Haemoglobin consists of four polypeptide chains, two alpha chains of 141 amino acid residues each and two beta chains of 146 amino acid residues each. Each chain harbours one haem, a cyclic tetrapyrrole. A single polypeptide chain a e combined with a single haem is called a subunit of haemoglobin or monomer 3
o of the molecule. In the complete molecule the four subunits are closely joined by o hydrogen bonding to form a tetramer. Functions of haem proteins include in oxygen binding, oxygen transport and electron transport.
Each polypeptide chain is divided into helical and non-helical areas. The helical areas are designated A to H. Amino acids can be designated by number or according to helical region. Each haem consists of protoporphyrin IX, with four nitrogen atoms co-ordinated to a ferrous ion, Fe2+. The Fe2+ is also co-ordinated to a nitrogen atom in a histidine residue of the globin part of the molecule. Oxygen binding causes alterations in the plane of the Fe2+, triggering a sequence of intermolecular rearrangements that are transmitted to the other subunits.
Haemoglobin is structurally adapted to bind strongly to oxygen in the lungs and to release it in oxygen depleted tissues. It can also transport some carbon dioxide back to the lungs for release.
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