Carbonic anhydrase alpha, isozyme IX. 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 strictly monomeric enzymes. The zinc ion is complexed by three histidine residues. This sub-family comprises the membrane protein CA IX. CA IX is functionally implicated in tumor growth and survival. CA IX is mainly present in solid tumors and its expression in normal tissues is limited to the mucosa of alimentary tract. CA IX is a transmembrane protein with two extracellular domains: carbonic anhydrase and, a proteoglycan-like segment mediating cell-cell adhesion. There is evidence for an involvement of the MAPK pathway in the regulation of CA9 expression.