Hydration of carbon dioxide by carbonic anhydrase: internal proton transfer of Zn2+-bound HCO3-

Proton transfer within HCO3- has been examined under various conditions through molecular orbital methods: partial retention of diatomic differential overlap and 4-31G self-consistent field programs. These conditions include the absence or presence of Zn2+, Zn2+(NH3)3, or a water ligand on Zn2+. In addition, 4-31G+ and some MP2/4-31G results are obtained. The use of Be2+ to simulate Zn2+ reproduces reaction pathways and energy barriers, except for marginal cases. The barrier of 35.6 kcal/mol for direct internal proton transfer is reduced to 3.5 kcal/mol when one water molecule, not bound to Zn2+, is included for proton relay and to 1.4 kcal/mol when two such water molecules are included. In the enzyme, either Thr-199 or solvent molecules could perform this relay function. Our results favor this facilitated proton transfer over a mechanism in which Zn2+-bound OH- attacks CO2, a bidentate intermediate forms, and the OH moiety of the resulting HCO3- dissociates from Zn2+, thus leaving one of the oxygens of the original CO2 as a ligand to Zn2+.