View source for Carbonic anhydrase

{{short description|Class of enzymes}}
{{Use dmy dates|date=January 2020}}
{{infobox enzyme
| Name       = Carbonate dehydratase
| EC_number  = 4.2.1.1
| CAS_number = 9001-03-0
| GO_code    = 0004089
| image      = Human carbonic anhydrase II PDB=6LUX.png
| caption    = Human carbonic anhydrase II with bound zinc and carbon dioxide. {{PDB|6LUX}}
| name       = Carbonic anhydrase
}}
{{Pfam_box
| Symbol = Carb_anhydrase
| Name = Eukaryotic-type carbonic anhydrase
| image = 
| width = 
| caption = 
| Pfam= PF00194
| InterPro= IPR001148
| SMART= 
| Prosite = PDOC00146
| SCOP = 1can
| TCDB = 
| OPM family= 
| OPM protein= 
| Membranome superfamily = 333
}}

The '''carbonic anhydrases''' (or '''carbonate dehydratases''') ({{EnzExplorer|4.2.1.1}}) form a family of [[enzyme]]s that [[catalyst|catalyze]] the interconversion between [[carbon dioxide]] and [[water]] and the [[Dissociation (chemistry)|dissociated]] ions of [[carbonic acid]] (i.e. [[bicarbonate]] and [[hydrogen ion]]s).<ref name="pmid9336012">{{cite journal|vauthors=Badger MR, Price GD|year=1994 |title=The role of carbonic anhydrase in photosynthesis|journal=Annu. Rev. Plant Physiol. Plant Mol. Biol.|volume=45|pages=369–392|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]]. The enzyme maintains [[Acid–base homeostasis|acid-base balance]] and helps transport carbon dioxide.<ref name=":1">{{Cite web |title=PDB101: Molecule of the Month: Carbonic Anhydrase |url=http://pdb101.rcsb.org/motm/49 |access-date=3 December 2018 |website=RCSB: PDB-101}}</ref>

Carbonic anhydrase helps maintain [[acid–base homeostasis]], regulate [[pH]], and fluid balance. Depending on its location, the role of the enzyme changes slightly. For example, carbonic anhydrase produces acid in the stomach lining. In the kidney, the control of bicarbonate ions influences the water content of the cell. The control of bicarbonate ions also influences the water content in the eyes. Inhibitors of carbonic anhydrase are used to treat glaucoma, the excessive build-up of water in the eyes. Blocking this enzyme shifts the fluid balance in the eyes to reduce fluid build-up thereby relieving pressure.<ref name=":1" /><ref>{{Cite book |vauthors=Supuran CT |veditors=Supuran CT, Scozzafava A, Conway J |date=27 May 2004 |title=Carbonic Anhydrases |chapter=Carbonic Anhydrases: Catalytic and Inhibition Mechanisms, Distribution and Physiological Roles |chapter-url=https://www.taylorfrancis.com/chapters/edit/10.1201/9780203475300-5/carbonic-anhydrases-catalytic-inhibition-mechanisms-distribution-physiological-roles-claudiu-supuran |publisher=Taylor & Francis |isbn=9780203475300 |doi=10.1201/9780203475300 |access-date=19 July 2022 |archive-date=11 April 2023 |archive-url=https://web.archive.org/web/20230411161011/https://www.taylorfrancis.com/chapters/edit/10.1201/9780203475300-5/carbonic-anhydrases-catalytic-inhibition-mechanisms-distribution-physiological-roles-claudiu-supuran |url-status=dead }}</ref>

Carbonic anhydrase is critical to hemoglobin function via the [[Bohr effect]] which catalyzes the hydration of carbon dioxide to form carbonic acid and rapidly dissociate into water.<ref>{{cite journal |vauthors=Occhipinti R, Boron WF |title=Role of Carbonic Anhydrases and Inhibitors in Acid-Base Physiology: Insights from Mathematical Modeling |journal=International Journal of Molecular Sciences |volume=20 |issue=15 |pages=3841 |date=August 2019 |pmid=31390837 |doi=10.3390/ijms20153841|doi-access=free |pmc=6695913 }}</ref> Essentially an increase in carbon dioxide results in lowered blood pH, which lowers [[oxygen-hemoglobin binding]].<ref>{{Cite web |url=http://www.pathwaymedicine.org/bohr-effect |title=Bohr Effect|website=www.pathwaymedicine.org|language=en|access-date=23 November 2019}}</ref> The opposite is true where a decrease in the concentration of carbon dioxide raises the blood pH which raises the rate of oxygen-hemoglobin binding. Relating the Bohr effect to carbonic anhydrase is simple: carbonic anhydrase speeds up the reaction of carbon dioxide reacting with water to produce hydrogen ions (protons) and bicarbonate ions.

To describe equilibrium in the carbonic anhydrase reaction, [[Le Chatelier's principle]] is used. Most tissue is more acidic than lung tissue because carbon dioxide is produced by cellular respiration in these tissues, where it reacts with water to produce protons and bicarbonate. Because the carbon dioxide concentration is higher, the equilibrium shifts to the right, to the bicarbonate side. The opposite is seen in the lungs, where carbon dioxide is being released, reducing its concentration, so the equilibrium shifts to the left, favoring carbon dioxide production.<ref>{{Cite web|url=https://www.chemguide.co.uk/physical/equilibria/lechatelier.html|title=Le Chatelier's Principle|website=www.chemguide.co.uk|access-date=23 November 2019}}</ref>

== Occurrence and function ==
Carbonic anhydrase was initially isolated and characterised from red blood cells in 1933, with simultaneous reports by Meldrum and Roughton (at Cambridge University in the United Kingdom) and by Stadie and O’Brien (at the University of Pennsylvania in the United States),<ref>{{cite book |vauthors=Forster RE |chapter=Remarks on the Discovery of Carbonic Anhydrase |date=2000 |title=The Carbonic Anhydrases: New Horizons |pages=1–11 | series = Experientia Supplementum | volume = 90 | veditors=Chegwidden WR, Carter ND, Edwards YH |place=Basel |publisher=Springer/Birkhauser |language=en |doi=10.1007/978-3-0348-8446-4_1 |isbn=978-3-0348-9570-5}}</ref><ref>{{cite book |vauthors=Starr I |chapter= William Christopher Stadie, 1886-1959 |date=1989 |title=Biographical Memoirs |pages=513–518 | series =  | volume =  | veditors= |place=Washington, DC |publisher=National Academy of Sciences Press |language=en |url=https://www.nasonline.org/publications/biographical-memoirs/memoir-pdfs/stadie-william.pdf |isbn=}}</ref> both while searching for a "catalytic factor... necessary for rapid transit of the HCO<sub>3</sub><sup>−</sup> [bicarbonate anion] from the erythrocyte to... pulmonary capillar[ies]".<ref name=":22">{{cite book |vauthors=Dodgson SJ |chapter=The Carbonic Anhydrases |date=1991 |title=The Carbonic Anhydrases: Cellular Physiology and Molecular Genetics |pages=3–14 |veditors=Dodgson SJ, Tashian RD, Gros G, Carter ND |place=Boston, MA |publisher=Springer US |language=en |doi=10.1007/978-1-4899-0750-9_1 |isbn=978-1-4899-0750-9}}</ref>

=== Regulation of pH ===
Carbonic anhydrase plays an essential role in regulating the blood pH, which speeds up the CO<sub>2</sub> + H<sub>2</sub>O <chem> <=> </chem> HCO<sub>3</sub><sup>−</sup> + H<sup>+</sup> reaction to ensure the equilibrium balance is rapidly maintained. The equilibrium reaction is influenced by the proportion of bicarbonate and H<sup>+</sup> to carbon dioxide.<ref name=":02">{{Cite journal |vauthors=Deutsch HF |date=1987-01-01 |title=Carbonic anhydrases |journal=International Journal of Biochemistry |language=en |volume=19 |issue=2 |pages=101–113 |doi=10.1016/0020-711X(87)90320-X |issn=0020-711X}}</ref> The HCO<sub>3</sub><sup>−</sup> is a conjugate base that neutralizes acids, and the H<sup>+</sup> is a conjugate acid that neutralizes bases by [[Acid–base homeostasis|Acid-base homeostasis]]. The HCO<sub>3</sub><sup>−</sup> and H<sup>+</sup> are ideal for buffering pH in the blood and tissues because the pKa is close to the physiological pH = 7.2 – 7.6. Since HCO<sub>3</sub><sup>−</sup> and H<sup>+</sup> are regulated in the kidneys and plasma carbon dioxide is regulated in the lungs, both actions in the kidneys and lungs are important to maintain the stability of blood pH. Therefore, carbonic anhydrase helps with the H+ secretion into the [[Nephron|lumen of the renal tubule]] and the reabsorption of  HCO<sub>3</sub><sup>−</sup> in the kidneys. Also, it helps the carbon dioxide transport from the lung tissue to the alveoli in the pulmonary capillary, where the carbon dioxide will be excreted during exhalation.<ref name=":02" />

Carbonic anhydrase is a very ancient enzyme found in both domains of prokaryotes that exists in six different classes among most of the living organisms.<ref name=":12">{{cite book |vauthors=Chegwidden WR, Carter ND |chapter=Introduction to the carbonic anhydrases |date=2000 |title=The Carbonic Anhydrases |pages=13–28 |veditors=Chegwidden WR, Carter ND, Edwards YH |place=Basel |publisher=Birkhäuser |language=en |doi=10.1007/978-3-0348-8446-4_2 |isbn=978-3-0348-8446-4}}</ref> These families are not similar in sequence or structure because they evolved independently of each other, but all evolved the same Zn<sup>2+</sup> active site structure, showing a great example of convergent evolution.{{citation needed|date = March 2024}}

==Background==
An enzyme is a substance that acts as a catalyst in living organisms which helps to speed up chemical reactions.<ref name=":2">{{Cite web|url=https://www.britannica.com/science/enzyme|title=Britannica Dictionary}}</ref> Carbonic anhydrase is one important enzyme that is found in red blood cells, gastric mucosa, pancreatic cells, and even renal tubules. It was discovered in the year 1932 and it has been categorized into three general classes.<ref>{{cite journal |vauthors=Maren TH |s2cid=19954840 |title=Carbonic anhydrase: chemistry, physiology, and inhibition |journal=Physiological Reviews |volume=47 |issue=4 |pages=595–781 |date=October 1967 |pmid=4964060 |doi=10.1152/physrev.1967.47.4.595|doi-access=free }}</ref> Class one being alpha carbonic anhydrase which is found in mammals, class two being beta carbonic anhydrase which is found in bacteria and plants and lastly, class three which is gamma carbonic anhydrase which is found in methanogen bacteria in hot springs.<ref>{{cite book |vauthors=Bertini I, Gray H, Stiefel E, Valentine J |title=Biological Inorganic Chemistry: Structure and Reactivity |year=2007 |publisher=University Science Books |location=Sausalito, California |isbn=978-1-891389-43-6 |edition=1st |at=section IX.1.3.1. p. 180}}</ref> The three classes of carbonic anhydrase all have the same active site with a Zn metal centre; however, they are not structurally similar to each other. The main role of carbonic anhydrase in humans is to catalyze the conversion of carbon dioxide to carbonic acid and back again. However, it can also help with CO<sub>2</sub> transport in the blood which in turn helps respiration. It can even function in the formation of [[hydrochloric acid]] by the stomach.[Citation needed] Therefore, the role of carbonic anhydrase depends on where it is found in the body.

=== Structure ===
In mammalian CA II, the active site consists of the following: a hard Lewis acid Zn<sup>+2</sup> metal atom coordinated to [[Histidine|His]] -94, -96, and -119 residues 109˚ apart from one another and a hydroxide ion (pKa=6.8; 120° in T<sub>d</sub>  configuration, a hydrophobic pocket adjacent to Zinc-bound hydroxide consisting of by Val-143 at its base and Val-121, Trp-209, and Leu-198 at its neck, a Proton Shuttling Residue (PSR) His-64 H<sup>+</sup> shuttles H<sup>+</sup> in and out of active site via conformational switching, and a hydrogen bonding network consisting of Thr-199 hydroxyl group and Glu-106 the carboxyl group which stabilizes the Zinc-bound hydroxide by facilitating the orientation of water molecules in the active side to a specific geometric configuration. CA II has a turnover frequency of 10<sup>6</sup> s<sup>−1</sup> which is 10<sup>7</sup> times faster than the uncatalyzed reaction.

== Reaction ==
{{see also|carbonic acid}}
[[File:Carbonic anhydrase II mechanism.svg|thumb|class=skin-invert-image|Schematic of the cyclic reaction mechanism catalyzed by carbonic anhydrase II.<ref name="Lindskog_1997" />|288x288px]]
The reaction that shows the catalyzation of carbonic anhydrase in our tissues is
: CO<sub>2</sub> + H<sub>2</sub>O ⟶ {{chem|H|2|CO|3}} ⟶ H<sup>+</sup> + {{chem|HCO|3|-}}

The catalyzation of carbonic anhydrase in the lungs is shown by
: H<sup>+</sup> + {{chem|HCO|3|-}} ⟶ {{chem|H|2|CO|3}} ⟶ CO<sub>2</sub> + H<sub>2</sub>O

The reason for the reactions being in opposite directions for the tissues and lungs is because of the different pH levels found in them. Without the carbonic anhydrase catalyst, the reaction is very slow, however with the catalyst the reaction is 10<sup>7</sup> times faster.

The reaction catalyzed by carbonic anhydrase is

: {{chem|HCO|3|-}} + H<sup>+</sup> ⇌ CO<sub>2</sub> + H<sub>2</sub>O

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.

Carbonic anhydrase is one of the fastest enzymes, and its rate is typically limited by the [[diffusion]] rate of its [[Substrate (biochemistry)|substrate]]s. 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 | vauthors = Lindskog S | title = Structure and mechanism of carbonic anhydrase | journal = Pharmacology & Therapeutics | volume = 74 | issue = 1 | pages = 1–20 | year = 1997 | pmid = 9336012 | doi = 10.1016/S0163-7258(96)00198-2 }}</ref>

The uncatalyzed reverse reaction is relatively slow (kinetics in the 15-second range).  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 | vauthors = Thatcher BJ, Doherty AE, Orvisky E, Martin BM, Henkin RI | title = Gustin from human parotid saliva is carbonic anhydrase VI | journal = Biochemical and Biophysical Research Communications | volume = 250 | issue = 3 | pages = 635–41 | date = September 1998 | pmid = 9784398 | doi = 10.1006/bbrc.1998.9356 }}</ref>

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.

In the lungs carbonic anhydrase converts bicarbonate to carbon dioxide, suited for exhalation.

=== CO<sub>2</sub> transport ===
[[File:Active site of CAII (cropped).jpg|thumb|Active site of CAII enzyme (PDB code: 1CA2) which O = red, N = blue, Zn = grey and C = green]]
Carbon dioxide is transported in the blood in three forms:

# Dissolved as gas in plasma – 7-10%
# Bound to hemoglobin as carbaminohemoglobin in red blood cells – 20%
# Present as bicarbonate ion in plasma and transported as bicarbonate - 70%<ref>{{cite journal |vauthors=Yadav RR, Krishnamurthi K, Mudliar SN, Devi SS, Naoghare PK, Bafana A, Chakrabarti T |title=Carbonic anhydrase mediated carbon dioxide sequestration: promises, challenges and future prospects |journal=Journal of Basic Microbiology |volume=54 |issue=6 |pages=472–481 |date=June 2014 |pmid=24740638 |doi=10.1002/jobm.201300849 |s2cid=29177837}}</ref>

== Mechanism ==

[[Image:Carbonic anhydrase 1CA2 active site.png|thumb|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}}.]]
[[File:CAII enzyme.jpg|thumb|CAII enzyme (PDB code: 1CA2) showing the secondary structures and the hydrophobic pocket (right side of the Zn<sup>2+</sup>) formed by the beta sheet (blue) and the two valine and one tryptophan side chains]]
A [[zinc]] [[prosthetic group]] in the enzyme is coordinated in three positions by [[histidine]] [[Side chain|side-chains]]. The fourth coordination position is occupied by [[metal aquo complex|water]]. A fourth histidine is close to the water ligand, facilitating formation of Zn-OH center, which binds CO<sub>2</sub> to give a zinc bicarbonate.<ref>{{cite journal |vauthors=Parkin G |s2cid=9857226 |title=Synthetic analogues relevant to the structure and function of zinc enzymes |journal=Chemical Reviews |volume=104 |issue=2 |pages=699–767 |date=February 2004 |pmid=14871139 |doi=10.1021/cr0206263}}</ref> The construct is an example of general acid – general base [[catalysis]] (see the article "[[Acid catalysis]]"). The active site also features a pocket suited for carbon dioxide, bringing it close to the hydroxide group. Kinetic studies performed determine the following mechanism for the enzyme: In Step 1 & 2, the nucleophile O<sup>−</sup> on the hydroxide ion coordinated to Zn<sup>2+</sup> performs a nucleophilic attack on the partially electrophilic carbon on the CO<sub>2</sub> molecule. Here the Zn<sup>2+</sup> acts as a Lewis acid that lowers the pK<sub>a</sub> of the coordinated OH<sub>2</sub> ligand from ~7-8 down to 6.8  as T<sub>d</sub> , which drives the deprotonation of water to a hydroxide ion and the free proton is neutralized by the surrounding buffer. In step 3), a proton transfer (H<sup>+</sup>) occurs from the OH<sup>−1</sup> to the non-coordinated O<sup>−</sup> in CO<sub>3</sub><sup>−2</sup> coordinated to the Zn<sup>+2</sup> atom in the active site. Next, a bicarbonate ion is released and the catalytic site is regenerated through the binding of another water molecule in exchange for the bicarbonate ion . In step 4), the coordinated water ligand is deprotonated facilitated by the Zn<sup>+2</sup> to generate another hydroxide ion to start the cycle over again.<ref>{{Cite book| vauthors=Berg JM, Tymoczko JL, Stryer L |date=2013|title=Stryer Biochemie| doi=10.1007/978-3-8274-2989-6 |isbn=978-3-8274-2988-9}}</ref><ref>{{cite journal |vauthors=Goto M, Takahashi T, Yamashita F, Koreeda A, Mori H, Ohta M, Arakawa Y |title=Inhibition of the metallo-beta-lactamase produced from Serratia marcescens by thiol compounds |journal=Biological & Pharmaceutical Bulletin |volume=20 |issue=11 |pages=1136–1140 |date=November 1997 |pmid=9401719 |doi=10.1021/ar9501232 }}</ref>

== Families ==
[[Image:Carbonic anhydrase 1CA2.png|thumb|Ribbon diagram of human carbonic anhydrase II. [[Active site]] [[zinc]] ion visible at center. From {{PDB|1CA2}}.]]

At least five distinct CA families are recognized: α, β, γ, δ and ζ. 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.

=== α-CA ===

Eukaryotes, including [[vertebrate]]s, [[algae]], [[plants]], and [[fungi]], as well as some [[bacteria]] contain this family of CAs.

The CA enzymes found in [[mammal]]s are divided into four broad subgroups,{{citation needed|date=June 2021}} which, in turn consist of several homologous classes of genes:
* 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}})
* [[mitochondrion|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" human carbonic anhydrase isoforms ([[CA8|CA-VIII]], [[CA10|CA-X]], and [[CA11|CA-XI]]) ({{gene2|CA8|1382}}, {{gene2|CA10|1369}}, {{gene2|CA11|1370}}) whose functions remain unclear.<ref name="pmid9878252">{{cite journal | vauthors = 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 | date = December 1998 | pmid = 9878252 | doi = 10.1006/geno.1998.5585 }}</ref>
[[File:This image shows the ligands and pocket style of carbonic anhydrase.jpg|thumb|This image shows the ligands and pocket style of carbonic anhydrase.<ref>{{cite journal | vauthors = Boriack-Sjodin PA, Zeitlin S, Chen HH, Crenshaw L, Gross S, Dantanarayana A, Delgado P, May JA, Dean T, Christianson DW | display-authors = 6 | title = Structural analysis of inhibitor binding to human carbonic anhydrase II | language = en-US | journal = Protein Science | volume = 7 | issue = 12 | pages = 2483–9 | date = December 1998 | pmid = 9865942 | pmc = 2143894 | doi = 10.1002/pro.5560071201 | url = http://www.rcsb.org/pdb/explore/jmol.do?structureId=1bn1&bionumber=1&jmolMode=HTML5 }}</ref>]]

{| class="wikitable"
|+ Comparison of mammalian carbonic anhydrases
|-
! rowspan=2 | Isoform
! rowspan=2 | Gene
! rowspan=2 | [[Molecular mass|Molecular <br/>mass]]<ref name=boron>Unless else specified: {{cite book |author = Boron WF  |title=Medical Physiology: A Cellular And Molecular Approach |publisher=Elsevier/Saunders |year= 2005|isbn=978-1-4160-2328-9 }} Page 638</ref> <br/>(kDa)
! colspan=2 | Location
! rowspan=2 | Specific activity of <br/>human{{nbsp}}enzymes,{{efn|Except for mouse CA XV.}}<ref name="Hilvo_2008">
{{cite journal
 | vauthors = 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
 | display-authors = 6
 | title = Biochemical characterization of CA IX, one of the most active carbonic anhydrase isozymes
 | journal = The Journal of Biological Chemistry
 | volume = 283 | issue = 41 | pages = 27799–809
 | date = October 2008
 | pmid = 18703501 | doi = 10.1074/jbc.M800938200 | doi-access = free
}}</ref> (s<sup>−1</sup>)
! rowspan=2 | Sensitivity to <br/>[[sulfonamide]]s,{{efn|Acetazolamide in this table.}} <br/>K<sub>I</sub> (nM)<ref name="Hilvo_2008" />
|-
! Cell
! Tissue<ref name=boron/>
|-
| [[Carbonic anhydrase I|CA-I]] || {{gene2|CA1|1368}} || 29 || [[cytosol]] || [[red blood cell]] and [[gastrointestinal tract|GI tract]] || 2.0 × 10<sup>5</sup> || 250
|-
| [[Carbonic anhydrase II|CA-II]] || {{gene2|CA2|1373}} || 29 || [[cytosol]] || almost ubiquitous || 1.4 × 10<sup>6</sup> || 12
|-
| [[Carbonic anhydrase III|CA-III]] || {{gene2|CA3|1374}} || 29 || [[cytosol]] || 8% of soluble protein in Type I [[muscle]] || 1.3 × 10<sup>4</sup> || 240000
|-
| [[Carbonic anhydrase IV|CA-IV]] || {{gene2|CA4|1375}} || 35 || extracellular [[glycophosphatidylinositol|GPI]]-linked || [[gastrointestinal tract|GI tract]], [[kidney]], endothelium || 1.1 × 10<sup>6</sup> || 74
|-
| [[Carbonic anhydrase VA|CA-VA]] || {{gene|CA5A}} || 34.7 (predicted) || [[mitochondria]] || liver || 2.9 × 10<sup>5</sup> || 63
|-
| [[Carbonic anhydrase VB|CA-VB]] || {{gene|CA5B}}  || 36.4 (predicted) || [[mitochondria]] || widely distributed || 9.5 × 10<sup>5</sup> || 54
|-
| [[Carbonic anhydrase VI|CA-VI]] || {{gene2|CA6|1380}} || 39–42 || secretory || saliva and milk || 3.4 × 10<sup>5</sup> || 11
|-
| [[Carbonic anhydrase VII|CA-VII]] || {{gene2|CA7|1381}} || 29 || [[cytosol]] || widely distributed || 9.5 × 10<sup>5</sup> || 2.5
|-
| [[Carbonic anhydrase IX|CA-IX]] || {{gene2|CA9|1383}} || 54, 58 || [[cell membrane]]-associated || normal [[gastrointestinal tract|GI tract]], several cancers || 3.8 × 10<sup>5</sup> || 16
|-
| [[Carbonic anhydrase XII|CA-XII]] || {{gene2|CA12|1371}} || 44 || extracellularily located [[active site]] || [[kidney]], certain [[cancer]]s || 4.2 × 10<sup>5</sup> || 5.7
|-
| [[Carbonic anhydrase XIII|CA-XIII]]<ref name="Lehtonen 2004">{{cite journal | vauthors = Lehtonen J, Shen B, Vihinen M, Casini A, Scozzafava A, Supuran CT, Parkkila AK, Saarnio J, Kivelä AJ, Waheed A, Sly WS, Parkkila S | display-authors = 6 | title = Characterization of CA XIII, a novel member of the carbonic anhydrase isozyme family | journal = The Journal of Biological Chemistry | volume = 279 | issue = 4 | pages = 2719–27 | date = January 2004 | pmid = 14600151 | doi = 10.1074/jbc.M308984200 | doi-access = free }}</ref>|| {{gene2|CA13|14914}} || 29 || [[cytosol]] || widely distributed || 1.5 × 10<sup>5</sup> || 16
|-
| [[Carbonic anhydrase XIV|CA-XIV]] || {{gene2|CA14|1372}} || 54 || extracellularily located [[active site]] || [[kidney]], [[heart]], [[skeletal muscle]], [[brain]] || 3.1 × 10<sup>5</sup> || 41
|-
| CA-XV<ref name="Hilvo_2005">{{cite journal | vauthors = Hilvo M, Tolvanen M, Clark A, Shen B, Shah GN, Waheed A, Halmi P, Hänninen M, Hämäläinen JM, Vihinen M, Sly WS, Parkkila S | display-authors = 6 | title = Characterization of CA XV, a new GPI-anchored form of carbonic anhydrase | journal = The Biochemical Journal | volume = 392 | issue = Pt 1 | pages = 83–92 | date = November 2005 | pmid = 16083424 | pmc = 1317667 | doi = 10.1042/BJ20051102 }}</ref>|| {{gene2|CA15|80733}} || 34–36 || extracellular [[glycophosphatidylinositol|GPI]]-linked || [[kidney]], not expressed in human tissues || 4.7 × 10<sup>5</sup> || 72
|}
{{notelist}}

=== β-CA ===

Most [[prokaryote|prokaryotic]] and plant [[chloroplast]] CAs belong to the beta family.
Two [[sequence motif|signature pattern]]s 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 comes from [[methanogen]]s, methane-producing archaea 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 recently<ref name="Sawaya_2006">{{cite journal | vauthors = 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 = The Journal of Biological Chemistry | volume = 281 | issue = 11 | pages = 7546–55 | date = March 2006 | pmid = 16407248 | doi = 10.1074/jbc.M510464200 | doi-access = free }}</ref> come into question, however.

=== ζ-CA ===

The zeta 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 | vauthors = 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 = Journal of Bacteriology | volume = 186 | issue = 3 | pages = 623–30 | date = February 2004 | pmid = 14729686 | pmc = 321498 | doi = 10.1128/JB.186.3.623-630.2004 }}</ref> Recent 3-dimensional analyses<ref name="Sawaya_2006" /> suggest that ζ-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.

=== η-CA ===

The eta family of CAs was recently found in organisms of the genus ''[[Plasmodium]]''. These are a group of enzymes previously thought to belong to the alpha family of CAs, however it has been demonstrated that η-CAs have unique features, such as their metal ion coordination pattern.<ref name="pmid25168745">{{cite journal | vauthors = Del Prete S, Vullo D, Fisher GM, Andrews KT, Poulsen SA, Capasso C, Supuran CT | title = Discovery of a new family of carbonic anhydrases in the malaria pathogen Plasmodium falciparum—the η-carbonic anhydrases | journal = Bioorganic & Medicinal Chemistry Letters | volume = 24 | issue = 18 | pages = 4389–4396 | date = September 2014 | pmid = 25168745 | doi = 10.1016/j.bmcl.2014.08.015 | hdl = 10072/63103 | hdl-access = free }}</ref>

=== ι-CA ===

The iota class is the most recent class of CAs described. It has been discovered in the marine diatom ''[[Thalassiosira pseudonana]],'' and is widespread among marine phytoplankton.<ref name=":3">{{cite journal | vauthors = Jensen EL, Clement R, Kosta A, Maberly SC, Gontero B | title = A new widespread subclass of carbonic anhydrase in marine phytoplankton | journal = The ISME Journal | volume = 13 | issue = 8 | pages = 2094–2106 | date = August 2019 | pmid = 31024153 | doi = 10.1038/s41396-019-0426-8 | pmc = 6776030 }}</ref> In diatoms, the ι-CA is essential for the CO<sub>2</sub>-concentrating mechanisms and - in contrast to other CA classes -  it can use [[manganese]] instead of zinc as metal cofactor.<ref name=":3" /> Homologs of the ι-CA have been also confirmed in gram-negative bacteria, where can be present as a protein homodimer.<ref>{{cite journal | vauthors = Del Prete S, Nocentini A, Supuran CT, Capasso C | title = Burkholderia territorii | journal = Journal of Enzyme Inhibition and Medicinal Chemistry | volume = 35 | issue = 1 | pages = 1060–1068 | date = December 2020 | pmid = 32314608 | pmc = 7191908 | doi = 10.1080/14756366.2020.1755852 }}</ref>

== Structure and function ==

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.<ref>{{cite journal | vauthors = Krishnamurthy VM, Kaufman GK, Urbach AR, Gitlin I, Gudiksen KL, Weibel DB, Whitesides GM | title = Carbonic anhydrase as a model for biophysical and physical-organic studies of proteins and protein-ligand binding | journal = Chemical Reviews | volume = 108 | issue = 3 | pages = 946–1051 | date = March 2008 | pmid = 18335973 | pmc = 2740730 | doi = 10.1021/cr050262p }}</ref>

The primary function of the enzyme in animals is to interconvert carbon dioxide and bicarbonate to maintain [[Acid–base homeostasis|acid-base balance]] in blood and other tissues, and to help transport carbon dioxide out of tissues.

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.{{citation needed|date=February 2014}}

==Cadmium-containing carbonic anhydrase==
Marine [[diatom]]s have been found to express a new form of ζ carbonic anhydrase. [[Thalassiosira weissflogii|''T. weissflogii'']], a species of [[phytoplankton]] common to many marine ecosystems, was found to contain carbonic anhydrase with a [[cadmium]] ion in place of zinc.<ref name="Park et al">{{cite journal |vauthors=Park H, McGinn PJ, More FM |title=Expression of cadmium carbonic anhydrase of diatoms in seawater |journal=Aquatic Microbial Ecology|date=19 May 2008|volume=51|pages=183–193 |doi=10.3354/ame01192 |doi-access=free}}</ref> Previously, it had been believed that cadmium was a toxic metal with no biological function whatsoever. However, this species of phytoplankton appears to have adapted to the low levels of zinc in the ocean by using cadmium when there is not enough zinc.<ref name="Lane et al">{{cite journal |vauthors=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 |bibcode=2005Natur.435...42L |s2cid=52819760 |doi-access=free}}</ref> Although the concentration of cadmium in sea water is also low (about 1x10<sup>−16</sup> [[Molar concentration|molar]]), there is an environmental advantage to being able to use either metal depending on which is more available at the time. This type of carbonic anhydrase is therefore cambialistic, meaning it can interchange the metal in its active site with other metals (namely, zinc and cadmium).{{sfnp|Bertini |Gray |Stiefel |Valentine |2007 |p={{page needed|date=December 2022}}}}

===Similarities to other carbonic anhydrases===
The mechanism of cadmium carbonic anhydrase (CDCA) is essentially the same as that of other carbonic anhydrases in its conversion of carbon dioxide and water into bicarbonate and a proton.<ref name="Book on Cadmium">{{cite book |vauthors=Sigel A, Sigel H, Sigel RK |title=Cadmium from toxicity to essentiality|date=2013|publisher=Springer|location=Dordrecht|isbn=978-94-007-5179-8 }}</ref> Additionally, like the other carbonic anhydrases, CDCA makes the reaction go almost as fast as the diffusion rate of its substrates, and it can be inhibited by [[sulfonamide]] and [[sulfamate]] derivatives.<ref name="Book on Cadmium" />

===Differences from other carbonic anhydrases===
Unlike most other carbonic anhydrases, the active site metal ion is not bound by three histidine residues and a hydroxide ion. Instead, it is bound by two [[cysteine]] residues, one histidine residue, and a hydroxide ion, which is characteristic of β-CA.<ref name="Book on Cadmium" /><ref name="Xu et al">{{cite journal | vauthors = Xu Y, Feng L, Jeffrey PD, Shi Y, Morel FM | title = Structure and metal exchange in the cadmium carbonic anhydrase of marine diatoms | journal = Nature | volume = 452 | issue = 7183 | pages = 56–61 | date = March 2008 | pmid = 18322527 | doi = 10.1038/nature06636 | bibcode = 2008Natur.452...56X | s2cid = 4420280 }}</ref> Due to the fact that cadmium is a [[HSAB theory|soft acid]], it will be more tightly bound by [[HSAB theory|soft base]] ligands.{{sfnp|Bertini |Gray |Stiefel |Valentine |2007 |p={{page needed|date=December 2022}}}} The [[sulfur]] atoms on the cysteine residues are soft bases, thus binding the cadmium more tightly than the nitrogen on histidine residues would. CDCA also has a three-dimensional folding structure that is unlike any other carbonic anhydrase, and its amino acid sequence is dissimilar to the other carbonic anhydrases.<ref name="Book on Cadmium" /> It is a [[monomer]] with three domains, each one identical in amino acid sequence and each one containing an active site with a metal ion.<ref name="Xu et al" />

Another key difference between CDCA and the other carbonic anhydrases is that CDCA has a mechanism for switching out its cadmium ion for a zinc ion in the event that zinc becomes more available to the phytoplankton than cadmium. The active site of CDCA is essentially "gated" by a chain of nine [[amino acid]]s with [[glycine]] residues at positions 1 and 9. Normally, this gate remains closed and the cadmium ion is trapped inside. However, due to the flexibility and position of the glycine residues, this gate can be opened in order to remove the cadmium ion. A zinc ion can then be put in its place and the gate will close behind it.<ref name="Book on Cadmium" /> As a [[HSAB theory|borderline acid]], zinc will not bind as tightly to the cysteine ligands as cadmium would, but the enzyme will still be active and reasonably efficient. The metal in the active site can be switched between zinc and cadmium depending on which one is more abundant at the time. It is the ability of CDCA to utilize either cadmium or zinc that likely gives ''T. weissflogii'' a survival advantage.<ref name="Lane et al" />

===Transport of cadmium===
Cadmium is still considered lethal to phytoplankton in high amounts. Studies have shown that ''T. weissflogii'' has an initial toxic response to cadmium when exposed to it. The toxicity of the metal is reduced by the transcription and translation of [[phytochelatin]], which are proteins that can bind and transport cadmium. Once bound by phytochelatin, cadmium is no longer toxic, and it can be safely transported to the CDCA enzyme.<ref name="Park et al" /> It's also been shown that the uptake of cadmium via phytochelatin leads to a significant increase in CDCA expression.<ref name="Park et al" />

===CDCA-like proteins===
Other phytoplankton from different water sources have been tested for the presence of CDCA. It was found that many of them contain proteins that are homologous to the CDCA found in ''T. weissflogii''.<ref name="Park et al" /> This includes species from [[Great Bay (New Jersey)|Great Bay, New Jersey]] as well as in the [[Pacific Ocean]] near the [[equator]]. In all species tested, CDCA-like proteins showed high levels of expression even in high concentrations of zinc and in the absence of cadmium.<ref name="Park et al" /> The similarity between these proteins and the CDCA expressed by ''T. weissflogii'' varied, but they were always at least 67% similar.<ref name="Park et al" />

== Carbon capture and sequestration ==
{{further|Carbon capture and storage}}
Carbonic anhydrase could in principle prove relevant to [[Carbon capture and storage|carbon capture]]. Some carbonic anhydrases can withstand temperatures up to 107&nbsp;°C and extreme alkalinity (pH > 10).<ref name=":0">{{cite journal | vauthors = Alvizo O, Nguyen LJ, Savile CK, Bresson JA, Lakhapatri SL, Solis EO, Fox RJ, Broering JM, Benoit MR, Zimmerman SA, Novick SJ, Liang J, Lalonde JJ | display-authors = 6 | title = Directed evolution of an ultrastable carbonic anhydrase for highly efficient carbon capture from flue gas | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 111 | issue = 46 | pages = 16436–41 | date = November 2014 | pmid = 25368146 | pmc = 4246266 | doi = 10.1073/pnas.1411461111 | bibcode = 2014PNAS..11116436A | doi-access = free }}</ref> A pilot run with the more stable CA on a flue stream that consisted of 12–13% mol composition CO₂ had a capture rate of 63.6% over a 60-hour period with no noticeable effects in enzyme performance. CA was placed in a N-methyldiethanolamine (MDEA) solution where it served to increase the concentration difference (driving force) of CO<sub>2</sub> between the flue stream of the power plant and liquid phase in a liquid-gas contactor.<ref name=":0" />

== See also ==
* [[Carbonic anhydrase inhibitor]]

== References ==
{{reflist}}

== Further reading ==
{{refbegin}}
* {{cite journal | vauthors = Lyall V, Alam RI, Phan DQ, Ereso GL, Phan TH, Malik SA, Montrose MH, Chu S, Heck GL, Feldman GM, DeSimone JA | display-authors = 6 | title = Decrease in rat taste receptor cell intracellular pH is the proximate stimulus in sour taste transduction | journal = American Journal of Physiology. Cell Physiology | volume = 281 | issue = 3 | pages = C1005-13 | date = September 2001 | pmid = 11502578 | doi = 10.1152/ajpcell.2001.281.3.C1005 | title-link = intracellular pH }}
{{refend}}

== External links ==
*{{PDBe-KB2|P00918|Human Carbonic anhydrase 2}}	

{{Carbonic anhydrases|state=expand}}
{{Amphetamine}}
{{Carbon-oxygen lyases}}
{{Enzymes}}
{{Portal bar|Biology|border=no}}

{{DEFAULTSORT:Carbonic Anhydrase}}
[[Category:EC 4.2.1]]
[[Category:Zinc enzymes]]