Alternatives to the oxoferryl porphyrin cation radical as the proposed reactive intermediate of cytochrome P450: two-electron oxidized Fe(III) porphyrin derivatives.

The active intermediates in most heme enzyme-catalyzed oxidations such as epoxidation and hydroxylation have been attributed to the O=Fe(IV) porphyrin pi-cation radical, so-called compound I. This could be correct for many cases, however, alternatives to compound I have been proposed for several oxidations including aliphatic hydroxylation catalyzed by P450. Therefore, two-electron oxidized iron porphyrin complexes other than compound I have been reviewed as candidates for the active species responsible for oxidations catalyzed by heme enzymes.