Allosteric Effects
An allosteric activator

stabilizes the R state of the protein
decreases the binding affinity
both (a) and (c)
increases the binding affinity

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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
its binding pocket becomes too small to accommodate BPG
it is displaced from the heme by movement of the proximal histidine
it is displaced from the heme by oxygen

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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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