Question

In what sense does the MWC model invoke the concept of cooperativity?

Answer 1

In the MWC model, the oxygen binding seems, at first glance, to be totally noncooperative; the binding of an oxygen to a globin within a tense hemoglobin complex is explicitly assumed to have no effect on the affinities of other globins for oxygen. And the same is true of the binding of an oxygen to a globin within a relaxed complex. Instead, the cooperativity here is embodied in the notion that the whole hemoglobin complex flips between the tense and relaxed states as a unit.

This concerted conformation change has the effect of allowing the binding of the first oxygen to indirectly promote the binding of the second, and the second to promote the binding of the third, and the third to promote the binding of the fourth. This is because the binding of each oxygen makes the flip to the tight-binding state more favorable, and the flip to the tight-binding state makes the binding of the next oxygen more favorable.