Fig. 1

Catalytic mechanism of single- and two-component FDMs. S: substrate. A In single-component flavin-dependent monooxygenases (FDM), NAD(P)H serves as the reducing power to reduce the flavin cofactor FAD after binding the substrate, resulting in the formation of a reduced complex in the reductive half-reaction. Subsequently, the reduced complex reacts with molecular oxygen to hydroxylate the substrate and regenerate the oxidative flavin cofactor FAD in the oxidative half-reaction. B In two-component FDMs, FAD is bound with flavin reductase as the flavin cofactors and is reduced directly by NAD (P)H without the need to bind the substrate. Once reduced, the flavin cofactor FADH₂ is released and transferred to the monooxygenase component. The subsequent catalytic steps of the monooxygenase are similar to the oxidative half-reaction of single-component FDMs, where FADH₂ is used to hydroxylate the substrate. C Protein structure of a typical single-component FDM: PobA from Pseudomonas aeruginosa [50]. D Protein structures of typical two-component FDM: EcHpaB (right) and EcHpaC (left) from Escherichia coli [63, 99]