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：1973
卷号：70
期号：9
页码：2505-2508
DOI：10.1073/pnas.70.9.2505
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：It is shown that an "inverse" relationship between the pH dependencies of the rates of hydration of CO2 and dehydration of HCO3- by carbonic anhydrase (EC 4.2.1.1 ) is a direct consequence of the thermodynamic equilibrium between CO2 and HCO3- and independent of any assumptions about the catalytic mechanism. It is further shown that proposed mechanisms for carbonic anhydrase involving HCO3- as the substrate in the dehydration reaction and a proton transfer reaction, EH+ {rightleftharpoons} E + H+, as an obligatory step during catalysis obey the rule of microscopic reversibility. This includes mechanisms in which the proton dissociation is from a zinc-coordinated water molecule. Such mechanisms can be in accord with the observed rapid turnover rates of the enzyme, since rapid proton exchange can occur with the buffer components, EH+ + B {rightleftharpoons} E + BH+. Mechanisms in which H2CO3 is the substrate in dehydration avoid the proton-transfer step, but require that H2CO3 combines with enzyme more rapidly than in a diffusion-controlled reaction. Physico-chemical evidence for and against a zinc-hydroxide mechanism is discussed.
关键词：metalloenzymes ; enzyme mechanism ; hydration of CO2
