Carbonic anhydrases (CAs) are ubiquitous metalloenzymes. In mammals, 16 α-CA isozymes or CA-related proteins have been described: five cytosolic forms (CA I, CA II, CA III, CA VII, and CA XIII), five membrane bound isozymes (CA IV, CA IX, CA XII, CA XIV, and CA XV), two mitochondrial forms (CA VA and CA VB), and a secreted CA isozyme (CA VI). CAs catalyze the conversion of CO2 to the bicarbonate ion and protons to mediate pH and CO2 homeostasis.