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Proposed reaction mechanism of PPO. The oxidation of protoporphyrinogen IX to protoporphyrin IX occurs in three steps, whereby each time the FAD cofactor is reduced by the tetrapyrrol ring and reoxidised by an oxygen molecule that is reduced to hydrogen peroxide. The reaction always starts at the C20 atom of the tetrapyrrol ring by hydride transfer, followed by hydrogen rearrangements that take place in the whole ring system by enamine–imine tautomerisations (Jordan, 1991).
