Carbonic anhydrase
The carbonic anhydrases form a family of enzymes that catalyze the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid. The active site of most carbonic anhydrases contains a zinc ion. They are therefore classified as metalloenzymes. The enzyme maintains acid-base balance and helps transport carbon dioxide.
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. 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 of the patient to reduce fluid build up thereby relieving pressure.
The Bohr Effect is a way to describe hemoglobin's oxygen binding affinity. The Bohr Effect was described by Christian Bohr in the year 1904, and it refers to a shift in an oxygen dissociation curve that is caused by a change in concentration of carbon dioxide or a change in the pH. Essentially an increase in carbon dioxide results in lowered blood pH which lowers oxygen-hemoglobin binding. 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 protons and bicarbonate ions.
To describe equilibrium in the carbonic anhydrase reaction, Le Chatelier's principle is used. The tissues are more acidic than the lungs because carbon dioxide is produced by cellular respiration and it reacts with water in the tissues to produce the hydrogen protons. Because the carbon dioxide concentration is higher, equilibrium shifts to the right, to the bicarbonate side. The opposite is seen in the lungs where carbon dioxide is being released so its concentration is lower so equilibrium shifts to the left towards carbon dioxide to try and raise its concentration.
Background
An enzyme is known as a substance that acts as a catalyst in living organisms which helps to speed up chemical reactions. Carbonic anhydrase is one important enzyme that is found in red blood cells, gastric mucosa, pancreatic cells, and even renal tubules. It is a very old enzyme that was discovered in the year 1932 and it has been categorized into three general classes. 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. 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 CO2 transport in the blood which in turn helps respiration. It can even function in the formation of hydrochloric acid by the stomach. Therefore, the role of carbonic anhydrase depends on where it is found in the body.Reaction
The reaction that shows the catalyzation of carbonic anhydrase in our tissues is: CO2 + H2O H2CO3 H+ + HCO3−. The catalyzation of carbonic anhydrase in the lungs is shown by: H+ + HCO3− H2CO3 CO2 + H2O. 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 107 times faster.The reaction catalyzed by carbonic anhydrase is:
Carbonic acid has a pKa of around 6.36, 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. 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.
Mechanism
A zinc prosthetic group in the enzyme is coordinated in three positions by histidine side-chains. The fourth coordination position is occupied by water. 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. The active site also features a pocket suited for carbon dioxide, bringing it close to the hydroxide group.Families
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.
α-CA
s, algae and some bacteria have this family of CAs.The CA enzymes found in mammals are divided into four broad subgroups, which, in turn consist of several isoforms:
- the cytosolic CAs
- mitochondrial CAs
- secreted CAs
- membrane-associated CAs
| Isoform | Gene | Molecular mass | Location | Location | Specific activity of human enzymes | Sensitivity to sulfonamides KI |
| CA-I | 29 kDa | cytosol | red blood cell and GI tract | 2.0 × 105 | 250 | |
| CA-II | 29 kDa | cytosol | almost ubiquitous | 1.4 × 106 | 12 | |
| CA-III | 29 kDa | cytosol | 8% of soluble protein in Type I muscle | 1.3 × 104 | 240000 | |
| CA-IV | 35 kDa | extracellular GPI-linked | GI tract, kidney, endothelium | 1.1 × 106 | 74 | |
| CA-VA | 34.7 kDa | mitochondria | liver | 2.9 × 105 | 63 | |
| CA-VB | 36.4 kDa | mitochondria | widely distributed | 9.5 × 105 | 54 | |
| CA-VI | 39–42 kDa | secretory | saliva and milk | 3.4 × 105 | 11 | |
| CA-VII | 29 kDa | cytosol | widely distributed | 9.5 × 105 | 2.5 | |
| CA-IX | 54, 58 kDa | cell membrane-associated | normal GI tract, several cancers | 1.1 × 106 | 16 | |
| CA-XII | 44 kDa | extracellularily located active site | kidney, certain cancers | 4.2 × 105 | 5.7 | |
| CA-XIII | 29 kDa | cytosol | widely distributed | 1.5 × 105 | 16 | |
| CA-XIV | 54 kDa | extracellularily located active site | kidney, heart, skeletal muscle, brain | 3.1 × 105 | 41 | |
| CA-XV | 34–36 kDa | extracellular GPI-linked | kidney, not expressed in human tissues | 4.7 × 105 | 72 |
β-CA
Most prokaryotic and plant chloroplast CAs belong to the beta family.Two signature patterns for this family have been identified:
- C--D-S-R--x-
- --A--x--x--x-G-H-x-C-G
γ-CA
δ-CA
The delta class of CAs has been described in diatoms. The distinction of this class of CA has recently come into question, however.ζ-CA
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.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.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.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 anhydrase
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, 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.Similarities to other carbonic anhydrases
The mechanism of cadmium carbonic anhydrase 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 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. 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.