Cofactor Binding and Oxygen Equilibria in Haemoglobin

THE oxygen affinity of haemoglobin is profoundly affected by the presence of low concentrations of organic phosphates, in particular 2,3-diphosphoglycerate (DPG)^1–5. This phenomenon, which has now been widely recognized as a physiologically vital control mechanism^6–10, can be explained qualitatively by our observations² that deoxyhaemoglobin has a single strong binding site for the cofactor, which in consequence stabilizes it in relation to the oxygenated state. The binding site is associated with the β chains¹¹ and it has been identified as the cavity at the conjunction of the N-termini of these chains^5,12–18, lying therefore on the dyad axis of the tetramer.