Figure 7.9: Hill plots of oxygen binding for myoglobin and hemoglobin.

Interpretation: Wholly deoxygenated hemoglobin molecules are in the deoxy conformation, so as oxygen is added to a solution of such molecules, the binding initially occurs along the line corresponding to the weak-binding state. But partial oxygenation favors transition to the strong-binding oxy state. As oxygen is bound, more and more of the remaining available sites are in hemoglobin molecules that have this conformation. Therefore, the binding curve passes over to that for the strong-binding state. As the last few sites are filled, all the molecules have adopted the strong-binding form.