金属离子对GTP结合蛋白Cdc42Hs的内源性GTP酶活性的影响

不同的金属离子会对GTP结合蛋白Cdc42Hs的内源性GTP水解酶活性产生不同的影响。相对于生理条件下的辅基Mg^2 而言，Mn^2 对Cdc42Hs酶活力有所激活，表现在饱和浓度时，实验曲线指数项的表观速率常数kobs有2倍左右的提高，而其稳态反应速度要低于Mg^2 。Mg^2 和Mn^2 的实验曲线在本质上没有什么差别，都是一个指数项和一个一次项的叠加，说明Mn^2 和Mg^2 以相似的机制结合于Cdc42Hs。在Ca^2 存在时，实验曲线无明显的指数项出现，Ca^2 的存在仅使稳态反应速度有所，说明Ca^2 以不同于Mg^2 和Mn^2 的机理与Cdc42Hs结合，随着Mg^2 和Mn^2 离子浓度的增大，指数项的表观速率常数kobs逐步升高，稳态反应速度vs逐渐降低，进一步的动力学模型分析得到了这一反应的微观动力学数学和Mg^2 、Mn^2 与蛋白的结合常数。

EFFECTS OF METAL IONS ON THE INTRINSIC GTP-HYDROLYSIS ACTIVITIES OF SMALL GTPase Cdc42Hs

Different metal ions have different effects on the intrinsic GTP-hydrolysis activity of Cdc42Hs. Compared with Mg2 , the physiological cofactor, Mn2 has a slight activation effect on GTP-hydrolysis activity. The kobs of the exponential phase of the experiment curve of Mn2 is two-times higher than that of Mg2 under saturated condition. And the velocity of the steady state is lower than that of Mg2 . Essentially, there is no difference between the experiment curves of Mn2 and Mg2 . Both of them have an exponential phase and linear phase, which indicate that Mn2 and Mg2 adopt the same mechanism in binding to Cdc42Hs. In the case of Ca2 , there is no detectable exponential phase in the experiment curve. The presence of Ca2 only slows down the velocity of the steady state. This indicates that the binding mechanism of Ca2 to Cdc42Hs is different from that of Mn2 and Mg2 . With the increasing of Mn2 and Mg2 concentration, the kobs of the exponential phase increase, and the velocity of the steady state decrease. A detail kinetic analysis deduces the microscopic kinetic constants of the hydrolysis reaction and the disassociate constants of the metal ions with the protein.