Answer:
Oxygen binds with the hemoglobin to carry the blood to different body parts through the circulatory system. The binding of the oxygen to Hb is regulated by the Bisphosphoglycerate or BPG molecule. It is bounded to the deoxy form of hemoglobin.
BPG binds with deoxy Hb in the central cavity of the four subunits which decreases the affinity to hemoglobin. The binding of BPG stabilizes T form or deoxy-Hb as the central cavity is larger in T form in comparison to the R state. R state has a low affinity to oxygen due to relatively smaller central cavity.