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首页> 外文期刊>The 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：对氯巯基苯甲酸酯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修饰中国仓鼠二氢叶酸还原酶表现出单相动力学，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免受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，而在修饰酶的Km上增加了2倍。利用基团的结合能来增加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并未实现，而是用于稳定酶-过渡态复合物。

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