Sobieszek A
Biochemistry. 1985 Feb 26;24(5):1266-74. doi: 10.1021/bi00326a032.
Phosphorylation of vertebrate smooth muscle myosin or its isolated 20 000-dalton light chains by myosin light-chain kinase (MLCK) was found to follow first-order kinetics not only at low ([M] much less than Km) but also at high ([M] greater than or equal to Km) substrate concentration. This observation can most simply be explained by a product inhibition for which the Michaelis constants (Km) of the enzyme for the substrate (dephosphorylated myosin) and for the product (phosphorylated myosin) are approximately the same. For such a case, integration of the kinetic velocity equation gives an exponential formula similar to that of a true first-order reaction, the only difference being that its rate constant (k) depends additionally on the initial substrate concentration ([M]0). The standard kinetic constants (k, Km, Vmax) have been calculated by using this pseudo-first-order relationship. Independent evidence for the validity of the derived kinetic relationship was obtained from binding studies with myosin and MLCK. These showed that MLCK binds to phosphorylated and dephosphorylated myosin with approximately equal affinity (Ks = 30 X 10(-9) M). The possible applicability of the same kinetic relationship to other enzyme systems is discussed.
人们发现，肌球蛋白轻链激酶（MLCK）对脊椎动物平滑肌肌球蛋白或其分离出的20000道尔顿轻链的磷酸化不仅在低底物浓度（[M]远小于Km）下，而且在高底物浓度（[M]大于或等于Km）下均遵循一级动力学。这一观察结果最简单的解释是产物抑制作用，即该酶对底物（去磷酸化肌球蛋白）和产物（磷酸化肌球蛋白）的米氏常数（Km）大致相同。对于这种情况，动力学速度方程的积分给出了一个类似于真正一级反应的指数公式，唯一的区别在于其速率常数（k）还额外取决于初始底物浓度（[M]0）。利用这种假一级关系计算出了标准动力学常数（k、Km、Vmax）。通过肌球蛋白与MLCK的结合研究获得了支持所推导动力学关系有效性的独立证据。这些研究表明，MLCK以大致相等的亲和力（Ks = 30×10⁻⁹ M）与磷酸化和去磷酸化肌球蛋白结合。本文还讨论了相同动力学关系对其他酶系统的可能适用性。
