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| {{enzyme
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| | Name = Carbonate dehydratase
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| | EC_number = 4.2.1.1
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| | CAS_number = 9001-03-0
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| | IUBMB_EC_number = 4/2/1/1
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| | GO_code = 0004089
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| | image = Carbonic anhydrase.png
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| | width = 240px
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| | caption = [[Ribbon diagram]] of human carbonic anhydrase II, with zinc ion visible in the center
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| }}
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| {{Pfam_box
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| | Symbol = Carb_anhydrase
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| | Name = Eukaryotic-type carbonic anhydrase
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| | image =
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| | width =
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| | caption =
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| | Pfam= PF00194
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| | InterPro= IPR001148
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| | SMART=
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| | Prosite = PDOC00146
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| | SCOP = 1can
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| | TCDB =
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| | OPM family=
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| | OPM protein=
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| | PDB=
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| {{PDB3|1kop}}B:33-252 {{PDB3|1rj6}}B:22-278 {{PDB3|1rj5}}B:22-278
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| {{PDB3|1jcz}}A:32-289 {{PDB3|1jd0}}A:32-289 {{PDB3|1g6v}}A:5-253
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| {{PDB3|1v9e}}B:5-258 {{PDB3|1v9i}}C:5-258 {{PDB3|1cak}} :5-259
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| {{PDB3|1i9q}}A:5-259 {{PDB3|1yo0}}A:5-259 {{PDB3|1zsa}} :5-259
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| {{PDB3|1cnc}} :5-258 {{PDB3|1bnw}} :5-258 {{PDB3|1fql}}A:5-259
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| {{PDB3|1cim}} :5-258 {{PDB3|1h9n}} :5-259 {{PDB3|1g0e}}A:5-259
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| {{PDB3|1g46}}A:5-259 {{PDB3|1xev}}C:5-259 {{PDB3|1ydb}} :5-258
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| {{PDB3|1fsr}}B:5-259 {{PDB3|1cve}} :5-258 {{PDB3|1zsb}} :5-259
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| {{PDB3|1fr7}}A:5-259 {{PDB3|1bv3}}A:5-259 {{PDB3|1cnj}} :5-259
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| {{PDB3|1fsq}}B:5-259 {{PDB3|1cam}} :5-259 {{PDB3|1lzv}}A:5-259
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| {{PDB3|1if4}}A:5-259 {{PDB3|1t9n}}A:5-259 {{PDB3|1caj}} :5-259
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| {{PDB3|1f2w}}A:5-259 {{PDB3|2cba}} :5-259 {{PDB3|1bn1}} :5-259
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| {{PDB3|1cai}} :5-259 {{PDB3|1i9l}}A:5-259 {{PDB3|1g3z}}A:5-259
| |
| {{PDB3|1kwq}}A:5-259 {{PDB3|2cbe}} :5-259 {{PDB3|1raz}} :5-259
| |
| {{PDB3|1xpz}}A:5-259 {{PDB3|2abe}}A:5-259 {{PDB3|1ugg}} :5-259
| |
| {{PDB3|1rzd}} :5-259 {{PDB3|1te3}}X:5-259 {{PDB3|1dca}} :5-258
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| {{PDB3|1g52}}A:5-259 {{PDB3|1yda}} :5-258 {{PDB3|1can}} :5-259
| |
| {{PDB3|12ca}} :5-258 {{PDB3|1cvh}} :5-258 {{PDB3|1ca3}} :5-258
| |
| {{PDB3|1fqn}}A:5-259 {{PDB3|9ca2}} :5-258 {{PDB3|1if5}}A:5-259
| |
| {{PDB3|1h9q}} :5-258 {{PDB3|5ca2}} :5-258 {{PDB3|1heb}} :5-258
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| {{PDB3|1moo}}A:5-259 {{PDB3|4ca2}} :5-258 {{PDB3|1g45}}A:5-259
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| {{PDB3|1mua}} :5-258 {{PDB3|1lg6}}A:5-259 {{PDB3|1ca2}} :5-258
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| {{PDB3|1i91}}A:5-259 {{PDB3|1hva}} :5-258 {{PDB3|1ugc}} :5-259
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| {{PDB3|1xeg}}A:5-259 {{PDB3|1hca}} :5-258 {{PDB3|1avn}} :5-259
| |
| {{PDB3|1am6}} :5-259 {{PDB3|1fsn}}B:5-259 {{PDB3|1g1d}}A:5-259
| |
| {{PDB3|2cbc}} :5-259 {{PDB3|1fqr}}A:5-259 {{PDB3|1cng}} :5-259
| |
| {{PDB3|1g53}}A:5-259 {{PDB3|1ccu}} :5-258 {{PDB3|1teq}}X:5-259
| |
| {{PDB3|1caz}} :5-259 {{PDB3|1ccs}} :5-258 {{PDB3|1oq5}}A:5-259
| |
| {{PDB3|1bnq}} :5-258 {{PDB3|1bnu}} :5-258 {{PDB3|1cil}} :5-258
| |
| {{PDB3|1rze}} :5-259 {{PDB3|8ca2}} :5-258 {{PDB3|1ray}} :5-259
| |
| {{PDB3|1i8z}}A:5-259 {{PDB3|2cbd}} :5-259 {{PDB3|1eou}}A:5-259
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| {{PDB3|1okl}} :5-258 {{PDB3|1ze8}}A:5-259 {{PDB3|1g0f}}A:5-259
| |
| {{PDB3|4cac}} :5-258 {{PDB3|1cay}} :5-259 {{PDB3|1tg9}}A:5-259
| |
| {{PDB3|1hec}} :5-258 {{PDB3|1yo2}}A:5-259 {{PDB3|6ca2}} :5-258
| |
| {{PDB3|1okn}} :5-259 {{PDB3|1cvc}} :5-258 {{PDB3|1ttm}}A:5-259
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| {{PDB3|1cnb}} :5-258 {{PDB3|1th9}}A:5-259 {{PDB3|1lg5}}A:5-259
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| {{PDB3|1zsc}} :5-259 {{PDB3|2cbb}} :5-259 {{PDB3|1bcd}} :5-259
| |
| {{PDB3|1bnv}} :5-259 {{PDB3|1h4n}} :5-259 {{PDB3|1bic}} :5-259
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| {{PDB3|1g4j}}A:5-259 {{PDB3|1g54}}A:5-259 {{PDB3|1cal}} :5-259
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| {{PDB3|1tb0}}X:5-259 {{PDB3|1ugd}} :5-259 {{PDB3|1i9m}}A:5-259
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| {{PDB3|1cah}} :5-259 {{PDB3|1a42}} :5-258 {{PDB3|1thk}}A:5-259
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| {{PDB3|1kwr}}A:5-259 {{PDB3|1g4o}}A:5-259 {{PDB3|3ca2}} :5-258
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| {{PDB3|1ugf}} :5-259 {{PDB3|1cvb}} :5-258 {{PDB3|1rzb}} :5-259
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| {{PDB3|1lgd}}A:5-259 {{PDB3|1bn4}} :5-259 {{PDB3|1fqm}}A:5-259
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| {{PDB3|1cao}} :5-259 {{PDB3|1cni}} :5-259 {{PDB3|1cnx}} :5-258
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| {{PDB3|2ca2}} :5-258 {{PDB3|1bnm}} :5-259 {{PDB3|1yo1}}A:5-259
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| {{PDB3|1rzc}} :5-259 {{PDB3|1uge}} :5-259 {{PDB3|7ca2}} :5-258
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| {{PDB3|1uga}} :5-259 {{PDB3|1cra}} :5-259 {{PDB3|2ax2}}A:5-259
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| {{PDB3|1bnn}} :5-259 {{PDB3|2h4n}} :5-259 {{PDB3|1dcb}} :5-258
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| {{PDB3|1cny}} :5-258 {{PDB3|1i9o}}A:5-259 {{PDB3|1if8}}A:5-259
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| {{PDB3|1ydc}} :5-258 {{PDB3|1bnt}} :5-258 {{PDB3|5cac}} :5-258
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| {{PDB3|1rza}} :5-259 {{PDB3|1cnk}} :5-259 {{PDB3|1cnw}} :5-258
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| {{PDB3|1tbt}}X:5-259 {{PDB3|1i9p}}A:5-259 {{PDB3|1hea}} :5-258
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| {{PDB3|1if9}}A:5-259 {{PDB3|1i90}}A:5-259 {{PDB3|1cvf}} :5-258
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| {{PDB3|1fr4}}A:5-259 {{PDB3|1cvd}} :5-258 {{PDB3|1if7}}A:5-259
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| {{PDB3|1cin}} :5-258 {{PDB3|1cva}} :5-258 {{PDB3|1i9n}}A:5-259
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| {{PDB3|1ydd}} :5-258 {{PDB3|1bn3}} :5-259 {{PDB3|1teu}}X:5-259
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| {{PDB3|1hed}} :5-258 {{PDB3|1tg3}}A:5-259 {{PDB3|1cnh}} :5-259
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| {{PDB3|1okm}} :5-259 {{PDB3|1if6}}A:5-259 {{PDB3|1cct}} :5-258
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| {{PDB3|1g48}}A:5-259 {{PDB3|1xq0}}A:5-259 {{PDB3|1q4i}}A:5-259
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| {{PDB3|1crm}} :6-260 {{PDB3|1hcb}} :6-260 {{PDB3|1huh}} :6-260
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| {{PDB3|1hug}} :6-260 {{PDB3|1j9w}}A:6-260 {{PDB3|1czm}} :6-260
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| {{PDB3|1jv0}}B:6-260 {{PDB3|1bzm}} :6-260 {{PDB3|1azm}} :6-260
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| {{PDB3|2cab}} :6-260 {{PDB3|1z97}}A:5-259 {{PDB3|1z93}}A:5-259
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| {{PDB3|1flj}}A:5-259 {{PDB3|1dmx}}A:55-290 {{PDB3|1urt}} :55-290
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| {{PDB3|1dmy}}A:55-290 {{PDB3|1keq}}B:53-290 {{PDB3|2znc}} :22-278
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| {{PDB3|3znc}} :22-278 {{PDB3|1znc}}B:23-284
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| }}
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| The '''carbonic anhydrases''' (or '''carbonate dehydratases''') form a family of [[enzyme]]s that [[catalyst|catalyze]] the rapid interconversion of [[carbon dioxide]] and [[water]] to [[bicarbonate]] and [[Hydronium ion|proton]]s (or vice versa), a [[reversible reaction]] that occurs rather slowly in the absence of a catalyst.<ref name="pmid9336012">{{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 }}</ref> The [[active site]] of most carbonic anhydrases contains a [[zinc]] ion; they are therefore classified as [[metalloprotein|metalloenzymes]].
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| One of the functions 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.
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| == Reaction ==
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| The reaction catalyzed by carbonic anhydrase is:
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| :<math>\rm CO_2 + H_2O \xleftarrow{Carbonic\ anhydrase} H_2CO_3</math> (in [[Biological tissue|tissue]]s - high CO<sub>2</sub> concentration)<ref>Carbonic acid has a pK<sub>a</sub> 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 HCO<sub>3</sub><sup>-</sup> + H<sup>+</sup><math>\rightleftharpoons</math> H<sub>2</sub>CO<sub>3</sub> and H<sub>2</sub>CO<sub>3</sub><math>\rightleftharpoons</math> CO<sub>2</sub> + H<sub>2</sub>O</ref>
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| 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 (biochemistry)|substrate]]s.
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| Typical catalytic rates of the different forms of this enzyme ranging between 10<sup>4</sup> and 10<sup>6</sup> reactions per second.<ref name="Lindskog_1997">{{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 }}</ref>
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| The reverse reaction is relatively slow (kinetics in the 15-second range) in the absence of a catalyst. This is why a carbonated drink does not instantly degas when opening the container; however it will rapidly degas in the mouth when it comes in contact with carbonic anhydrase that is contained in saliva.<ref name="pmid9784398">{{cite journal | author = Thatcher BJ, Doherty AE, Orvisky E, Martin BM, Henkin RI | title = Gustin from human parotid saliva is carbonic anhydrase VI | journal = Biochem. Biophys. Res. Commun. | volume = 250 | issue = 3 | pages = 635–41 |date=September 1998 | pmid = 9784398 | doi = 10.1006/bbrc.1998.9356 | url = | issn = }}</ref>
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| An anhydrase is defined as an enzyme that catalyzes the removal of a water molecule from a compound, and so it is this "reverse" reaction that gives carbonic anhydrase its name, because it removes a water molecule from carbonic acid.
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| :<math>\rm HCO_3^- + H^+ \rightarrow H_2CO_3 \rightarrow CO_2 + H_2O</math> (in [[lung]]s and [[nephrons]] of the [[kidney]] - low CO<sub>2</sub> concentration, in plant cells)
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| ==Mechanism==
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| [[Image:Carbonic anhydrase 1CA2 active site.png|thumb|200px|Close-up rendering of active site of human carbonic anhydrase II, showing three [[histidine]] residues and a [[hydroxide]] group coordinating (dashed lines) the [[zinc]] ion at center. From {{PDB|1CA2}}.]]
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| 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 the bond.
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| A fourth histidine is placed close to the substrate of water and accepts a [[proton]], in an example of [[general acid]] - [[general base]] [[catalysis]] (see the article "[[Acid catalysis]]"). This leaves a [[hydroxide]] attached to the zinc.
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| 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.
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| ==CA families==
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| [[Image:Carbonic anhydrase 1CA2.png|thumb|200px|Ribbon diagram of human carbonic anhydrase II. [[Active site]] [[zinc]] ion visible at center. From {{PDB|1CA2}}.]]
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| There are at least five distinct CA families (α, β, γ, δ and ε). These families have no significant [[protein sequence|amino acid sequence]] similarity and in most cases are thought to be an example of [[convergent evolution]]. The α-CAs are found in humans.
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| ===α-CA===
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| The CA enzymes found in [[mammal]]s are divided into four broad subgroups,<ref name="pmid11875253">{{cite 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}}</ref> which, in turn consist of several isoforms: | |
| * the [[cytosol]]ic CAs (CA-I, [[Carbonic anhydrase II|CA-II]], CA-III, CA-VII and CA XIII) ({{gene2|CA1|1368}}, {{gene2|CA2|1373}}, {{gene2|CA3|1374}}, {{gene2|CA7|1381}}, {{gene2|CA13|14914}})
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| * [[mitochondrion|mitochondrial]] CAs (CA-VA and CA-VB) ({{gene|CA5A}}, {{gene|CA5B}})
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| * secreted CAs (CA-VI) ({{gene2|CA6|1380}})
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| * 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}})
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| 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.<ref name="pmid9878252">{{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 }}</ref>
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| {| class="wikitable" align="center"
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| |-
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| |+ Comparison of mammalian carbonic anhydrases
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| !Isoform
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| !Gene
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| ![[Molecular mass]]<ref name=boron>Unless else specified: {{cite book |author=Walter F., PhD. Boron |title=Medical Physiology: A Cellular And Molecular Approaoch |publisher=Elsevier/Saunders |location= |year= 2005|pages= |isbn=1-4160-2328-3 |oclc= |doi = }} Page 638</ref>
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| !Location (cell)
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| !Location (tissue)<ref name=boron/>
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| !Specific activity of human enzymes (except for mouse CA XV) (s<sup>−1</sup>)<ref name="Hilvo_2008">{{cite journal | author = Hilvo M, Baranauskiene L, Salzano AM, Scaloni A, Matulis D, Innocenti A, Scozzafava A, Monti SM, Di Fiore A, De Simone G, Lindfors M, Jänis J, Valjakka J, Pastoreková S, Pastorek J, Kulomaa MS, Nordlund HR, Supuran CT, Parkkila S | title = Biochemical characterization of CA IX, one of the most active carbonic anhydrase isozymes | journal = J. Biol. Chem. | volume = 283 | issue = 41 | pages = 27799–809 | year = 2008 | pmid = 18703501 | doi = 10.1074/jbc.M800938200 }}</ref>
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| !Sensitivity to [[sulfonamide]]s (acetazolamide in this table) K<sub>I</sub> (nM)<ref name="Hilvo_2008">{{cite journal | author = Hilvo M, Baranauskiene L, Salzano AM, Scaloni A, Matulis D, Innocenti A, Scozzafava A, Monti SM, Di Fiore A, De Simone G, Lindfors M, Jänis J, Valjakka J, Pastoreková S, Pastorek J, Kulomaa MS, Nordlund HR, Supuran CT, Parkkila S | title = Biochemical characterization of CA IX, one of the most active carbonic anhydrase isozymes. | journal = J. Biol. Chem. | volume = 283 | issue = 41 | pages = 27799–809 | year = 2008 | pmid = 18703501 | doi = 10.1074/jbc.M800938200 }}</ref>
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| | [[Carbonic anhydrase I|CA-I]] || {{gene2|CA1|1368}} || 29 kDa || [[cytosol]] || [[red blood cell]] and [[gastrointestinal tract|GI tract]] || 2.0 × 10<sup>5</sup> || 250
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| | [[Carbonic anhydrase II|CA-II]] || {{gene2|CA2|1373}} || 29 kDa || [[cytosol]] || almost ubiquitous || 1.4 × 10<sup>6</sup> || 12
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| | [[Carbonic anhydrase III|CA-III]] || {{gene2|CA3|1374}} || 29 kDa || [[cytosol]] || 8% of soluble protein in Type I [[muscle]] || 1.3 × 10<sup>4</sup> || 240000
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| | [[Carbonic anhydrase IV|CA-IV]] || {{gene2|CA4|1375}} || 35 kDa || extracellular [[glycophosphatidylinositol|GPI]]-linked || [[gastrointestinal tract|GI tract]], [[kidney]], endothelium || 1.1 × 10<sup>6</sup> || 74
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| | [[Carbonic anhydrase VA|CA-VA]] || {{gene|CA5A}} || 34.7 kDa (predicted) || [[mitochondria]] || liver || 2.9 × 10<sup>5</sup> || 63
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| | [[Carbonic anhydrase VB|CA-VB]] || {{gene|CA5B}} || 36.4 kDa (predicted) || [[mitochondria]] || widely distributed || 9.5 × 10<sup>5</sup> || 54
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| | [[Carbonic anhydrase VI|CA-VI]] || {{gene2|CA6|1380}} || 39-42 kDa || secretory || saliva and milk || 3.4 × 10<sup>5</sup> || 11
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| | [[Carbonic anhydrase VII|CA-VII]] || {{gene2|CA7|1381}} || 29 kDa || [[cytosol]] || widely distributed || 9.5 × 10<sup>5</sup> || 2.5
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| | [[Carbonic anhydrase IX|CA-IX]] || {{gene2|CA9|1383}} || 54, 58 kDa || [[cell membrane]]-associated || normal [[gastrointestinal tract|GI tract]], several cancers || 1.1 × 10<sup>6</sup> || 16
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| | [[Carbonic anhydrase XII|CA-XII]] || {{gene2|CA12|1371}} || 44 kDa || extracellularily located [[active site]] || [[kidney]], certain [[cancer]]s || 4.2 × 10<sup>5</sup> || 5.7
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| | [[Carbonic anhydrase XIII|CA-XIII]]<ref name="Lehtonen 2004">{{cite journal | author = Lehtonen J, Shen B, Vihinen M, Casini A, Scozzafava A, Supuran CT, Parkkila AK, Saarnio J, Kivelä AJ, Waheed A, Sly WS, Parkkila S | title = Characterization of CA XIII, a novel member of the carbonic anhydrase isozyme family | journal = J. Biol. Chem. | volume = 279 | issue = 4 | pages = 2719–27 | year = 2004 | pmid = 14600151 | doi = 10.1074/jbc.M308984200 }}</ref>|| {{gene2|CA13|14914}} || 29 kDa || [[cytosol]] || widely distributed || 1.5 × 10<sup>5</sup> || 16
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| | [[Carbonic anhydrase XIV|CA-XIV]] || {{gene2|CA14|1372}} || 54 kDa || extracellularily located [[active site]] || [[kidney]], [[heart]], [[skeletal muscle]], [[brain]] || 3.1 × 10<sup>5</sup> || 41
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| | [[Carbonic anhydrase XV|CA-XV]]<ref name="Hilvo_2005">{{cite journal | author = Hilvo M, Tolvanen M, Clark A, Shen B, Shah GN, Waheed A, Halmi P, Hänninen M, Hämäläinen JM, Vihinen M, Sly WS, Parkkila S | title = Characterization of CA XV, a new GPI-anchored form of carbonic anhydrase | journal = Biochem. J. | volume = 392 | pages = 83–92 | year = 2005 | pmid = 16083424 | doi = 10.1042/BJ20051102 | issue = Pt 1 | pmc = 1317667 }}</ref>|| {{gene2|CA15|80733}} || 34-36 kDa || extracellular [[glycophosphatidylinositol|GPI]]-linked || [[kidney]], not expressed in human tissues || 4.7 × 10<sup>5</sup> || 72
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| |}
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| ===β-CA===
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| Most [[prokaryote|prokaryotic]] and plant [[chloroplast]] CAs belong to the beta family.
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| Two [[sequence motif|signature pattern]]s for this family have been identified:
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| * C-[SA]-D-S-R-[LIVM]-x-[AP]
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| * [EQ]-[YF]-A-[LIVM]-x(2)-[LIVM]-x(4)-[LIVMF](3)-x-G-H-x(2)-C-G
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| ===γ-CA===
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| The gamma class of CAs come from [[methanogen]]s, [[methane-producing bacteria]] that grow in hot springs.
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| ===δ-CA===
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| The delta class of CAs has been described in [[diatom]]s. The distinction of this class of CA has recently<ref name="Sawaya_2006">{{cite 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 }}</ref> come into question, however.
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| ===ε-CA===
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| The epsilon class of CAs occurs exclusively in [[bacteria]] in a few [[chemolithotroph]]s and marine [[cyanobacteria]] that contain cso-[[carboxysome]]s.<ref name="pmid14729686">{{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 | pmc = 321498 }}</ref> Recent 3-dimensional analyses<ref name="Sawaya_2006" /> 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 [[protein sequence|amino acid sequence]] has since diverged considerably.
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| ==Pharmacological agents affecting CA==
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| :''See [[Carbonic anhydrase inhibitor]] | |
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| ==Structure and function of carbonic anhydrase==
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| 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.
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| 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.
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| There are at least 14 different isoforms in mammals. [[Plant]]s 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 CO<sub>2</sub> within the [[chloroplast]] in order to increase the carboxylation rate of the enzyme [[RuBisCO]]. This is the reaction that integrates CO<sub>2</sub> into [[organic carbon]] sugars during [[photosynthesis]], and can use only the CO<sub>2</sub> form of carbon, not carbonic acid or bicarbonate.
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| A [[cadmium]]-containing carbonic anhydrase was found to be expressed in marine [[diatom]]s during zinc limitation.<ref name="pmid15875011"/> 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. [[Cadmium]] has in general been thought of as a very toxic [[heavy metals|heavy metal]] without biological function. This peculiar carbonic anhydrase form hosts the only known beneficial cadmium-dependent biological reaction.<ref name="pmid15875011">{{cite journal | author = Lane TW, Saito MA, George GN, Pickering IJ, Prince RC, Morel FM | title = Biochemistry: a cadmium enzyme from a marine diatom | journal = Nature | volume = 435 | issue = 7038 | pages = 42 |date=May 2005 | pmid = 15875011 | doi = 10.1038/435042a | laysummary = http://www-ssrl.slac.stanford.edu/research/highlights_archive/cd-ca.html | laysource = SSRL Science Highlight }}</ref>
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| ==Industrial applications==
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| Modified carbonic anhydrase enzymes have been used to replace [[methyl diethanolamine]] ("MDEA") in [[Carbon dioxide sequestration|carbon dioxide capture]]. Quebec based company CO2 solutions promotes this technology.
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| ==References==
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| {{reflist}}
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| == Further reading ==
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| {{refbegin}}
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| * {{cite journal | author = Lyall V, Alam RI, Phan DQ, Ereso GL, Phan TH, Malik SA, Montrose MH, Chu S, Heck GL, Feldman GM, DeSimone JA | title = Decrease in rat taste receptor cell [[intracellular pH]] is the proximate stimulus in sour taste transduction | journal = Am. J. Physiol., Cell Physiol. | volume = 281 | issue = 3 | pages = C1005–13 |date=September 2001 | pmid = 11502578 | doi = }}
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| * {{cite web | url = http://www.rcsb.org/pdb/101/motm.do?momID=49 | title = Carbonic Anhydrase | author = Goodsell D | date = 2004-01-01 | format = | work = PDB Molecule of the Month | publisher = Protein Data Bank | accessdate = 2011-05-28 }}
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| {{refend}}
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| {{Carbon-oxygen lyases}}
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| {{Carbonic anhydrases}}
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| {{DEFAULTSORT:Carbonic Anhydrase}}
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| [[Category:EC 4.2.1]]
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| [[Category:Enzymes]]
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| [[Category:Zinc enzymes]]
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