Remember that Fe2+ ion is coordinated by the 4 N of the tetrapyrrolic ring system and by the N of proximal histidine. However, observe that the haem group in deoxyhaemoglobin is not completely flat, but suffers a slight deformation due to iron getting closer to the proximal histidine.
When the oxygen molecule binds, it pulls the ferrous ion, so that this becomes more coplanar with the protoporphyrin ring:
As a result of such a shift of Fe2+ while binding oxygen, proximal histidine also shifts, and this in turn induces a slight conformational change in the remainder of the protein. As a consequence, each subunit in haemoglobin alternates between two conformations, the “tense” conformation or T form when it is deoxygenated and the “relaxed” conformation or R form when it is oxygenated.
Let's see it, for instance, with the subunit:
The conformational change also afects the interactions among the 4 subunits :
This is the molecular basis of the allosterism observed in haemoglobin, which produces cooperativity in oxygen binding and release. We will come back to this topic after studying how is the interaction among the 4 subunits.