Final answer:
When O₂ binds to hemoglobin, it causes a structural shift particularly in the position of helix E, transitions the heme iron from high-spin to low-spin and induces conformation changes from the T to R state, improving oxygen affinity and resulting in the characteristic sigmoidal oxygen dissociation curve.
Step-by-step explanation:
When O₂ binds to hemoglobin, the structural change that occurs is a shift in the position of helix E. This is due to the fact that oxygen binding to hemoglobin facilitates the rupture of salt bridges, which in turn alters the secondary, tertiary, and quaternary structures of hemoglobin, leading to a conformational change from the T (tense) state to the R (relaxed) state, thus increasing hemoglobin's affinity for oxygen.
As part of this process, the iron ion in heme transitions from a high-spin state to a low-spin state after oxygen binds, which changes the size of the iron ion and allows it to move within the plane of the porphyrin ring, contributing to the overall structural change. This structural change is crucial for the cooperative binding of oxygen, as evidenced by the sigmoidal oxygen dissociation curve of hemoglobin.