通过稳态测量估算酶-底物复合物的解离常数。对Km的pH独立性的解释。
Estimation of the dissociation constants of enzyme-substrate complexes from steady-state measurements. Interpretation of pH-independence of Km.
作者信息
Cornish-Bowden A
出版信息
Biochem J. 1976 Feb 1;153(2):455-61. doi: 10.1042/bj1530455.
Abstract
If the Michaelis constant of an enzyme-catalysed reaction is independent of pH under conditions where the catalytic constant varies with pH, it is equal to the thermodynamic dissociation constant of the enzyme-substrate complex. This is true for realistic mechanisms in which binding and catalytic steps, are clearly distinguished, as well as for the simpler mechanisms that have been considered previously. It is also true for a mechanism in which a bell-shaped pH profile for the catalytic constant results from a change of rate-limiting step with pH. The relaxation time for ionization of a typical group in unbuffered solutions at 25 degrees C is of the order of 0.1 ms at the longest, and is much shorter in buffered solutions. Thus ionizations in almost all enzyme mechanisms can properly be treated as equilibria, provided that ionization is not accompanied by a slow, compulsory change in conformation.
摘要
如果在催化常数随pH变化的条件下，酶催化反应的米氏常数与pH无关，那么它就等于酶-底物复合物的热力学解离常数。对于能清楚区分结合步骤和催化步骤的实际反应机制，以及之前考虑过的更简单的机制而言，都是如此。对于一种催化常数呈钟形pH曲线是由限速步骤随pH变化而导致的机制，也是如此。在25摄氏度的无缓冲溶液中，典型基团电离的弛豫时间最长约为0.1毫秒，而在缓冲溶液中则短得多。因此，几乎所有酶反应机制中的电离都可以适当地当作平衡来处理，前提是电离不伴随着构象的缓慢、强制变化。
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