Nuclear magnetic relaxation dispersion in protein solutions. IV. Proton relaxation at the active site of carbonic anhydrase.

摘要: Abstract Bovine and human B erythrocyte carbonic anhydrases substituted with Co2+ cause an enhancement of the nuclear magnetic relaxation rate solvent water protons (T1-1) at high pH that is decreased by addition anhydrase inhibitors such as azide Ethoxzolamide. The part T1-1 which can be inhibited Ethoxzolamide due to exchangeable located active site enzyme. This inhibitable pH-dependent a pK 7.0 ± 0.2 for bovine cobalt enzyme 8.2 From field dependence T1-1, correlation time dipolar interaction proton electronic spin 10-11 sec, proton-cobalt distance 2.5 2.9 A, residence time, τm, small compared 10-5 sec are calculated. We conclude from contributes either on hydroxide ion bound or molecule 1 hydrogen bonded nearby residue. In contrast similarity enzymes, there non-inhibitable, pH-independent contribution resulting substitution, relatively large very enzyme, not understood. contributions (diamagnetic) native zinc enzymes have also been measured. As in case apotransferrin (Koenig, S. H., Schillinger, W. E., J. Biol. Chem., 244, 3283 (1969)), results agree qualitatively but quantitatively theory.