Haemoglobin: Structure and Types

Key points

• • Each haemoglobin molecule has four subunits, each a globin protein chain wrapped around a haem group, and each haem holds one iron atom that binds one oxygen molecule.
• • One haemoglobin molecule can therefore carry up to four oxygen molecules, and the binding is cooperative, which gives the oxygen curve its S-shape.
• • Adult haemoglobin (HbA) has two alpha and two beta chains, while a minor adult form (HbA2) has two alpha and two delta chains.
• • Foetal haemoglobin (HbF) has two alpha and two gamma chains and binds oxygen more tightly than adult haemoglobin, which helps the foetus draw oxygen across the placenta.
• • Faults in the globin chains cause disease: a single DNA base change swaps glutamate for valine at position 6 of the beta chain to give sickle cell haemoglobin (HbS), and reduced chain production causes thalassaemia.
• • The iron in haem must stay in the ferrous (Fe2+) state to carry oxygen; oxidation to the ferric (Fe3+) state forms methaemoglobin, whose oxidised haem cannot carry oxygen.