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113Cd-Substituted human and bovine erythrocyte carbonic anhydrases have been studied by 113Cd NMR as a function of pH and bicarbonate concentration. Plots of chemical shift versus pH give sigmoidal titration curves in the pH range of the study, 6.9 to 10.5. The pKa values vary from 9.2 to 9.7, which correlates well with available activity profiles for the Cd-enzymes. Because the samples contain no buffers and no anions other than hydroxide, the results point to the existence of high and low pH forms of the enzymes in rapid exchange and differing in inner sphere coordination. When bicarbonate is added to the samples, upfield shifts are produced which eventually level off. Only a single CN- binds to the metal for all three enzymes. These observations are best explained by a rapid exchange among three species in which the open coordination site of the metal ion is occupied by hydroxide, water, or bicarbonate, as in the scheme: E--OH- in equilibrium or formed from E--H2O in equilibrium or formed from E--HCO-3.
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