Deoxyhemoglobin is held in the T-state partly by ion pair interactions between the C-terminal residues of the subunits. Upon oxygenation, the earliest structural event that initiates the T-to-R transition is:

• A Rupture of the salt bridge between His146 beta and Asp94 beta, releasing a proton
• B Rotation of the alpha1-beta2 interface by 15 degrees, opening the central cavity
• C Expulsion of 2,3-BPG from the central cavity between the beta chains
• D Movement of the ferrous iron into the plane of the porphyrin ring, tugging the proximal histidine and the F helix ✓

Explanation

In deoxyhemoglobin the high-spin Fe2+ sits slightly out of the porphyrin plane toward the proximal histidine. Binding of oxygen shrinks the iron to its low-spin form, allowing it to move into the porphyrin plane. This pulls the F8 histidine and the F helix, transmitting strain through the subunit interfaces, breaking terminal salt bridges, and rotating the dimers into the R-state. Salt bridge rupture and BPG expulsion are consequences, not the initiating event.

Reference: Lehninger Principles of Biochemistry, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.