|MadSci Network: Biochemistry|
Sorry about the delay - once one oxygen binds, it gets successively easier to bind the second, third and fourth. The S-shaped curve indicates this cooperativity, but it levels out in the end because although it is successively easier to bind more oxygen, the increase in affinity is not as extreme once you get to the 3rd or fourth oxygen. There is a significant increase in affinity in going from 1 to 2 oxygen molecules bound.
The sigmoidal curve also reflects a hybrid between a 100% high-affinity (R) state (which would have an extremely steep hyperbolic curve), and a 100% low-affinity (T) state (which would have a very shallow hyperbolic curve).
Try the links in the MadSci Library for more information on Biochemistry.