Final answer:
The conversion of hemoglobin subunits from deoxy to oxy state is primarily explained by tertiary structure changes, caused by the binding of oxygen to the iron ion in each subunit, triggering structural changes that increase affinity for additional oxygen.
Step-by-step explanation:
Conversion of hemoglobin subunits from the deoxy to oxy state is explained by the tertiary structure changes. This structural change is triggered by the binding of oxygen to the iron at the heart of the heme group in each subunit. As oxygen binds, it causes the iron to move into the plane of the porphyrin ring. This movement facilitates a change in the quaternary structure and ruptures the salt bridges between the subunits, increasing the molecule's affinity for oxygen and allowing further uptake of O₂.
In summary, the iron ion at the center of each subunit undergoes a change in its environment which changes its oxidation state, thus prompting a series of structural changes at various levels of the protein's structure, particularly the tertiary and quaternary structures. The affinity of hemoglobin for oxygen (the R state) increases, showing cooperativity among the subunits.