Catalyzes the reversible hydration of carbon dioxide (By similarity). Involved in the regulation of fluid secretion into the anterior chamber of the eye (By similarity). Essential for bone resorption and osteoclast differentiation (By similarity). Contributes to intracellular pH regulation in the duodenal upper villous epithelium during proton-coupled peptide absorption (By similarity). Stimulates the chloride-bicarbonate exchange activity of SLC26A6 (By similarity). H(+) + hydrogencarbonate = CO2 + H2O Inhibited by acetazolamide. Interacts with SLC4A4. Interaction with SLC4A7 regulates SLC4A7 transporter activity (By similarity). Colocalized with SLC26A6 at the surface of the cell membrane in order to form a bicarbonate transport metabolon. Displaced from the cytosolic surface of the cell membrane by PKC in phorbol myristate acetate (PMA)-induced cells. One minor and two major forms were isolated chromatographically. Belongs to the alpha-carbonic anhydrase family.