Haemoglobin (Hb) is the oxygen-carrying molecule in red blood cells.
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Structure
Haemoglobin is a tetrameric protein molecule composed of four subunits. Each subunit consists of an iron-containing cyclic haem component linked to a polypeptide chain, the polypeptides are together known as globin. Each haemoglobin molecule comprises two pairs of polypeptide chains.
In haemoglobin A (HbA), which is the predominant form in adults, the two polypeptide chains are termed the α chains and β chains, and so haemoglobin A is styled α2β2. A small percentage (2.5%) of the normal haemoglobin in adults is haemoglobin A2 (HbA2) which contains δ chains instead of β chains, thus α2δ2.
Physiology
Oxygen reversibly binds to the iron (Fe2+) atom within each haem moiety, forming oxyhaemoglobin, following the release of the oxygen it becomes deoxyhaemoglobin. The affinity with which oxygen binds to the haemoglobin 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.
Pathology
Carbon monoxide (CO) binds to haemoglobin forming carboxyhaemoglobin (COHb). Carbon monoxide has a stronger affinity than oxygen for haemoglobin, thus displacing the oxygen from the haemoglobin, such that less oxygen is available to the tissues.