Recall that in deoxymyoglobin, the Fe(II) atom has only five ligands and lies 0.6 Å below the plane of the heme, in the direction of His F8. When O2 binds, the iron is pulled back toward the porphyrin plane and is now only 0.2 Å out of the plane. This small movement causes a number of small structural changes in the tertiary structure. Here we shall focus on the realignment of the imidazole ring of the proximal histidine, His F8.
His F8 is tightly packed by its surroundings. The close contacts force the imidazole ring away from a preferred perpendicular alignment with the plane of the heme.