Carbonic anhydrase I (CA1, Khalifah 1971, Pesando 1975, Simonsson et al. 1982, Ren & Lindskog 1992) and carbonic anhydrase II (CA2, Tibell et al. 1984, Jones & Shaw 1983, Ghannam et al. 1986) hydrate carbon dioxide (CO2) to yield bicarbonate (HCO3-) and a proton (H+). During the reaction a hydroxyl group bound by the zinc ion (Zn2+) attacks the CO2 molecule in the active site to directly form HCO3- (reviewed in Lindskog 1997). The HCO3- is displaced by water, which is then deprotonated by a histidine residue to recreate the Zn2+:hydroxyl group. Depending on the concentrations of reactants the reaction is reversible.