And it's this surface to surface interaction change that causes the cooperative behavior between the different polypeptide chains and the different heme groups found inside that polypeptide chain. So once again, as oxygen binds onto the heme group, as shown in the following diagram, it decreases the size of that iron atom, pulling it into the plane of the proteporphinen. And since the iron atom is attached onto the proximal histidine, it pulls on that proximal histidine. And that in turn pulls and shifts that entire polypeptide chain as shown in this diagram. So this shift causes a conformational change in the surface to surface interaction between the adjacent subunits. And this is precisely what leads to the cooperative behavior of the hemoglobin molecule.