The Iron Moves Toward the Plane of the Heme When Oxygen Is Bound

The iron of unoxygenated myoglobin lies 0.03 nm (0.3 A) outside the plane of the heme ring, toward His F8. The heme therefore "puckers" slightly. When O2 occupies the sixth coordination position, the iron moves to within 0.01 nm (0.1 A) of the plane of the heme ring. Oxygenation of myoglobin thus is accompanied by motion of the iron, of His F8, and of residues linked to His F8.

Figure 6-1. Heme. The pyrrole rings and methylene bridge carbons are coplanar, and the iron atom (Fe2+) resides in almost the same plane. The fifth and sixth coordination positions of Fe2+ are directed perpendicular to—and directly above and below—the plane of the heme ring. Observe the nature of the substituent groups on the p carbons of the pyrrole rings, the central iron atom, and the location of the polar side of the heme ring (at about 7 o'clock) that faces the surface of the myoglobin molecule.

Figure 6-2. A model of myoglobin at low resolution. Only the a-carbon atoms are shown. The a-helical regions are named A through H. (Based on Dickerson RE in: The Proteins, 2nd ed. Vol 2. Neurath H [editor]. Academic Press, 1964. Reproduced with permission.)

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