Carbonic anhydrase I (CA1, Khalifah 1971, Simonsson et al. 1982, Ren and Lindskog 1992), carbonic anyhydrase II (CA2, Tibell et al. 1984, Jones and Shaw 1983, Pesando 1975, Ghannam et al. 1986), carbonic anhydrase III (CA3, Carter et al. 1979, Tu et al. 1990, Tu et al. 1994, Tu et al. 1998, Silverman et al. 1993), carbonic anhydrase VII (CA7, Bootorabi et al. 2010, Gitto et al. 2010) hydrate carbon dioxide to yield bicarbonate and a proton. Carbonic anhydrase deprotonates water to yield a zinc-hydroxyl group and a proton which is transferred to external buffer molecules via histidine or glutamate residues in carbonic anhydrase. The hydroxyl group reacts with carbon dioxide in the active site to yield bicarbonate. A water molecule displaces the bicarbonate and the reaction cycle begins again (reviewed in Lindskog 1997). Depending on the concentrations of reactants the reaction is reversible.
CA2 and CA7 have high catalytic activity, CA1 has low activity (10% of the activity of CA2), and CA3 has very low activity (1% of the activity of CA2). CA1 and CA2 are found in erythrocytes. CA2 is also found in kidney, lung, and white muscle where it facilitates diffusion of carbon dioxide. CA3 is found in red muscle where it participates in resistance against oxidative stress.