Proximal ligand control of heme iron coordination structure and reactivity with hydrogen peroxide: investigations of the myoglobin cavity mutant H93G with unnatural oxygen donor proximal ligands.

The role of the proximal heme iron ligand in activation of hydrogen peroxide and control of spin state and coordination number in heme proteins is not yet well understood. Although there are several examples of amino acid sidechains with oxygen atoms which can act as potential heme iron ligands, the occurrence of protein-derived oxygen donor ligation in natural protein systems is quite rare. The sperm whale myoglobin cavity mutant H93G Mb (D. Barrick, Biochemistry 33 (1994) 6546) has its proximal histidine ligand replaced by glycine, a mutation which leaves an open cavity capable of accommodation of a variety of unnatural potential proximal ligands. This provides a convenient system for studying ligand-protein interactions. Molecular modeling of the proximal cavity in the active site of H93G Mb indicates that the cavity is of sufficient size to accommodate benzoate and phenolate in conformations that allow their oxygen atoms to come within binding distance of the heme iron. In addition, benzoate may occupy the cavity in an orientation which allows one carboxylate oxygen atom to ligate to the heme iron while the other carboxylate oxygen is within hydrogen bonding distance of serine 92. The ferric phenolate and benzoate complexes have been prepared and characterized by UV-visible and MCD spectroscopies. The benzoate adduct shows characteristics of a six-coordinate high-spin complex. To our knowledge, this is the first known example of a six-coordinate high-spin heme complex with an anionic oxygen donor proximal ligand. The benzoate ligand is displaced at alkaline pH and upon reaction with hydrogen peroxide. The phenolate adduct of H93G Mb is a five-coordinate high-spin complex whose UV-visible and MCD spectra are distinct from those of the histidine 93 to tyrosine (H93Y Mb) mutant of sperm whale myoglobin. The phenolate adduct is stable at alkaline pH and exhibits a reduced reactivity with hydrogen peroxide relative to that of both native ferric myoglobin, and the exogenous ligand-free derivative of ferric H93G Mb. These observations indicate that the identity of the proximal oxygen donor ligand has an important influence on both the heme iron coordination number and the reactivity of the complex with hydrogen peroxide.