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

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

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

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

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