Insights into the enzymatic formation, chemical features, and biological role of the flavin-N5-oxide

Flavoenzymes are versatile catalysts that mostly facilitate redox reactions such as the oxygenation of organic substrates. Commonly, flavin monooxygenases employ a flavin-C4a-(hydro)peroxide as oxygenating species. Recently, however, a modified N5-functionalized flavin cofactor featuring a distinct nitrone moiety - the flavin-N5-oxide - was reported for the first time as oxygenating species in the bacterial enzyme EncM that catalyzes the dual oxidation of a reactive poly-β-ketone substrate. Meanwhile, additional flavoenzymes have been reported that form the flavin-N5-oxide. Here, we highlight aspects of the discovery and characterization of this novel flavin redox state with a focus on recent findings that shed more light onto its chemical features and enzymatic formation. We furthermore provide a rationale for the oxygenase functionality of EncM by contrast with structurally related flavin oxidases and dehydrogenases from the vanillyl alcohol oxidase/p-cresol methylhydroxylase flavoprotein (VAO/PCMH) superfamily. In addition, the possible biological roles of the flavin-N5-oxide are discussed.