¹¹³Cd nuclear magnetic resonance of mammalian erythrocyte carbonic anhydrases

¹¹³Cd NMR of the ¹¹³Cd(II)-substituted zinc metalloenzymes, human and bovine erythrocyte carbonic anhydrases, is reported. The high activity isozymes, human erythrocyte carbonic anhydrase, isozyme C (HCAC) and bovine erythrocyte carbonic anhydrase, isozyme B (BCAB), are characterized by relatively narrow ¹¹³Cd resonances around 220 ppm to lowfield of Cd(ClO₄)₂. These resonances are relatively invariant between pH 5.5 and 10. In contrast, the low activity isozyme, human carbonic anhydrase B (HCAB), shows no detectable or extremely broad ¹¹³Cd resonance at all pH values in the absence of anion inhibitors. Binding of halide anions, Cl^- or I^-, to the active site cadmium ion in HCAB results in the appearance of a ¹¹³Cd resonance at 230 to 240 ppm. The initial absence of resonance in ¹¹³Cd(II)HCAB appears to reflect an intermediate rate (10³ to 10⁴ s^-1) of chemical exchange of ligands at the active site ¹¹³Cd ion. This exchange may modulate the chemical shift by ~100 ppm. The exchanging ligand is most likely coordinated solvent. Exchange is interrupted by substitution of the monodentate anions which form complexes of long lifetimes. In contrast, the narrow resonance observed for the high activity, isozymes appears to reflect an initially rapid (>10⁵ s^-1) solvent ligand exchange rate, a conclusion supported by the temperature dependence of the linewidth of the ¹¹³Cd resonance in BCAB. The extreme sensitivity of the active site ¹¹³Cd ion's chemical shift to minor changes in the coordination sphere is shown by the isolation of the bovine isozyme A for which the resonance of the ¹¹³Cd derivative undergoes an 88-ppm shift on I^- binding. These findings are discussed as they relate to the currently proposed mechanisms of action for carbonic anhydrases.