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.

In deoxymyoglobin, the Fe-imidazole N bond is tilted 8° from the perpendicular to the plane of the porphyrin. This slight tilt is more apparent if the Fe-N bond is oriented vertically as shown here. Since the porphyrin is not perpendicular to the screen, it wobbles slightly as it rotates.

The two imidazole carbons nearest the heme are shown in two shades of green.

Note the close contact between the H atom on the light green carbon and the porphyrin ring. Because the imidazole ring is tilted, the hydrogen on the dark green carbon does not make contact with the porphyrin ring. Keep these spatial relationships in mind as we switch to oxymyoglobin.

Wireframe model showing linkages to heme iron in oxymyoglobin. Oxygen binding requires that the Fe move into the heme plane. However, the Fe can move only if the imidazole group moves. Toggle spacefill.

As the Fe moves into the heme plane, the imidazole ring rotates about 8°, with the H atom on the light green C atom serving as the fulcrum. Thus, His F8 acts a lever, forcing the entire F helix to slide about 1 Å across the heme plane. Furthermore, the polypeptide segments connecting the F helix to the E and G helices must move in concert with the F helix. This slight change in the tertiary structure is trival as far as the biological function of myoglobin is concerned.

Toggle between deoxy and oxy configurations. The 0.4 Å change in the position of the iron atom illustrated here is the fundamental cause of the change in the quaternary structure of hemoglobin underlying its allosteric behavior.