Appleton D W, Sarkar B
Proc Natl Acad Sci U S A. 1974 May;71(5):1686-90. doi: 10.1073/pnas.71.5.1686.
The catalytic activity of carbonic anhydrase (EC 4.2.1.1) is linked to the ionization of a group in close proximity to the essential zinc ion. Studies have been undertaken to delineate the ionizations germane to the active-site chelate system. Several imidazole ligand systems were studied in order to approach a representative chelate. The simplest involved the complexation of Zn(II) by imidazole and by N-methylimidazole. As well, two bidentate systems, Zn(II)-4,4'-bis-imidazoylmethane and Co(II)-cyclic-L-histidyl-L-histidine were investigated. It was found that in a species containing metal-bound water and imidazole coordinated by means of the pyridinium nitrogen, the most acidic group was the pyrrole N-H in the imidazole ring. By the use of N-methylimidazole, the pK(a) of a metal-bound water molecule in a tri-imidazole ligand field was found to be 9.1. Noting the preference for labilization of the pyrrole hydrogen, the catalytic features of carbonic anhydrase are reexamined assuming that the pK(enz) is associated with the N-H ionization, and not with the ionization of metal-bound water.
碳酸酐酶（EC 4.2.1.1）的催化活性与紧邻必需锌离子的一个基团的电离有关。已开展研究以描绘与活性位点螯合体系相关的电离情况。为了接近具有代表性的螯合物，研究了几种咪唑配体体系。最简单的是锌（II）与咪唑以及N - 甲基咪唑的络合。此外，还研究了两种双齿体系，即锌（II） - 4,4'-双咪唑基甲烷和钴（II） - 环 - L - 组氨酰 - L - 组氨酸。研究发现，在一个含有与金属结合的水以及通过吡啶鎓氮配位的咪唑的物种中，最酸性的基团是咪唑环中的吡咯N - H。通过使用N - 甲基咪唑，发现在三咪唑配体场中与金属结合的水分子的pK(a)为9.1。注意到对吡咯氢的去稳定化的偏好，假设pK(enz)与N - H电离相关，而非与金属结合水的电离相关，对碳酸酐酶的催化特性进行了重新审视。
