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. The affinity with which oxygen binds to the hemoglobin is modified by pH, body temperature and the concentration within red cells of a molecule known as 2,3-bisphosphoglycerate (2,3-BPG).

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

Article information

rID: 72000
System: Haematology
Section: Pathology
Synonyms or Alternate Spellings:
  • Oxyhemoglobin
  • Adult haemoglobin
  • HbA2
  • HbA
  • Hemoglobins
  • Haemoglobins
  • Hemoglobin

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