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首页> 美国卫生研究院文献>Biochemical Journal >Activation mechanism and modification kinetics of Chinese hamster dihydrofolate reductase by p-chloromercuribenzoate.
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Activation mechanism and modification kinetics of Chinese hamster dihydrofolate reductase by p-chloromercuribenzoate.

机译：对氯汞苯甲酸对中国仓鼠二氢叶酸还原酶的激活机理和修饰动力学。

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摘要

Substrate effects on the activation kinetics of Chinese hamster dihydrofolate reductase by p-chloromercuribenzoate (pCMB) have been studied. On the basis of the kinetic equation of substrate reaction in the presence of pCMB, all modification kinetic constants for the free enzyme and enzyme-substrate binary and ternary complexes have been determined. The results of the present study indicate that the modification of Chinese hamster dihydrofolate reductase by pCMB shows single-phase kinetics, and that changes in the enzyme activity and tertiary structure proceed simultaneously during the modification process. Both substrates, NADPH and 7,8-dihydrofolate, protect dihydrofolate reductase against modification by pCMB. In the presence of a saturating concentration of NADPH, the value of kcat for 7,8-dihydrofolate in the enzyme-catalysed reaction increased four-fold on modification of Cys-6, accompanied by a two-fold increase in Km for the modified enzyme. The utilization of the binding energy of a group to increase kcat rather than reduce Km implies that the full binding energy of the group is not realized in the formation of the enzyme-substrate complex, but is used to stabilize the enzyme-transition-state complex.
机译：研究了底物对对氯mercuribenzoate（pCMB）激活中国仓鼠二氢叶酸还原酶动力学的影响。基于在pCMB存在下底物反应的动力学方程，已经确定了游离酶和酶-底物二元和三元复合物的所有修饰动力学常数。本研究结果表明，pCMB对中国仓鼠二氢叶酸还原酶的修饰表现出单相动力学，并且酶活性和三级结构的变化在修饰过程中同时进行。两种底物NADPH和7,8-二氢叶酸均可保护二氢叶酸还原酶不受pCMB的修饰。在饱和浓度的NADPH存在下，酶催化反应中的7，8-二氢叶酸的kcat值在Cys-6修饰时增加了4倍，而修饰酶的Km增加了2倍。 。利用基团的结合能增加kcat而不是降低Km意味着该基团的完全结合能在酶-底物复合物的形成中没有实现，而是用于稳定酶-过渡态复合物。

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