Ishikawa H, Ogino H, Oshida H
Department of Chemical Engineering, University of Osaka Prefecture, Japan.
Biochem J. 1991 Nov 15;280 ( Pt 1)(Pt 1):131-7. doi: 10.1042/bj2800131.
For the reaction S in equilibrium P catalysed by a dimeric enzyme, the reaction schemes are considered on the basis of the KNF model. For each of the ten possible schemes, the rate equation is derived on the basis of the combined steady-state and rapid-equilibrium assumptions. The curves of the plots of initial velocity v versus the substrate concentration [S] and the Hill coefficients h calculated from the rate equations depend strongly on the reaction scheme and the parameter X1. This parameter is defined by log (KS2/KS1) and is a measure of the relative affinities of the first and second protomers for the substrate. When X1 less than 0, v-[S] curves for some schemes exhibit negative co-operativity (h less than 1.0) and v-[S] curves for other schemes are similar to that of the Michaelis-Menten scheme, indicating that, even if there is interaction between the distinct protomers, sigmoidal rate behaviour is not necessarily observed. When X1 greater than 0, all the reaction schemes except one, which shows substrate-inhibition kinetic behaviour, exhibit sigmoidal kinetic behaviour (h greater than 1.0), and at the limit of X1 much greater than 0 the Hill coefficients attain the maximum possible value of 2.0. Furthermore, we have found that, even if X1 = 0, the v-[S] curve for almost all the schemes considered in the present work does not necessarily agree with that for the Michaelis-Menten scheme. This means that the deviation of the v-[S] curve from a hyperbola can be observed even if there is no interaction between the distinct protomers.
对于由二聚体酶催化的反应S⇌P，基于KNF模型考虑反应机制。对于十种可能的机制中的每一种，基于稳态和快速平衡假设的组合推导速率方程。由速率方程计算得到的初始速度v与底物浓度[S]的关系曲线以及希尔系数h强烈依赖于反应机制和参数X1。该参数由log(KS2/KS1)定义，是第一个和第二个亚基对底物相对亲和力的一种度量。当X1小于0时，某些机制的v-[S]曲线表现出负协同性（h小于1.0），而其他机制的v-[S]曲线与米氏方程的曲线相似，这表明，即使不同亚基之间存在相互作用，也不一定会观察到S形速率行为。当X1大于0时，除了一种表现出底物抑制动力学行为的机制外，所有反应机制都表现出S形动力学行为（h大于1.0），并且在X1远大于0的极限情况下，希尔系数达到最大可能值2.0。此外，我们发现，即使X1 = 0，本研究中考虑的几乎所有机制的v-[S]曲线也不一定与米氏方程的曲线一致。这意味着，即使不同亚基之间没有相互作用，也可以观察到v-[S]曲线偏离双曲线的情况。
