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FIGURE 15.25 • The six liganding positions of an iron ion. Four ligands lie in the same plane; the remaining two are, respectively, above and below this plane. In myoglobin, His F8 is the fifth ligand; in oxymyoglobin, O2 becomes the sixth.

O2 Binding to Mb

Iron ions, whether ferrous or ferric, prefer to interact with six ligands, four of which share a common plane. The fifth and sixth ligands lie above and below this plane (see Figure 15.25). In heme, four of the ligands are provided by the nitrogen atoms of the four pyrroles. A fifth ligand is donated by the imidazole side chain of amino acid residue His F8. When myoglobin binds O2 to become oxymyoglobin, the O2 molecule adds to the heme iron ion as the sixth ligand (Figure 15.25). O2 adds end on to the heme iron, but it is not oriented perpendicular to the plane of the heme. Rather, it is tilted about 60° with respect to the perpendicular. In deoxymyoglobin, the sixth ligand position is vacant, and in metmyoglobin, a water molecule fills the O2 site and becomes the sixth ligand for the ferric atom. On the oxygen-binding side of the heme lies another histidine residue, His E7. While its imidazole function lies too far away to interact with the Fe atom, it is close enough to contact the O2. Therefore, the O2-binding site is a sterically hindered region. Biologically important properties stem from this hindrance. For example, the affinity of free heme in solution for carbon monoxide (CO) is 25,000 times greater than its affinity for O2. But CO only binds 250 times more tightly than O2 to the heme of myoglobin, because His E7 forces the CO molecule to tilt away from a preferred perpendicular alignment with the plane of the heme (Figure 15.26). This diminished affinity of myoglobin for CO guards against the possibility that traces of CO produced during metabolism might occupy all of the heme sites, effectively preventing O2 from binding. Nevertheless, CO is a potent poison and can cause death by asphyxiation.

O2 Binding Alters Mb Conformation

What happens when the heme group of myoglobin binds oxygen? X-ray crystallography has revealed that a crucial change occurs in the position of the iron atom relative to the plane of the heme. In deoxymyoglobin, the ferrous ion has but five ligands, and it lies 0.055 nm above the plane of the heme, in the direction of His F8. The iron: porphyrin complex is therefore dome-shaped.

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