Tlapak-Simmons V L, Reinhart G D
Department of Biochemistry and Biophysics, Texas A & M University, College Station, Texas 77843-2128 USA.
Biophys J. 1998 Aug;75(2):1010-5. doi: 10.1016/S0006-3495(98)77589-7.
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.
分别在5至9摄氏度和6至40摄氏度的范围内研究了嗜热脂肪芽孢杆菌磷酸果糖激酶（PFK）变构性质对pH值和温度的依赖性。在整个pH值和温度范围内，变构配体MgADP和磷酸烯醇丙酮酸（PEP）对催化常数（kcat）均无影响。当pH值升高时，底物6-磷酸果糖和变构配体的解离常数，以及这些配体之间耦合自由能的绝对值均增加，这表明尽管每种配体的亲和力有所降低，但PEP的抑制作用和MgADP的激活作用仍会增强。然而，随着pH值的升高，组成耦合焓和熵的绝对值大幅减小，这表明变构配体结合所产生的分子扰动幅度与这些扰动的功能后果幅度并不相关。温度和pH值通过改变主要相互补偿的ΔH和TDeltaS项对耦合自由能的相对贡献，来影响所观察到的变构行为。
