Ligands methionine

Myoglobin is a protein of molecular weight of about 17,000 with the protein chain containing 153 amino acid residues folded about the single heme group. This restricts access to the iron atom (by a second heme) and reduces the likelihood of formation of a hematin-like Fe(III) dimer. The micro environment is similar to that in Cytochrome c, but there is no sixth ligand (methionine) to complete the coordination sphere of the iron atom. Thus there is a site to which a dioxygen molecule may reversibly bind. 

In the case of His/CN systems, structurally pertinent to myoglobin cyanide, peroxidase cyanide, cytochrome c cyanide and the CN derivative of a cytochrome c mutant, where the axial ligand methionine is substituted with alanine (Ala80-cyt c-CN), the simple assignment of the four methyl protons, which is an almost trivial task nowadays, provides direct structural information on the axial ligands, which can be used for structural analysis in solution. The chemical shifts for each... 

Fig. 5.20. Hyperfine shifts of methyl protons in (A) Met80Ala cytochrome c-CN-, and (B) cytochrome b5. The former is a histidine-cyanide ferriheme protein, since the axial ligand methionine is substituted with alanine, the latter is a bis-histidine ferriheme protein (labeling as in Fig. 5.7B).

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