Carbonic Anhydrase 4 (CA4) belongs to the alpha-carbonic anhydrase family. Alpha-carbonic anhydrase is a large family of zinc metalloenzymes that catalyze the reversible hydration of carbon dioxide. Carbonic anhydrase 4 is a glycosylphosphatidyl-inositol-anchored membrane isozyme expressed on the luminal surfaces of pulmonary (and certain other) capillaries and proximal renal tubules. Carbonic anhydrase 4 may stimulate the sodium/bicarbonate transporter activity of SLC4A4 that acts in pH homeostasis. It may have a role in inherited renal abnormalities of bicarbonate transport. Furthermore, Carbonic anhydrase 4 is essential for acid overload removal from the retina and retina epithelium and acid release in the choriocapillaris.