Delocalization over the heme and the axial ligands of one of the two oxidizing equivalents stored above the ferric state in the peroxidase and catalase Compound-I intermediates: Indirect participation of the proximal axial ligand of iron in the oxidation reactions catalyzed by heme-based peroxidases and catalases?

A comparison of the exchange interactions arising in the peroxidase and catalase Compound I intermediates and their iron(IV)-oxo porphyrin pi-cation radical models, both of which are two oxidizing equivalents above the ferric state, suggests that in the models the oxidizing equivalent is localized on the porphyrin ring, while in the proteins it is partly delocalized onto the proximal ligand. Thus, the proximal axial ligand of iron participates indirectly in the oxidation reactions catalyzed by the enzymes. Possible roles of the axial ligand in the catalytic mechanism of these heme-based enzymes are discussed.