4036
• Main Authors: Reinhart, Gregory D., Hartleip, Sharon B., Symcox, Marina M.
• Format: Article
• Language: English
• Subjects: Allosteric Regulation; Analytical, structural and metabolic biochemistry; Animals; Biochemistry; Biological and medical sciences; Dehydrogenases; Entropy; Enzyme substrates; Enzymes; Enzymes and enzyme inhibitors; Free energy; Fundamental and applied biological sciences. Psychology; General aspects, investigation methods; isocitrate dehydrogenase (NAD super(+)); Isocitrates; Kinetics; Ligands; Liver; Liver - enzymology; Logarithms; Male; Models, Theoretical; Phosphofructokinase-1 - metabolism; Rats; Thermodynamics
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.86.11.4032

The coupling free energy between an allosteric ligand and a substrate, Δ 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 (Δ Hax) favoring activation and the coupling entropy (Δ Sax) favoring inhibition. For phosphofructokinase at 25 degrees C, the absolute value of TΔ Saxis greater than the absolute value of Δ Hax, and net inhibition of rat liver phosphofructokinase by MgATP is realized. For isocitrate dehydrogenase, Δ Haxdominates; however, the net activation is substantially mitigated by the magnitude of TΔ 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.