文章基本信息

标题：Catalysis by acetylcholinesterase: evidence that the rate-limiting step for acylation with certain substrates precedes general acid-base catalysis
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作者：T L Rosenberry
期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：1975
卷号：72
期号：10
页码：3834-3838
DOI：10.1073/pnas.72.10.3834
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Inferences about the catalytic mechanism of acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7 ) are frequently made on the basis of a presumed analogy with chymotrypsin, EC 3.4.21.1 . Although both enzymes are serine hydrolases, several differences in the steady-state kinetic properties of the two have been observed. In this report particular attention is focused on the second-order reaction constant, kcat/Kapp. While the reported pH dependence and deuterium oxide isotope effect associated with this parameter for chymotrypsin are generally consistent with simple models involving rate-limiting general acid-base catalysis, this study finds a more complicated situation with acetylcholinesterase. The apparent pKa of kcat/Kapp for acetylcholinesterase varies between 5.5 and 6.3 for neutral substrates and involves nonlinear inhibition by [H+]. Deuterium oxide isotope effects for kcat/Kapp range from 1.1 for acetylcholine to 1.9 for p-nitrophenyl acetate. The bimolecular reaction rate appears rate-limiting for acetylcholine at low concentrations, while a rate-limiting induced-fit step is proposed to account for apparent pKa values and low deuterium oxide isotope effects associated with low concentrations of phenyl acetate and isoamyl acetate.