And so when the binding takes place, there is a change in surface to surface interaction between these two adjacent polypeptide chains. Notice the distance here and the distance here basically changes. And this changes the electrostatic interaction between these two polypeptide chains. 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.