The haemoglobin A molecule has four polypeptide chains (2 a and 2 P chains), each of which has a covalently bound haem group consisting of a porphyrin ring with a central iron atom in the ferrous (Fe2+) state. A single oxygen molecule can bind to the central iron atom of each haem group. The cross links between the polypeptide chains consist of non covalent electrostatic interactions (salt links). There are two distinct types of salt link in the haemoglobin molecule (Figure HA.1).
• The aipi (a2p2) interaction is responsible for stabilization of the molecule
• The aip2 (a2pi) interaction is located close to the haem group and undergoes conformational change with oxygenation, this causing the P chains to rotate apart by about 0.7 nm
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