Carbonic anhydrase II or Carbonic anhydrase IV (CA II (for Carbonic anhydrase II), CA IV (for Carbonic anhydrase IV))

Carbonic anhydrase II is a highly active, cytosolic zinc metalloenzyme primarily found in erythrocytes, renal tubules, and many other tissues, catalyzing the reversible hydration of carbon dioxide to form bicarbonate and protons, which is fundamental for acid–base homeostasis, CO₂ transport, and various physiological processes. Carbonic anhydrase IV is a related but distinct membrane-bound isozyme, tethered to cell surfaces by a glycosylphosphatidylinositol (GPI) anchor, with significant activity in kidneys and capillaries, contributing to extracellular acidification, pH regulation, and bicarbonate reabsorption; both CA II and IV have critical structural features including a zinc-dependent active site, with catalysis near the diffusion-controlled limit. Both are clinically important therapeutic targets for conditions such as glaucoma and metabolic disorders, with several approved drugs acting as inhibitors of their enzymatic activity. Their functions and tissue distribution differ, so precise identification of the isoform is important in both research and therapy.

Other names

CA2 (gene symbol for Carbonic anhydrase II)CA4 (gene symbol for Carbonic anhydrase IV)Carbonic anhydrase IICarbonic anhydrase IVCarbonate dehydratase II (II)Carbonate dehydratase IV (IV)HCA II, HCA IV (for human isozymes)EC 4.2.1.1 (Enzyme Commission number for both isozymes)These names may appear with or without the "carbonic" prefix and sometimes simply as "CA II" or "CA IV"

Competitive inhibition of the zinc-dependent hydration/dehydration reaction of CO₂ to bicarbonate, usually by binding to the active site via sulfonamide or similar functional groups. Drugs decrease catalytic activity, reducing formation of aqueous humor (eye), urine acidification, or seizure susceptibility (brain).