Carbonic Anhydrase

Carbonic Anhydrase

There are three recognised classes of carbonic anhydrase enzymes, alpha, beta and gamma, which have no significant sequence identity, and have structurally distinct overall folds.  Yet, despite their structural differences, the active sites of all three classes function with a single zinc atom that is essential for catalysis.  These enzymes are of ancient origin, and appear to have evolved independently from one another, thereby providing an excellent example of convergent evolution.  The three classes have differing distributions in different organisms: in mammals, all the isozymes so far discovered belong to the alpha-class; plants produce mainly the beta-class; prokaryotes encode all three classes of enzyme, with the beta- and gamma-classes predominating.

An Overview of Carbonic Anhydrase

Carbonic Anhydrase (CA) is a zinc-containing metalloenzyme. So far, at least eight isozymes have been found in mammals. Their structures, distribution, and properties are related to the secretion of H- and bicarbonate in various epithelial cells. By catalyzing the hydration of CO2 and some lipids and aldehydes, they participate in a variety of ion exchange and maintain the homeostasis of the body. Carbonic anhydrase is one of the main protein components of red blood cells and is second only to hemoglobin in erythrocytes. Carbonic anhydrase is found in the red blood cells of vertebrates and in various tissues of many animals as well as in the leaves of plants. It can rapid convert carbonated and bicarbonate ions in red blood cells and act on the secretion of hydrochloric acid in the stomach, generally speaking, by regulating the pH of body fluid. In addition, it is related to photosynthesis of plants.

Inhibition of Carbonic Anhydrase

The main inhibitor of CA is sulfanilamide. The inhibition of CA by surfactants such as DDT may be related to the facilitation of group dissociation. Different CAs have different sensitivities to sulfanilamide inhibitors. Hydrophobicity and drug affinity are related to the phenyl filling of phenylalanine side chain at CAⅢ-198, resulting in low catalysis and low sensitivity. In addition, surfactants can inhibit the formation of polymer at high concentration of CA, thus reducing the competitive inhibition of CA renaturation protein formation by the polymer and promoting protein refolding. Finally, the active concentration range of CA is enlarged during the start-up stage.

Carbonic Anhydrase and diseases

CA catalyzes the production of HCO3 by CO2 and H2O in ciliary epithelial cells, which is secreted into aqueous humor through the membrane of the chamber. Because the liquid in aqueous humor should keep the electric neutral Na to increase the secretion of aqueous humor, at the same time, it moves Cl- to aqueous humor, thus causing the aqueous humor to form hyperosmotic pressure. Therefore, it promotes the flow of H2O to aqueous humor and maintains aqueous humor balance and normal pH value. However, the intraocular pressure of glaucoma patients is increased due to the lack of reflux of aqueous humor. CA inhibitor (CAIs) can inhibit the activity of CA, reduce the production of HCO3 and decrease intraocular pressure, which is mainly used in the treatment of glaucoma and intraocular pressure.

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