Carbonic anhydrase

The α - carbonic anhydrase (abbreviation α -CA ) are enzymes which catalyze the hydration of carbon dioxide to form carbonic acid, and vice versa. They occur in animals, plants, bacteria and green algae. Carbon dioxide can be easily transported in the form of bicarbonate in the body, so a reversible transformation is useful. Also is controlled by the reaction, the pH of the blood plasma and the gastric acid. In animals eleven paralogous isoforms of the enzyme are known, and without remaining enzyme activity.

The eleven isoforms are on the localization divided into:

• Cytosolic (CA -I, CA -II, CA -III, CA VII, CA XIII)
• The mitochondria ( CA VA, CA VB)
• Secreted ( CA VI)
• Membrane- associated ( CA IV, CA IX, CA XII, CA XIV)

• 4.1 Other Functions
• 4.2 Reaction mechanism of carbonic anhydrase

Catalysed equilibrium

Carbon dioxide hydrates to form carbonic acid, and vice versa. Carbonic acid dissociates in aqueous solutions to bicarbonate and a proton:

Carbonic anhydrase is an enzyme that catalyzes very fast: it can be up to 106 molecules per second carbon dioxide hydrate, and accelerates the reaction of the 107 -fold. Your kcat value is 1,000,000 per second. The Michaelis constant Km amounts to 0.012 M.

For the reverse reaction kcat and Km = 400,000 per second is = 0.026 M.

Construction

CA was the first enzyme discovered that requires a metal cofactor in its active center. The protein contains as a cofactor a zinc ion which requires the actual catalytic activity of the enzyme. It is bound to three imidazole of the amino acid histidine. The fourth coordination site is occupied by a hydroxo ligand.

In rare cases, cadmium is as a cofactor in the active site of the enzyme bound (eg Thalassioria weissflogii ).

Functions in the organism

Breathing

During respiration falls carbon dioxide ( CO2) as a metabolic end product. Carbon dioxide is released into the blood and have to be transported to the lungs ( in complex organisms). In plants of carbon dioxide needed for photosynthesis and this was added through the stomata of the foliage. This eventually reaches the thylakoids and is implemented in the course of photosynthesis into glucose. In aqueous solutions, such as in the blood or in the cytoplasm, however, CO2 reacts with water to form bicarbonate. Although these reactions proceed spontaneously even without a catalyst, carbonic anhydrase is found in almost all organisms. If CO2 is absorbed or emitted depends on the pH of the cytoplasm. In the animal organism CA is found in erythrocytes ( red blood cells ), where it is involved in the Kohlenstoffdioxidtransport. Carbon dioxide diffuses into the capillaries in the red cells, where it is converted to carbonic acid with water and finally responds to bicarbonate ions and protons. The bicarbonate is exchanged in the so-called Hamburger -shift against chloride from the plasma to maintain the electroneutrality.

Eye

Carbonic anhydrase is regarded as a key enzyme for the production of aqueous humor in the eye. An excessive production or an impaired outflow of aqueous humor is regarded as one of the pathogenetic basis of glaucoma. The pharmacological inhibition of carbonic anhydrase with agents such as dorzolamide is therefore now an established option in the treatment of glaucoma.

Renal function

CA is inter alia important for the regulation of the acid-base balance through the kidney. This regulation is important that the primary urine filtered bicarbonate is reabsorbed to about 90 percent. Otherwise, would create a acidosis. Without the CA the bicarbonate reabsorption would not work from the primary. The mechanism: from the proximal tubule cells of the kidney are excreted by intracellularly by extracellularly ( in the lumen of the renal tubule ) protons (H ) in exchange for Na . The effect of CA filtered bicarbonate reacts with protons to form carbonic acid, then water and carbon dioxide. The carbon dioxide can pass (in contrast to bicarbonate ) easily through the cell membrane of the tubule lumen (primary ) in the tubule. There, the intracellular CA catalyzes the reverse reaction. Protons and hydrogen carbonate from the carbon dioxide formed in the tubule. The hydrogen is released from the cell into the blood, the proton is again for the same reaction. Can be inhibited by the drug acetazolamide carbonic anhydrase. The inhibition results in loss of bicarbonate in the urine and thus acidosis. This can be used for the treatment of metabolic alkalosis.

Stomach function

It is the key enzyme for the production of stomach acid. Carbonic anhydrase is located in the parietal cells ( ) of the stomach epithelium. Protons are transported to the apical membrane of epithelial cells by a proton- potassium ATPase (V- ATPase) in the gastric lumen. While potassium is transported into the cell in the exchange. In the cell, the CA protons and bicarbonate from water and carbon dioxide ago. At the basolateral membrane bicarbonate is then placed in the blood vessel system in exchange for chloride from the cell. The chloride is applied to the apical membrane chloride channel of a lumen in the stomach where it forms with the protons stomach acid.

The V- ATPase and carbonic anhydrase in the gastric parietal cells is inhibited by proton pump inhibitors such as omeprazole or pantoprazole.

CA -IV

In the ciliary body of the eye, the carbonic anhydrase CA IV is involved in the production of aqueous humor. Inhibition by acetazolamide, an excessive production ( relatively compared to removal from the eye ) reduce aqueous humor and thus help to reduce an excessive intraocular pressure glaucoma. Mutations in the CA4 gene can lead to a form of retinitis pigmentosa ( RP17 ).

Plant

Plants with Crassulacean acid metabolism and C4 metabolism need Carboanyhdrasen so that the dissolved carbon dioxide in the cytosol is converted rapidly to bicarbonate (HCO3-). HCO3- is used as the substrate of another enzyme, serving as a first preliminary Kohlenstoffdioxidfixierung.

Other Features

In the pancreas, the carbonic anhydrase of bicarbonate secretion is

The sulfonamides now rarely used also inhibit carbonic anhydrase.

In freshwater fish allows the carbonic anhydrase in specialized cells of the Kiemenepithels Salzresorption and thus serves osmoregulation.

Reaction mechanism of carbonic anhydrase

• To the zinc (II) ion, which is coordinated to the three Stickstoffdonorzentren of the imidazole ring of the three amino acids histidine, a molecule can anneal water. As a result of coordination to the zinc ion of the pKa value of the water molecule is lowered - so far, that even at the physiological pH value, as is to be found in the vicinity of carbonic anhydrase, a proton may be eliminated (1).

• In a next step (2) a carbon dioxide molecule to the active site of the enzyme superimposed in such a way that it can react with the hydroxide.
• The lone pairs of electrons on the oxygen of the OH group is nucleophilic attack on the Kohlenstoffdioxidmolekül so that a hydrogen carbonate ion is formed ( 3)
• Finally, a water molecule can be bound to the active site, so that the Hydrogenkarbonation is released (4).

Through this regeneration of the active site can begin a new cycle.