Shearwin K E, Winzor D J
Department of Biochemistry, University of Queensland, St. Lucia, Australia.
Arch Biochem Biophys. 1988 Feb 1;260(2):532-9. doi: 10.1016/0003-9861(88)90478-x.
Expressions for the effects of thermodynamic nonideality arising from the use of high concentrations of small substrate in enzyme kinetic studies are derived. Their application to experimental results for the hydrolysis of sucrose by yeast invertase (pH 4.9, 37 degrees C) signifies that the progressive decrease in initial velocity at high sucrose concentration is consistent with the occurrence of isomeric expansion during the transition of an enzyme-substrate complex to its activated state. Ultracentrifuge studies on the yeast enzyme preparation are then used to establish the physical acceptability of the volume change required to account for the kinetic effects in these terms: the postulated expansion of 1.3 liter/mol would represent a mere 0.16% increase in hydrated volume (or a corresponding increase in extent of asymmetry). Finally, although originally interpreted to signify an effect of sucrose on water concentration, published results for the invertase-sucrose system [J. M. Nelson and M. P. Schubert (1928) J. Amer. Chem. Soc. 50, 2188-2193] also find a rational explanation in terms of the present analysis based on effects of thermodynamic nonideality in enzyme kinetic studies.
推导了酶动力学研究中使用高浓度小分子底物时产生的热力学非理想性效应的表达式。将其应用于酵母转化酶水解蔗糖的实验结果（pH 4.9，37℃）表明，在高蔗糖浓度下初始速度的逐渐降低与酶 - 底物复合物向其活化状态转变过程中发生的异构膨胀一致。然后，对酵母酶制剂进行超速离心研究，以确定从这些方面解释动力学效应所需的体积变化在物理上的可接受性：假设的1.3升/摩尔的膨胀仅代表水合体积增加0.16%（或不对称程度相应增加）。最后，尽管最初将其解释为蔗糖对水浓度的影响，但基于酶动力学研究中热力学非理想性效应的当前分析，也能对转化酶 - 蔗糖系统已发表的结果[J.M.尼尔森和M.P.舒伯特（1928年）《美国化学会志》50，2188 - 2193]找到合理的解释。
