Carbonic anhydrase alpha, isozymes I, II, and III and XIII. Carbonic anhydrases (CAs) are zinc-containing enzymes that catalyze the reversible hydration of carbon dioxide in a two-step mechanism: a nucleophilic attack of a zinc-bound hydroxide ion on carbon dioxide, followed by the regeneration of the active site by ionization of the zinc-bound water molecule and removal of a proton from the active site. They are ubiquitous enzymes involved in fundamental processes like photosynthesis, respiration, pH homeostasis and ion transport. Most alpha CAs are monomeric enzymes. The zinc ion is complexed by three histidines. This vertebrate subgroup comprises isozymes I, II, and III, which are cytoplasmic enzymes. CA I, for example, is expressed in erythrocyes of many vertebrates; CA II is the most active cytosolic isozyme; while it is being expressed nearly ubiquitously, it comprises 95% of the renal carbonic anhydrase and is required for renal acidification; CA III has been implicated in protection from the damaging effect of oxidizing agents in hepatocytes. CAXIII may play important physiological roles in several organs.