Hydration of CO2 by carbonic anhydrase: intramolecular proton transfer between Zn2+-bound H2O and histidine 64 in human carbonic anhydrase II.

The energy barrier for the intramolecular proton transfer between zinc-bound water and His 64 in the active site of human carbonic anhydrase II (HCA II) has been studied at the partial retention of diatomic differential overlap (PRDDO) level. The most important stabilizing factor for the intramolecular proton transfer is the zinc ion, which lowers the pKa of zinc-bound water and electrostatically repels the proton. The energy barrier of 127.5 kcal/mol for proton transfer between a water dimer is completely removed in the presence of the zinc ion. The zinc ligands, which donate electrons to the zinc ion, raise the barrier slightly to 34 kcal/mol for a 4-coordinated zinc complex including three imidazole ligands from His 94, His 96, and His 119 and to 54 kcal/mol for the 5-coordinated zinc complex including the fifth water ligand. A few model calculations indicate that these energy barriers are expected to be reduced to within experimental range (approximately 10 kcal/mol) when large basis set, correlation energies, and molecular dynamics are considered. The proton-transfer group, which functions as proton receiver in the intramolecular proton transfer, helps to attract the proton; and the partially ordered active site water molecules are important for proton relay function.