DELOCALIZATION OVER THE HEME AND THE AXIAL LIGANDS OF ONE OF THE 2 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 an d catalase Compound I intermediates and their iron(IV)-oxo porphyrin p i-cation radical models, both of which are two oxidizing equivalents a bove the ferric state, suggests that in the models the oxidizing equiv alent 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 cata lyzed by the enzymes. Possible roles of the axial ligand in the cataly tic mechanism of these heme-based enzymes are discussed.