Molecular Mechanism of Oxygen Activation by P-450

In this review we have discussed the mechanistic features of P-450 reactions by comparing enzymic reactions with those of model porphyrin systems. Model porphyrin systems have allowed the complete dissection of the proposed mechanism of oxygen activation and transfer by P-450. Putative intermediates have been characterized, and each primitive transformation has now been observed. The similarities and differences in the oxygen activation between HRP and P -450 have also been discussed, especially homolysis and heterolysis of the acylperoxo-iron(II1) complex. In both cases, we consider the active species to be oxofenyl porphyrin cation radicals; however, many different reactivities exist between peroxidases and P-450. Ortiz de Montellano et al. have proposed that the position of substrates in the active site might depend on the spatial characteristics of the individual enzymes and influence the detailed course of the reaction (139). These propositions should be carefully examined. Scheme XXV illustrates all of the intermediates that have been observed and/or proposed in the oxygen activation mechanism by P-450 and that have been prepared by the model systems.