Proton nuclear magnetic resonance studies of histidines in horse carbonic anhydrase I

The 250 MHz 1H-NMR spectrum of horse carbonic anhydrase I (or B) (carbonate hydro-lyase, EC 4.2.1.1) was measured as a function of pH under various conditions. Eight resonances corresponding to histidine C-2 protons and four resonances corresponding to histidine C-4 protons were identified and assigned to individual histidine residues in the enzyme molecule. Substantial similarities between horse and human carbonic anhydrases I were demonstrated. While the human enzyme has three titratable histidine residues in its active site, the horse enzyme has only two, His-67 in the human enzyme being replaced by Gln in the horse enzyme (Jabusch, J.R., Bray, R.P. and Deutsch, H.F. (1980) J. Biol. Chem. 255, 9196-9204). This substitution has small but significant effects on the behaviour of the other active-site histidines. His-64 and His-200. However, His-64 has an anomalously low pKa value also in horse isoenzyme I, as previously observed in human isoenzyme I (Campbell, I.D., Lindskog, S. and White, A.I. (1974) J. Mol. Biol. 90, 469-489).