III. The BPG Binding Site in DeoxyHb

Opening image: Human deoxyHbA.  2,3-bisphosphoglycerate (BPG) binds only to the deoxyhemoglobin. Thus, BPG lowers the oxygen affinity of hemoglobin by shifting the T <=> R equilibrium to the left.
  Top view of binding pocket for BPG. Four pairs of positively charged groups (magenta) form ionic bonds with BPG. These groups reside in the gap between the beta chains. Consequently, only one binding site for BPG exists, and it exists only in the deoxy conformation.
  Side view of deoxyHb.
  Toggle rotation. Note that a gap also exists between the alpha chains. However, BPG cannot bind to that gap because there is no complemetary ring of positive charges.
  Toggle between deoxy and oxy conformations. In oxyHb the beta chains are closer together and the BPG binding site no longer exists. The Hb molecule cannot bind oxygen and BPG at the same time.

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