Catalytic mechanism of P450 enzymes. P450s are usually mono-oxygenases, catalyzing the insertion of one of the atoms of molecular oxygen into a substrate, the second atom of oxygen being reduced to water. The most frequently catalyzed reaction is hydroxylation (O insertion) using the very reactive and electrophilic iron-oxo intermediate (species [C], bottom row). The hydroperoxo form of the enzyme (species [B]-) is also an electrophilic oxidant catalyzing OH+ insertion. Nucleophilic attack can be catalyzed by species [A]2- and [B]- ; reduction, isomerization or dehydration are catalyzed by the oxygen-free forms of the enzyme. This, together with the variety of the apoproteins and intrinsic reactivity of all their substrates explains the extraordinary diversity of reactions catalyzed by P450 enzymes.