Carbonic anhydrase alpha, isozymes VI, IX, XII and XIV. 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. There are three evolutionary distinct groups - alpha, beta and gamma carbonic anhydrases - which show no significant sequence identity or structural similarity. Alpha CAs are mostly monomeric enzymes. The zinc ion is complexed by three histidine residues. This sub-family comprises the secreted CA VI, which is found in saliva, for example, and the membrane proteins CA IX, XII, and XIV.
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