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. 1974 May;71(5):1686–1690. doi: 10.1073/pnas.71.5.1686

The Activity-Related Ionization in Carbonic Anhydrase

David W Appleton 1,2, Bibudhendra Sarkar 1,2
PMCID: PMC388303  PMID: 4209558

Abstract

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 pKa 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 pKenz is associated with the N-H ionization, and not with the ionization of metal-bound water.

Keywords: metalloenzymes, enzyme mechanism, imidazole chelates

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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