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carboxyhemoglobin
Carboxyhemoglobin (British English: Carboxyhaemoglobin) (COHb) is a stable complex of carbon monoxide and hemoglobin that forms in red blood cells upon contact with carbon monoxide (CO). Large quantities of CO hinders the ability of Hb to deliver oxygen to the body. CO is produced in normal metabolism and is also a common chemical. Tobacco smoking (through carbon monoxide inhalation) raises the blood levels of COHb by a factor of several times from its normal concentrations.
==Affinity of Hb for CO==
Hemoglobin contains four heme groups each capable of reversibly binding to one oxygen molecule.〔Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002.〕 Oxygen binding to any of these sites causes a conformational change in the protein, facilitating binding to each of the other sites. Carbon monoxide binds to hemoglobin at the same sites as oxygen, but approximately 200 times more tightly.〔Berg, Jeremy. Biochemistry. 7. W.H. Freeman Company, 2011.〕 Normally, oxygen would bind to hemoglobin in the lungs and be released in areas with low oxygen partial pressure (e.g. active muscles).〔Schmidt-Nielsen K. 1997. Animal Physiology, fifth ed. Cambridge University Press, Cambridge, UK.〕 When carbon monoxide binds to hemoglobin, it cannot be released as easily as oxygen. The slow release rate of carbon monoxide causes an accumulation of CO-bound hemoglobin molecules as exposure to carbon monoxide continues. Because of this, fewer hemoglobin particles are available to bind and deliver oxygen, thus causing the gradual suffocation associated with carbon monoxide poisoning.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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