Zinc Solid-State NMR Spectroscopy of Human Carbonic Anhydrase:  Implications for the Enzymatic Mechanism

The pH dependence of the 67Zn solid-state nuclear magnetic resonance spectroscopy of human carbonic anhydrase (CAII) has been investigated to characterize the nature of the fourth ligand. CAII, through the Zn2+-bound hydroxide, catalyzes the deceptively simple reaction: CO2 + H2O ⇆ HCO3 - + H+. The accepted mechanism for CAII would predict that water would be bound to the Zn2+ at pH 5 and hydroxide would be bound at pH 8.5. The measured values for the electric field gradient (EFG) or quadrupole coupling constant (Cq) for CAII are independent of pH within the limits of the experimental error, i.e., 9.8 ± 0.2 MHz. The EFG interaction has been predicted by ab initio electronic structure calculations for water and hydroxide bound to the zinc, including various levels of hydrogen bonding. After comparing the predicted Cq's with the experimental values, we conclude that the species present from pH 5−8.5 is the hydroxide form. The NMR data presented here is not consistent with the accepted mechanism for CAII. We show that the NMR data is consistent with an alternative mechanism of CAII.