Chemical modification of carbonic anhydrase II with acrolein

We have reacted acrolein with human carbonic anhydrase II using conditions reported to result in maximal formylethylation of exposed histidine and lysine residues (Pocker, Y., and Janji?, N. (1988) J. Biol. Chem. 263, 6169-6176). Pocker and Janji? proposed that the decrease by 95-98% in the steady-state turnover number for the hydration of CO2 caused by this chemical modification is due predominantly to the alkylation of one residue, the imidazole side chain of histidine 64. We measured the rate of 18O exchange between CO2 and water catalyzed by these enzymes at chemical equilibrium using membrane inlet mass spectrometry. The catalyzed rate of interconversion of CO2 and HCO3- at chemical equilibrium was the same for the acrolein-modified and the unmodified carbonic anhydrases, but the rate of release of 18O-labeled water from the active site had decreased by as much as 85% for the acrolein-modified enzyme. The 18O-exchange kinetics catalyzed by the acrolein-modified carbonic anhydrase II was similar to that catalyzed by a mutant human carbonic anhydrase II in which histidine at residue 64 was replaced with alanine. Moreover, modification of this mutant carbonic anhydrase II with acrolein did not alter to a significant extent its 18O-exchange pattern. These results support the proposal of Pocker and Janji? and the suggested role of histidine 64 in carbonic anhydrase II as a proton shuttle residue that transfers a proton from zinc-bound water to buffer in solution.