Figure 1. The catalytic cycle of cytochrome P450.

The binding of substrate R-H (1) causes a decrease of the redox potential of about 100 mV, which allows the first electron transfer from cytochrome P450-reductase (CPR) (2). The reduction of Fe ³⁺ to Fe ²⁺ makes suitable O ₂ binding (3), which now can accept a second electron from either CPR or cytochrome b ₅ (cyt b ₅) (4), to form a hydroperoxyl intermediate known as compound 0. The O ₂ ⁻² complex reacts with surrounding protons to form the highly reactive oxyferryl intermediate, also known as compound I (5). The Fe-ligated O atom (6) is transferred to the substrate forming a hydroxylated form of the substrate (7). The product is finally released (8), replaced by a molecule of water. Three uncoupling reactions are shown as dashed lines, with the respective products: the autoxidation shunt O ₂ ⁻², the peroxide shunt (H ₂O ₂), and the oxidase shunt (H ₂O).