Fig. 1: Catalytic mechanism of flavin-containing monooxygenases.

The recurring molecular structure represents the isoalloxazine moiety of the FAD cofactor where R corresponds to the ribityl adenosine tail. E stands for enzyme. First, oxidized FAD (E-FAD) is reduced by NADPH. The reduced enzyme (E-FADH₂) readily reacts with O₂ forming the oxygenating enzyme intermediate C4a-flavin(hydro)peroxide (E-FADOO(H)). Two mechanisms are possible from here, the S/N oxidation or the BV oxidation, both followed by subsequent release of H₂O and NADP⁺. In absence of substrate the enzyme undergoes a futile hydrogen peroxide-producing cycle called uncoupling.