Final answer:
The transition from the T state to the R state in hemoglobin is triggered by the binding of oxygen, which results in structural changes that increase hemoglobin's affinity for oxygen.
Step-by-step explanation:
In hemoglobin, the transition from the T (tense) state to the R (relaxed) state is triggered by the binding of oxygen. In the lungs, where the partial pressure of oxygen is high, oxygen molecules bind to one of the hemoglobin chains, leading to the rupture of salt bridges between the four subunits. This initial binding of oxygen promotes further oxygen binding due to the cooperative nature of hemoglobin, represented by a sigmoid curve in the hemoglobin-oxygen association. The rupture of the salt bridges consequently alters the secondary, tertiary, and quaternary structures of hemoglobin, allowing rotation of one alpha/beta subunit with respect to another, compressing the tetramer. The release of 2,3-bisphosphoglycerate (2,3-BPG) occurs, which increases hemoglobin's affinity for oxygen and promotes the transition into the R state, facilitating oxygen transport throughout the body.