2.4. The catalytic cycle of CYP450

During the catalytic cycle of procarcinogen activation carried on by CYP450, short-lived spincorrelated RP intermediates are produced, which can either recombine or continue the catalytic process. The cycle initiates at resting state with the enzyme in the ferric form where a water molecule is implemented as the sixth ligand to iron in the active site. The ferric state of the resting enzyme has its five valence electrons occupying its orbitals, equilibrating the lowspin and high-spin state [46]. CYP450 at the beginning has ferric iron predominantly in the low-spin state. The substrate induces conformational changes displacing the water molecule from the distal axial coordinate position of the heme iron. This fact results in iron displacement from the porphyrin plane, which makes the heme a better electron sink and triggers electron transference [46], changing the oxidation/reduction potential in the spin state of the heme iron from the low-spin form towards the high-spin form, which is a better electron acceptor. Thus, the electron donor, which could be an iron/sulfur protein or a flavoprotein, reduces the highspin iron(III) heme to the high-spin iron(II) state. This intermediate compound has a singlet ground state and is a resonating mixture of the ferrous and ferric forms. This conjugation requires that the dioxygen will be in a singlet state so that the empty orbital of the oxygen can mix with the second occupied orbital of the iron [46]. It is transferred in the oxygen from the activated CYP450, that is, compound I, to the heteroatom of the substrate. It has been described in the general catalytic mechanism as an odd-electron process, which involves the transfer of one electron or hydrogen atom to generate an intermediate complex that collapses by recombination [47].
