Allosteric Effects
Bisphosphoglycerate (BPG) cannot bind to the oxygenated R state of hemoglobin because

BPG binds to the R state with the same affinity as the T state
it is displaced from the heme by movement of the proximal histidine
it is displaced from the heme by oxygen
its binding pocket becomes too small to accommodate BPG

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Allosteric Effects
The conformational changes from the T to the R state is initiated by

movement of the proximal histidine towards the heme
binding of oxygen to the heme
movement of the F-helix, which contains the proximal His
reorganization of protein-protein contacts between the individual subunits

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Allosteric Effects
O2 binding to hemoglobin results in

100-fold lower affinity for the last O2 bound than for the first
extensive protein conformational change
100-fold higher affinity for the last O2 bound than for the first
both (a) and (b)

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