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Last revised by Dr Daniel J Bell on 27 Jul 2022

Hemoglobin (Hb) is the oxygen-carrying molecule in red blood cells.

Hemoglobin is a tetrameric protein molecule composed of four subunits. Each subunit consists of an iron-containing cyclic heme component linked to a polypeptide chain, the polypeptides are together known as globin. Each hemoglobin molecule comprises two pairs of polypeptide chains.

In hemoglobin A (HbA), which is the predominant form in adults, the two polypeptide chains are termed the α chains and β chains, and so hemoglobin A is styled α2β2. A small percentage (2.5%) of the normal hemoglobin in the adult is hemoglobin A2 (HbA2) which contains δ chains instead of β chains, thus α2δ2.

Oxygen reversibly binds to the iron (Fe2+) atom within each heme moiety, forming oxyhemoglobin, following release of the oxygen it becomes deoxyhemoglobin. The affinity with which oxygen binds to the hemoglobin is modified by pH (Bohr effect 2), body temperature and the concentration within red cells of a molecule known as 2,3-bisphosphoglycerate (2,3-BPG), dependent - in turn - on the partial pressure of oxygen (pO2) 3,4. When oxygen is released

Carbon monoxide (CO) binds to hemoglobin forming carboxyhemoglobin (COHb). Carbon monoxide has a stronger affinity than oxygen for hemoglobin, thus displacing the oxygen from the hemoglobin, such that less oxygen is available to the tissues. 

See: carbon monoxide poisoning

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