Upon binding oxygen, the ferrous iron of hemoglobin undergoes a change that initiates the conformational transition from the T-state to the R-state. This initiating event is:
- A Movement of the iron atom into the plane of the porphyrin ring, pulling the F8 histidine toward the heme ✓
- B Oxidation of Fe2+ to Fe3+, shrinking the ionic radius
- C Displacement of the distal histidine E7 away from the heme pocket
- D Dissociation of the alpha-beta dimers followed by random reassociation
Correct answer: A. Movement of the iron atom into the plane of the porphyrin ring, pulling the F8 histidine toward the heme
Explanation
Oxygen binding converts iron from the high-spin to the low-spin state, decreasing its radius so it can move into the porphyrin plane. This pulls the coordinated F8 histidine and the attached F helix, triggering rupture of T-state salt bridges and the quaternary shift to the R-state. Iron stays ferrous in functional hemoglobin; oxidation gives methemoglobin. The distal histidine does not coordinate iron, and the alpha-beta dimers remain associated throughout.
Reference: Lehninger Principles of Biochemistry, 7th ed.
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