Catalyzes the reversible hydration of carbon dioxide into bicarbonate and protons and thus is essential to maintaining intracellular and extracellular pH. May stimulate the sodium/bicarbonate transporter activity of SLC4A4 that acts in pH homeostasis. It is essential for acid overload removal from the retina and retina epithelium, and acid release in the choriocapillaris in the choroid. H(+) + hydrogencarbonate = CO2 + H2O Inhibited by acetazolamide. Interacts with SLC4A4. Belongs to the alpha-carbonic anhydrase family.