Question

The Hb O2 binding curve provides an excellent example of the principle that living systems operate on the basis of weak non covalent interactions. Starting with the curve itself, explain how processes involving weak non covalent interactions within the structure of Hb are responsible for the sigmoid O2 binding curve.

Answer 1

Haemoglobin is made up of four symmetrical subunits and four heme groups. Iron associated with the heme binds oxygen. The binding of oxygen to the heme group of one subunit has the effect of increasing the affinity of a neighbouring subunit. Oxygenation cannot occur unless some of these salt links are broken so that the iron atom can move into the plane of the heme group. The number of salt links that need to be broken for the binding of an oxygen molecule depends on whether it is the first, second, third, or fourth to be bound. More salt links must be broken to permit the entry of the first oxygen molecule than of subsequent ones. Because energy is required to break salt links, the binding of the first oxygen molecule is energetically less favourable than that of subsequent oxygen molecules.

The sigmoidal shape of the binding curve allows more oxygen to be delivered from the lungs to the tissues than if the curve was hyperbolic. The non‐covalent interactions between subunits, quaternary structure, are responsible for haemoglobin working as a good oxygen transporter, binding oxygen in a cooperative way suggesting oxygen binding to one chain helps the binding of oxygen in other three chains too.