The carbonic anhydrases (or carbonate dehydratases) form a family of enzymes that catalyze the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid (i.e. bicarbonate and hydrogen ions). The active site of most carbonic anhydrases contains a zinc ion. They are therefore classified as metalloenzymes.
Carbonic anhydrase helps 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, and if the enzyme does not work properly, a buildup of fluid can lead to glaucoma.
Ribbon diagram of human carbonic anhydrase II, with zinc ion visible in the center
|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / QuickGO|
|Eukaryotic-type carbonic anhydrase|
|SCOPe||1can / SUPFAM|
The reaction catalyzed by carbonic anhydrase is:
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.
Carbonic anhydrase is one of the fastest 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.
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.
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.
A zinc prosthetic group in the enzyme is coordinated in three positions by histidine side-chains. The fourth coordination position is occupied by water. A fourth histidine is close to the water ligand, facilitating formation of Zn-OH center, which binds CO2 to give a zinc bicarbonate. 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.
Carbonic anhydrase was initially found in the red blood cells of cows.
At least five distinct CA families are recognized: α, β, γ, δ and ζ. 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.
|Isoform||Gene||Molecular mass||Location (cell)||Location (tissue)||Specific activity of human enzymes (except for mouse CA XV) (s−1)||Sensitivity to sulfonamides (acetazolamide in this table) KI (nM)|
|CA-I||CA1||29 kDa||cytosol||red blood cell and GI tract||2.0 × 105||250|
|CA-II||CA2||29 kDa||cytosol||almost ubiquitous||1.4 × 106||12|
|CA-III||CA3||29 kDa||cytosol||8% of soluble protein in Type I muscle||1.3 × 104||240000|
|CA-IV||CA4||35 kDa||extracellular GPI-linked||GI tract, kidney, endothelium||1.1 × 106||74|
|CA-VA||CA5A||34.7 kDa (predicted)||mitochondria||liver||2.9 × 105||63|
|CA-VB||CA5B||36.4 kDa (predicted)||mitochondria||widely distributed||9.5 × 105||54|
|CA-VI||CA6||39–42 kDa||secretory||saliva and milk||3.4 × 105||11|
|CA-VII||CA7||29 kDa||cytosol||widely distributed||9.5 × 105||2.5|
|CA-IX||CA9||54, 58 kDa||cell membrane-associated||normal GI tract, several cancers||1.1 × 106||16|
|CA-XII||CA12||44 kDa||extracellularily located active site||kidney, certain cancers||4.2 × 105||5.7|
|CA-XIII||CA13||29 kDa||cytosol||widely distributed||1.5 × 105||16|
|CA-XIV||CA14||54 kDa||extracellularily located active site||kidney, heart, skeletal muscle, brain||3.1 × 105||41|
|CA-XV||CA15||34–36 kDa||extracellular GPI-linked||kidney, not expressed in human tissues||4.7 × 105||72|
The zeta class of CAs occurs exclusively in bacteria in a few chemolithotrophs and marine cyanobacteria that contain cso-carboxysomes. Recent 3-dimensional analyses 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 amino acid sequence has since diverged considerably.
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.
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 are 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.
Marine diatoms have been found to express a new form of ζ carbonic anhydrase. 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. 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. Although the concentration of cadmium in sea water is also low (about 1x10−16 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).
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. 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.
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. Due to the fact that cadmium is a soft acid, it will be more tightly bound by soft base ligands. 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. 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.
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 acids 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. As a 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.
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. It's also been shown that the uptake of cadmium via phytochelatin leads to a significant increase in CDCA expression.
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. This includes species from 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. The similarity between these proteins and the CDCA expressed by T. weissflogii varied, but they were always at least 67% similar.
Carbonic anhydrase could in principle prove relevant to carbon capture. Some carbonic anhydrases can withstand temperatures up to 107 °C and extreme alkalinity (pH > 10). 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 CO2 between the flue stream of the power plant and liquid phase in a liquid-gas contactor.
Acetazolamide, sold under the trade name Diamox among others, is a medication used to treat glaucoma, epilepsy, altitude sickness, periodic paralysis, idiopathic intracranial hypertension (raised brain pressure of unclear cause), and heart failure. It may be used long term for the treatment of open angle glaucoma and short term for acute angle closure glaucoma until surgery can be carried out. It is taken by mouth or injection into a vein.Common side effects include numbness, ringing in the ears, loss of appetite, vomiting, and sleepiness. It is not recommended in those with significant kidney problems, liver problems, or who are allergic to sulfonamides. Acetazolamide is in the diuretic and carbonic anhydrase inhibitor families of medication. It works by decreasing the amount of hydrogen ions and bicarbonate in the body.Acetazolamide came into medical use in 1952. It is on the World Health Organization's List of Essential Medicines, which lists the safest and most effective medicines needed in a health system. Acetazolamide is available as a generic medication. The wholesale cost in the developing world is about US$1.40–16.93 per month. In the United States the wholesale cost is about US$125.34 per month.Brinzolamide
Brinzolamide (trade names Azopt, Alcon Laboratories, Befardin, Fardi Medicals, ) is a carbonic anhydrase inhibitor used to lower intraocular pressure in patients with open-angle glaucoma or ocular hypertension.CA10
Carbonic anhydrase-related protein 10 is an enzyme that in humans is encoded by the CA10 gene.This gene encodes a protein that belongs to the carbonic anhydrase family of zinc metalloenzymes, which catalyze the reversible hydration of carbon dioxide in various biological processes. The protein encoded by this gene is an acatalytic member of the alpha-carbonic anhydrase subgroup, and it is thought to play a role in the central nervous system, especially in brain development. Multiple transcript variants encoding the same protein have been found for this gene.Carbonic anhydrase 4
Carbonic anhydrase 4 is an enzyme that in humans is encoded by the CA4 gene.Carbonic anhydrase 7
Carbonic anhydrase 7 (CA7) is an enzyme that in humans is encoded by the CA7 gene.Carbonic anhydrase 9
Carbonic anhydrase IX (CA9/CA IX) is an enzyme that in humans is encoded by the CA9 gene. It is one of the 14 carbonic anhydrase isoforms found in humans and is a transmembrane dimeric metalloenzyme with an extracellular active site that facilitates acid secretion in the gastrointestinal tract. CA IX is overexpressed in many types of cancer including clear cell renal cell carcinoma (RCC) as well as carcinomas of the cervix, breast and lung where it promotes tumor growth by enhancing tumor acidosis.Carbonic anhydrase II
Carbonic anhydrase II (gene name CA2), is one of sixteen forms of human α carbonic anhydrases . Carbonic anhydrase catalyzes reversible hydration of carbon dioxide. Defects in this enzyme are associated with osteopetrosis and renal tubular acidosis.
Renal carbonic anhydrase allows the reabsorption of bicarbonate ions in the proximal tubule.
Loss of carbonic anhydrase activity in bones impairs the ability of osteoclasts to promote bone resorption, leading to osteopetrosis.Carbonic anhydrase III, muscle specific
Carbonic anhydrase 3 is an enzyme that in humans is encoded by the CA3 gene.Carbonic anhydrase III (CAIII) is a member of a multigene family (at least six separate genes are known) that encode carbonic anhydrase isozymes. These carbonic anhydrases are a class of metalloenzymes that catalyze the reversible hydration of carbon dioxide and are differentially expressed in a number of cell types. The expression of the CA3 gene is strictly tissue-specific and present at high levels in skeletal muscle and much lower levels in cardiac and smooth muscle. CA3 is insufficient in muscles of Myasthenia Gravis patients. A proportion of carriers of Duchenne muscle dystrophy have a higher CA3 level than normal. Autoantibodies to CA3 have been found to be significantly higher in patients with rheumatoid arthritis, systemic lupus erythematosus, and type 1 diabetes. The gene spans 10.3 kb and contains seven exons and six introns.Carbonic anhydrase inhibitor
Carbonic anhydrase inhibitors are a class of pharmaceuticals that suppress the activity of carbonic anhydrase. Their clinical use has been established as anti-glaucoma agents, diuretics, antiepileptics, in the management of mountain sickness, gastric and duodenal ulcers, idiopathic intracranial hypertension, neurological disorders, or osteoporosis.Casuarinin
Casuarinin is an ellagitannin. It is found in the pericarp of pomegranates (Punica granatum). It is also found in Casuarina and Stachyurus species and in Alnus sieboldiana.It is an isomer of casuarictin. It is a highly active carbonic anhydrase inhibitor.Clofenamide
Clofenamide (or diumide) is a low-ceiling sulfonamide diuretic.Clopamide
Clopamide (trade name Brinaldix) is a piperidine diuretic.Diclofenamide
Diclofenamide (or dichlorphenamide) is a sulfonamide and a carbonic anhydrase inhibitor of the meta-disulfamoylbenzene class.Ethoxzolamide
Ethoxzolamide (alternatively known as ethoxyzolamide) is a sulfonamide medication that functions as a carbonic anhydrase inhibitor. It is used in the treatment of glaucoma and duodenal ulcers, and as a diuretic. It may also be used in the treatment of some forms of epilepsy.Granatin B
Granatin B is an ellagitannin found in the fruit of Punica granatum (pomegranate). It is a molecule having an enantiomeric dehydrohexahydroxydiphenoyl group.It is a highly active carbonic anhydrase inhibitor.Methazolamide
Methazolamide (trade name Neptazane) is a potent carbonic anhydrase inhibitor. It is indicated in the treatment of increased intraocular pressure (IOP) in chronic open-angle glaucoma and secondary glaucoma. Also it is used preoperatively in acute angle-closure (narrow-angle) glaucoma where lowering the IOP is desired before surgery.Methyclothiazide
Methyclothiazide is a thiazide diuretic.Punicalin
Punicalin is an ellagitannin. It can be found in Punica granatum (pomegranate) or in the leaves of Terminalia catappa, a plant used to treat dermatitis and hepatitis. It is also reported in Combretum glutinosum, all three species being Myrtales, the two last being Combretaceae.
It is a highly active carbonic anhydrase inhibitor.Sultiame
Sultiame, also known as sulthiame, is a sulfonamide and inhibitor of the enzyme carbonic anhydrase. It is used as an anticonvulsant.
|Available protein structures:|
|Pfam||structures / ECOD|
|PDB||RCSB PDB; PDBe; PDBj|
|4.2.2: Acting on polysaccharides|
|4.2.3: Acting on phosphates|