Opening image: Human deoxyHb
2,3-Bisphosphoglycerate (BPG) binds only to deoxyhemoglobin. Therefore, BPG lowers the oxygen affinity of hemoglobin by shifting the T ⇌ R equilibrium to the left.
Top view of BPG pocket. Four pairs of positively charged groups (CPK) form ionic bonds with BPG. These groups reside in the gap between the β chains. Consequently, only one binding site for BPG exists, and it exists only in the deoxy- conformation.
Click for a side view of binding pocket.
Click to load a model of 2,3-bisphosphoglycerate (BPG). BPG has one negatively charged carboxylate and two negatively charged phosphate groups.
spin. Note that a gap also exists between the α chains. However, BPG cannot bind to that gap because there is no complemetary ring of positive charges.
We can see this if we the model by the formal charge on the atoms and compare the interface of the to the interface of the .
between deoxy- and oxy- conformations. Be sure to check out a side view of the BPG complex.
The BPG binding site no longer exists in oxyHb because the β chains close together when oxygen binds. Hemoglobin cannot bind oxygen and BPG at the same time.