Myoglobin ligand-protein interaction

Figure 34. Myoglobin-ligand interaction contour maps in the heme xy plane at z = 3.2 A (the iron is at the origin) showing protein relaxation a cross marks the iron atom projection onto the plane. Distances are in A and contours in kcal the values shown correspond to 90, 45,10, 0, and —3 kcal/mol relative to the ligand at infinity. The highest contours are closest to the atoms whose projections onto the plane of the figure are denoted by circles. Panel I X-ray structure. Panels 1I-IV sidechain rotations discussed in text.

Fig. 12. Schematic views of bis-histidyl ferri-, ferro-, and CO-ferro-heme-hemopexin. Unlike myoglobin with one open distal site, heme bound to hemopexin is coordinated to two strong field ligands, either of which a priori may be displaced by CO. This may well produce coupled changes in protein conformation like the Perutz mechanism for 02-binding by hemoglobin (143). The environment of heme bound to hemopexin and to the N-domain may be influenced by changes in the interactions of porphyrin-ring orbitals with those of aromatic residues in the heme binding site upon reduction and subsequent CO binding.

The function of myoglobin depends on the protein s ability not only to bind oxygen but also to release it when and where it is needed. Function in biochemistry often revolves around a reversible protein-ligand interaction of this type. A quantitative description of this interaction is therefore a central part of many biochemical investigations. 

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