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.
- 1. Kim E. Barrett, Susan M. Barman, Scott Boitano, Heddwen Brooks. Ganong's Review of Medical Physiology 25th Edition. (2015) ISBN: 9780071848978