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.
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|
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.
The reaction catalyzed by carbonic anhydrase is:
- HCO3− + H+ CO2 + H2O
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.
- the cytosolic CAs (CA-I, CA-II, CA-III, CA-VII and CA XIII) (CA1, CA2, CA3, CA7, CA13)
- mitochondrial CAs (CA-VA and CA-VB) (CA5A, CA5B)
- secreted CAs (CA-VI) (CA6)
- membrane-associated CAs (CA-IV, CA-IX, CA-XII, CA-XIV and CA-XV) (CA4, CA9, CA12, CA14)
|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.
Structure and functionEdit
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.
Cadmium-containing carbonic anhydraseEdit
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).
Similarities to other carbonic anhydrasesEdit
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.
Differences from other carbonic anhydrasesEdit
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.
Transport of cadmiumEdit
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.
Carbon capture and sequestrationEdit
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.
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