Question

Explain the allosteric regulation of hemoglobin by BPG, by referring to the structural changes in hemoglobin induced during binding

Answer 1

Solution

• The Bisphosphoglycerate (BPG) binds to deoxyhemoglobin with larger bonding affinity, such that it makes the T state of hemoglobin protein more stable or increases oxygen affinity of the Hb protein.

• The BPG binds to hemoglobin in the center of the tetramer to stabilize the T state.

• The hemoglobin wants to change into a more favorable R state due to the fact that the T state is quite unstable. Pure hemoglobin (without BPG) was tested and found to bind oxygen much more vigorously than hemoglobin in blood.

• A Hb protein with BPG has a lower affinity for oxygen binding in the tissue which allows it to be a better oxygen transporter than a pure hemoglobin, which does not have 2,3-BPG.

• But When BPG is present, it transports about 66% of oxygen while the pure hemoglobin only transports about 8%. The reason is that the BPG binds inside of the hemoglobin and somehow stabilizes its T state (the state that has less affinity for oxygen).

• When enough oxygen has bonded to hemoglobin, a transition occurs from the T state to the R state, which releases the BPG.

• BPG stays in the hemoglobin until enough oxygen has come to replace it. This keeps the oxygen in its T state until it is ready to transition to the R state (where its affinity for oxygen increases dramatically).

• The biological function of BPG is to control bonding between hemoglobin and oxygen molecules for oxygen to be released to body tissues.