Carbonic anhydrase
The carbonic anhydrases (or carbonate dehydratases) (EC 4.2.1.1) form a family of enzymes that catalyze the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid (i.e. bicarbonate and hydrogen ions). The active site of most carbonic anhydrases contains a zinc ion. They are therefore classified as metalloenzymes. The enzyme maintains acid-base balance and helps transport carbon dioxide.
| Carbonic anhydrase | |||||||||
|---|---|---|---|---|---|---|---|---|---|
Human carbonic anhydrase II with bound zinc and carbon dioxide. PDB: 6LUX | |||||||||
| Identifiers | |||||||||
| EC no. | 4.2.1.1 | ||||||||
| CAS no. | 9001-03-0 | ||||||||
| Databases | |||||||||
| IntEnz | IntEnz view | ||||||||
| BRENDA | BRENDA entry | ||||||||
| ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDB PDBe PDBsum | ||||||||
| Gene Ontology | AmiGO / QuickGO | ||||||||
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| Eukaryotic-type carbonic anhydrase | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
| Symbol | Carb_anhydrase | ||||||||
| Pfam | PF00194 | ||||||||
| InterPro | IPR001148 | ||||||||
| PROSITE | PDOC00146 | ||||||||
| SCOP2 | 1can / SCOPe / SUPFAM | ||||||||
| Membranome | 333 | ||||||||
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Carbonic anhydrase helps maintain acid–base homeostasis, regulate pH, and fluid balance. Depending on its location, the role of the enzyme changes slightly. For example, carbonic anhydrase produces acid in the stomach lining. In the kidney, the control of bicarbonate ions influences the water content of the cell. The control of bicarbonate ions also influences the water content in the eyes. Inhibitors of carbonic anhydrase are used to treat glaucoma, the excessive build-up of water in the eyes. Blocking this enzyme shifts the fluid balance in the eyes to reduce fluid build-up thereby relieving pressure.
Carbonic anhydrase is critical to hemoglobin function via the Bohr effect which catalyzes the hydration of carbon dioxide to form carbonic acid and rapidly dissociate into water. Essentially an increase in carbon dioxide results in lowered blood pH, which lowers oxygen-hemoglobin binding. The opposite is true where a decrease in the concentration of carbon dioxide raises the blood pH which raises the rate of oxygen-hemoglobin binding. Relating the Bohr effect to carbonic anhydrase is simple: carbonic anhydrase speeds up the reaction of carbon dioxide reacting with water to produce hydrogen ions (protons) and bicarbonate ions.
To describe equilibrium in the carbonic anhydrase reaction, Le Chatelier's principle is used. Most tissue is more acidic than lung tissue because carbon dioxide is produced by cellular respiration in these tissues, where it reacts with water to produce protons and bicarbonate. Because the carbon dioxide concentration is higher, the equilibrium shifts to the right, to the bicarbonate side. The opposite is seen in the lungs, where carbon dioxide is being released, reducing its concentration, so the equilibrium shifts to the left, favoring carbon dioxide production.