Catalyzes the reversible hydration of carbon dioxide into bicarbonate and protons and thus is essential to maintaining intracellular and extracellular pH (PubMed:15563508, PubMed:17652713, PubMed:7625839, PubMed:16807956, PubMed:16686544, PubMed:17705204, PubMed:17127057, PubMed:17314045, PubMed:19186056, PubMed:19206230, PubMed:18618712). May stimulate the sodium/bicarbonate transporter activity of SLC4A4 that acts in pH homeostasis (PubMed:15563508). It is essential for acid overload removal from the retina and retina epithelium, and acid release in the choriocapillaris in the choroid (PubMed:15563508). H(+) + hydrogencarbonate = CO2 + H2O Activated by histamine, L-adrenaline, D-phenylalanine, L- and D-histidine. Inhibited by coumarins, saccharin, sulfonamide derivatives such as acetazolamide and Foscarnet (phosphonoformate trisodium salt). Interacts with SLC4A4. Expressed in the endothelium of the choriocapillaris in eyes (at protein level). Not expressed in the retinal epithelium at detectable levels. The disease is caused by variants affecting the gene represented in this entry. Defective acid overload removal from retina and retinal epithelium, due to mutant CA4, is responsible for photoreceptor degeneration, indicating that impaired pH homeostasis is the most likely cause underlying the RP17 phenotype. Belongs to the alpha-carbonic anhydrase family.