The cooperative binding of oxygen by hemoglobin relies on a conformational change from the T-state to the R-state. The initial trigger for this transition upon oxygen binding to a heme subunit is:

• A Expulsion of 2,3-bisphosphoglycerate from the central cavity of the tetramer
• B Rotation of the alpha1-beta2 interface by 15 degrees, disrupting the C-terminal salt bridges
• C Movement of the iron atom into the plane of the porphyrin ring, pulling the proximal histidine and the F helix ✓
• D Protonation of the distal histidine, which stabilizes the bound oxygen molecule

Explanation

Oxygen binding pulls the heme iron into the plane of the porphyrin ring. Because the proximal histidine (F8) is coordinated to the iron, it shifts, pulling the F helix and initiating the T to R transition. Option B describes a later step in the quaternary shift. Option A is a consequence of the R-state, not the initial trigger. Option D describes the role of the distal histidine (E7), which stabilizes O2 but does not trigger the quaternary change.

Reference: Lehninger Principles of Biochemistry, 7th ed.

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