Obfuscation of allosteric structure-function relationships by enthalpy-entropy compensation.
The pH and temperature dependence of the allosteric properties of phosphofructokinase (PFK) from Bacillus stearothermophilus have been studied from 5 to 9 and 6 to 40 degrees C, respectively. Throughout this pH and temperature range the allosteric ligands MgADP and phospho(enol)pyruvate (PEP) have no effect on kcat. The dissociation constants of the substrate, fructose 6-phosphate, and the allosteric ligands, as well as the absolute value of the coupling free energies between these ligands, all increase when the pH is raised, indicating that the inhibition by PEP and the activation by MgADP increase despite each ligand's somewhat lower affinity. However, the constituent coupling enthalpies and entropies substantially diminish in absolute value as pH is increased, suggesting that the magnitudes of molecular perturbations engendered by the binding of allosteric ligands do not correlate with the magnitudes of the functional consequences of those perturbations. Temperature and pH exert their influence on the observed allosteric behavior by changing the relative contributions made by the largely compensating DeltaH and TDeltaS terms to the coupling free energy.
Department of Biochemistry and Biophysics, Texas A & M University, College Station, Texas 77843-2128 USA.
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Tlapak-Simmons
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Reinhart
D000494:Allosteric Regulation; D000495:Allosteric Site; D019277:Entropy; D001411:Geobacillus stearothermophilus / Q000201:enzymology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008024:Ligands; D010732:Phosphofructokinase-1 / Q000737:chemistry* / Q000378:metabolism*; D013816:Thermodynamics
D008024:Ligands; D010732:Phosphofructokinase-1