Carbonic anhydrase
Encyclopedia : C : CA : CAR : Carbonic anhydrase
| Carbonic anhydrase | |
|---|---|
| Systematic name | carbonate hydro-lyase |
| Other names | carbonate dehydratase; carbonate anhydrase; carbonic acid anhydrase |
| EC number | |
| CAS number | 9001-03-0 |
| [Chemical infoboxDisclaimer and references] | |
Carbonic anhydrase (carbonate dehydratase) is a family of metalloenzymes (enzymes that contain one or more metal atoms as a functional component of the enzyme) that catalyze the rapid interconversion of carbon dioxide and water into carbonic acid, protons, and bicarbonate ions. This reaction occurs spontaneously in aqueous solution, but does so slowly. Carbonic anhydrase can increase the rate of the reaction about a million-fold. The active site of most carbonic anhydrases contains a zinc ion.
Several forms of carbonic anhydrase occur in nature. In the best studied α-carbonic anhydrase form present in animals, this zinc ion is coordinated by the imidazole rings of 3 histidine residues, His94, His96 and His119. The primary function of the enzyme in animals is to interconvert carbon dioxide and bicarbonate to maintain acid-base balance in blood and other tissues and to help transport carbon dioxide out of tissues. Plants contain a different form called β-carbonic anhydrase which is an evolutionarily distinct enzyme but participates in the same reaction and also uses a zinc ion in its active site. In plants, carbonic anhydrase helps raise the concentration of CO2 within the chloroplast to increase the carboxylation rate of the enzyme Rubisco. This is the reaction which integrates CO2 into organic carbon sugars during photosynthesis, and can only use the CO2 form of carbon, not carbonic acid nor bicarbonate.
In 2000, a [cadmium containing carbonic anhydrase] was found to be expressed in marine diatoms during zinc limitation. In the open ocean, zinc is often in such low concentrations that it can limit the growth of phytoplankton like diatoms, thus a carbonic anhydrase using a different metal ion would be beneficial in these environments. Before this discovery, cadmium has generally been thought of as a very toxic heavy metal without biological function. As of 2005, this peculiar carbonic anhydrase form hosts the only known beneficial cadmium-dependent biological reaction.
Reactions catalyzed by carbonic anhydrase:
There are at least five evolutionarily distinct CA families (α, β, γ, δ and ε). These families have no significant sequence identity and are thought to be an example of convergent evolution.
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The rate of reaction of carbonic anhydrase is limited by the diffusion rate of its substrates, meaning carbonic anhydrase is part of an elite group of enzymes catalysing exceptionally rapid reactions.
Acetazolamide (trade name Diamox®) is an inhibitor of carbonic anhydrase. It is used for glaucoma, epilepsy (rarely), benign intracranial hypertension, and altitude sickness.CA families
α-CA
The CA enzymes found in mammals are divided into four broad subgroups:
β-CA
Most prokaryotic and plant chloroplast CAs belong to the beta family.
Two signature patterns for this family have been identified:
γ-CA
The gamma class of CAs come from methane-producing bacteria that grow in hot springs.External link
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