| Carbonic Anhydrase |
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Information AboutCarbonic Anhydrase |
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Carbonic anhydrase (carbonate dehydratase) is a family of of most carbonic anhydrases contains a Zinc ion. STRUCTURE AND FUNCTION OF CA 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. Plant s 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 Diatom s 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. Reaction catalyzed by carbonic anhydrase: : Carbonic acid has a pKa of around 6.36 (the exact value depends on the medium) so at pH 7 a small percentage of the bicarbonate is protonated. See Carbonic Acid for details concerning the equilibria HCO3- + H+ H2CO3 and H2CO3 CO2 + H2O (in Tissue s - high CO2 concentration) The reaction Rate of carbonic anhydrase is one of the fastest of all enzymes, and its rate is typically limited by the Diffusion rate of its Substrate s. The reverse reaction is also relatively slow (kinetics in the 15 second range), which is why a soda pop doesn't instantly degas when opening the can/bottle, but will rapidly degas in your mouth when carbonic anhydrase is added with your saliva. : (in Lungs and Nephrons of the Kidney - low CO2 concentration, in plant cells) MECHANISM residues (in pink) and a Hydroxide group (red and white) coordinating the Zinc ion (purple). From .]] A Zinc Prosthetic Group in the enzyme is coordinated in three positions by Histidine Side Chain s. The fourth coordination position is occupied by water. This causes polarisation of the hydrogen-oxygen bond, making the oxygen slightly more negative, thereby weakening it. A fourth histidine is placed close to the substrate of water and accepts a Proton , in an example of general acid-general base catalysis. This leaves a Hydroxide attached to the zinc. The active site also contains specificity pocket for carbon dioxide, bringing it close to the hydroxide group. This allows the electron rich hydroxide to attack the carbon dioxide, forming bicarbonate. CA FAMILIES Zinc ion visible at center. From .]] There are at least five distinct CA families (α, β, γ, δ and ε). These families have no significant Amino Acid Sequence similarity and in most cases are thought to be an example of Convergent Evolution . α-CA The CA enzymes found in Mammal s are divided into four broad subgroups:
β-CA Most Prokaryotic and plant Chloroplast CAs belong to the beta family. Two Signature Pattern s for this family have been identified:
γ-CA The gamma class of CAs come from Methane-producing Bacteria that grow in hot springs. δ-CA The delta class of CAs has been described in Diatom s. The distinction of this class of CA has recentlySawaya MR, Cannon GC, Heinhorst S, Tanaka S, Williams EB, Yeates TO, Kerfeld CA. 2006. The structure of beta-carbonic anhydrase from the carboxysomal shell reveals a distinct subclass with one active site for the price of two . J Biol Chem. 281(11):7546-55 come into question, however. ε-CA The epsilon class of CAs occurs exclusively in has since diverged considerably. PHARMACOLOGICAL AGENTS AFFECTING CA EXTERNAL LINK REFERENCES |
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