Carbonic anhydrase

The carbonic anhydrases (or carbonate dehydratases) form a family of enzymes that catalyze the rapid conversion of carbon dioxide to bicarbonate and protons, a reaction that occurs rather slowly in the absence of a catalyst.cite journal | author = Badger MR, Price GD | title = The role of carbonic anhydrase in photosynthesis | journal = Annu. Rev. Plant Physiol. Plant Mol. Bio. | volume = 45 | issue = | pages = 369–392 | year = 1994| doi = 10.1146/annurev.pp.45.060194.002101 ] The active site of most carbonic anhydrases contains a zinc ion; they are therefore classified as metalloenzymes.

tructure and function of carbonic anhydrase

Several forms of carbonic anhydrase occur in nature. In the best-studied "α-carbonic anhydrase" form present in animals, the 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.

There exist at least 14 different isoforms in mammals. Plants contain a different form called "β-carbonic anhydrase", which, from an evolutionary standpoint, is a 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 in order to increase the carboxylation rate of the enzyme RuBisCO. This is the reaction that integrates CO₂ into organic carbon sugars during photosynthesis, and can use only the CO₂ form of carbon, not carbonic acid or bicarbonate.

In 2000, a [http://www-ssrl.slac.stanford.edu/research/highlights_archive/cd-ca.html 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.

The reaction catalyzed by carbonic anhydrase is::$m\; CO\_2\; +\; H\_2O\; ightarrow^\{Carbonic\; anhydrase\}\; HCO\_3^-\; +\; H^+$(in tissues - high CO₂ concentration) [Carbonic acid has a pK_a 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⁺$ightleftharpoons$ H₂CO₃ and H₂CO₃$ightleftharpoons$ CO₂ + H₂O]

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 substrates.Typical catalytic rates of the different forms of this enzyme ranging between 10⁴ and 10⁶ reactions per second.cite journal | author = Lindskog S | title = Structure and mechanism of carbonic anhydrase | journal = Pharmacol. Ther. | volume = 74 | issue = 1 | pages = 1–20 | year = 1997 | pmid = 9336012 | doi = 10.1016/S0163-7258(96)00198-2 ]

The reverse reaction is also relatively slow (kinetics in the 15-second range), which is why a carbonated drink does not instantly degas when opening the container, but will rapidly degas in one's mouth when carbonic anhydrase is added with saliva. :$m\; HCO\_3^-\; +\; H^+\; ightarrow\; H\_2CO\_3\; ightarrow\; CO\_2\; +\; H\_2O$ (in lungs and nephrons of the kidney - low CO₂ concentration, in plant cells)

Mechanism

A zinc prosthetic group in the enzyme is coordinated in three positions by histidine side chains. 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

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. The α-CAs are found in humans.

α-CA

The CA enzymes found in mammals are divided into four broad subgroupscite journal | author = Breton S | title = The cellular physiology of carbonic anhydrases | journal = JOP | volume = 2 | issue = 4 Suppl | pages = 159–64 | year = 2001 | pmid = 11875253 | doi = | issn = | url = http://www.joplink.net/prev/200107/4.html] , which, in turn consist of several isoforms:
* the cytosolic CAs (CA-I, CA-II, CA-III, CA-VII and CA XIII) (gene2|CA1|1368, gene2|CA2|1373, gene2|CA3|1374, gene2|CA7|1381, gene2|CA13|14914)
* mitochondrial CAs (CA-VA and CA-VB) (gene|CA5A, gene|CA5B)
* secreted CAs (CA-VI) (gene2|CA6|1380)
* membrane-associated CAs (CA-IV, CA-IX, CA-XII, CA-XIV and CA-XV) (gene2|CA4|1375, gene2|CA9|1383, gene2|CA12|1371, gene2|CA14|1372)

There are three additional "acatalytic" CA isoforms (CA-VIII, CA-X, and CA-XI) (gene2|CA8|1382, gene2|CA10|1369, gene2|CA11|1370) whose functions remain unclear.cite journal | author = Lovejoy DA, Hewett-Emmett D, Porter CA, Cepoi D, Sheffield A, Vale WW, Tashian RE | title = Evolutionarily conserved, "acatalytic" carbonic anhydrase-related protein XI contains a sequence motif present in the neuropeptide sauvagine: the human CA-RP XI gene (CA11) is embedded between the secretor gene cluster and the DBP gene at 19q13.3 | journal = Genomics | volume = 54 | issue = 3 | pages = 484–93 | year = 1998 | pmid = 9878252 | doi = 10.1006/geno.1998.5585 ]

β-CA

Most prokaryotic and plant chloroplast CAs belong to the beta family.Two signature patterns for this family have been identified:
* C- [SA] -D-S-R- [LIVM] -x- [AP]
* [EQ] - [YF] -A- [LIVM] -x(2)- [LIVM] -x(4)- [LIVMF] (3)-x-G-H-x(2)-C-G

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 diatoms. The distinction of this class of CA has recentlycite journal | author = Sawaya MR, Cannon GC, Heinhorst S, Tanaka S, Williams EB, Yeates TO, Kerfeld CA | title = The structure of beta-carbonic anhydrase from the carboxysomal shell reveals a distinct subclass with one active site for the price of two | journal = J. Biol. Chem. | volume = 281 | issue = 11 | pages = 7546–55 | year = 2006 | pmid = 16407248 | doi = 10.1074/jbc.M510464200 ] come into question, however.

ε-CA

The epsilon class of CAs occurs exclusively in bacteria in a few chemolithotrophs and marine cyanobacteria that contain cso-carboxysomes.cite journal | author = So AK, Espie GS, Williams EB, Shively JM, Heinhorst S, Cannon GC | title = A novel evolutionary lineage of carbonic anhydrase (epsilon class) is a component of the carboxysome shell | journal = J. Bacteriol. | volume = 186 | issue = 3 | pages = 623–30 | year = 2004 | pmid = 14729686 | doi = 10.1128/JB.186.3.623-630.2004 ] Recent 3-dimensional analyses suggest that ε-CA bears some structural resemblance to β-CA, particularly near the metal ion site. Thus, the two forms may be distantly related, even though the underlying amino acid sequence has since diverged considerably.

Pharmacological agents affecting CA

:"See Carbonic anhydrase inhibitors"

External links

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References

9. Lyall V, Alam RI, Phan DQ, Ereso GL, Phan TH, Malik SA, Montrose MH, Chu S, Heck GL, Feldman GM, DeSimone JA.Decrease in rat taste receptor cell intracellular pH is the proximate stimulus in sour taste transduction.Am J Physiol Cell Physiol. 2001 Sep;281(3):C1005-13.