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# carbon monoxide

carbon monoxide, chemical compound, CO, a colorless, odorless, tasteless, extremely poisonous gas that is less dense than air under ordinary conditions. It is very slightly soluble in water and burns in air with a characteristic blue flame, producing carbon dioxide; it is a component of producer gas and water gas, which are widely used artificial fuels. It is a reducing agent, removing oxygen from many compounds and is used in the reduction of metals, e.g., iron (see blast furnace), from their ores. At high pressures and elevated temperatures it reacts with hydrogen in the presence of a catalyst to form methanol. Carbon monoxide is formed by combustion of carbon in oxygen at high temperatures when there is an excess of carbon. It is also formed (with oxygen) by decomposition of carbon dioxide at very high temperatures (above 2,000°C;). It is present in the exhaust of internal-combustion engines (e.g., in automobiles) and is generated in coal stoves, furnaces, and gas appliances that do not get enough air (because of a faulty draft or for other reasons).

Carbon monoxide is an extremely poisonous gas. Breathing air that contains as little as 0.1% carbon monoxide by volume can be fatal; a concentration of about 1% can cause death within a few minutes. The gas is especially dangerous because it is not easily detected by human senses. Early symptoms of carbon monoxide poisoning include drowsiness and headache, followed by unconsciousness, respiratory failure, and death. First aid for a victim of carbon monoxide poisoning requires access to fresh air; administration of artificial respiration and, if available, oxygen; and, as soon as possible, expert medical attention. When carbon monoxide is inhaled, it reacts with hemoglobin, the red blood pigment that normally carries oxygen to all parts of the body. Because carbon monoxide is attracted to the hemoglobin about 210 times as strongly as is oxygen, it takes the place of oxygen in the blood, causing oxygen starvation throughout the body. Carbon monoxide detectors for homes are now readily available.

Carbon monoxide from automobile and industrial emissions is a dangerous pollutant that may contribute to the greenhouse effect and global warming. In urban areas carbon monoxide, along with aldehydes, react photochemically to produce peroxy radicals. Peroxy radicals react with nitrogen oxide to increase the ratio of NO2 to NO, which reduces the quantity of NO that is available to react with ozone (see smog). Carbon monoxide is also a constituent of tobacco smoke.

Inorganic compound, a highly toxic, colourless, odourless, flammable gas, chemical formula CO. It is produced when carbon (including coal and coke) or carbon-containing fuel (including petroleum hydrocarbons; e.g., gasoline, fuel oil) does not burn completely to carbon dioxide, because of insufficient oxygen. CO is present in the exhaust gases of internal combustion engines and furnaces. It is toxic because it binds to hemoglobin in blood much more strongly than does oxygen and thus interferes with transport of oxygen from lungs to tissues (see hypoxia; respiration). Symptoms of CO poisoning range from headache, nausea, and syncope to coma, weak pulse, respiratory failure, and death. CO is used industrially as a fuel and in synthesis of numerous organic compounds, including methanol, ethylene, and aldehydes.

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In enzymology, a carbon-monoxide dehydrogenase (acceptor) is an enzyme that catalyzes the chemical reaction

CO + H2O + A $rightleftharpoons$ CO2 + AH2

The 3 substrates of this enzyme are CO, H2O, and A, whereas its two products are CO2 and AH2.

This enzyme belongs to the family of oxidoreductases, specifically those acting on the aldehyde or oxo group of donor with other acceptors. The systematic name of this enzyme class is carbon-monoxide:acceptor oxidoreductase. Other names in common use include anaerobic carbon monoxide dehydrogenase, carbon monoxide oxygenase, carbon-monoxide dehydrogenase, and carbon-monoxide:(acceptor) oxidoreductase. This enzyme participates in methane metabolism. It has 4 cofactors: iron, Zinc, Nickel, and Iron-sulfur.

## Structural studies

As of late 2007, 13 structures have been solved for this class of enzymes, with PDB accession codes , , , , , , , , , , , , and .

## References

• Bonam D, Ludden PW "Purification and characterization of carbon monoxide dehydrogenase, a nickel, zinc, iron-sulfur protein, from Rhodospirillum rubrum". J. Biol. Chem. 262 2980–7.
• Diekert G, Ritter M "Purification of the nickel protein carbon monoxide dehydrogenase of Clostridium thermoaceticum". FEBS. Lett. 151 41–4.
• Drennan CL, Heo J, Sintchak MD, Schreiter E, Ludden PW "Life on carbon monoxide: X-ray structure of Rhodospirillum rubrum Ni-Fe-S carbon monoxide dehydrogenase". Proc. Natl. Acad. Sci. U. S. A. 98 11973–8.
• Dobbek H, Svetlitchnyi V, Gremer L, Huber R, Meyer O "Crystal structure of a carbon monoxide dehydrogenase reveals a [Ni-4Fe-5S] cluster". Science. 293 1281–5.