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Carolin Mügge, Thomas Heine, Álvaro Gómez Baraibar, Willem J. H. van Berkel, Caroline E. Paul, Dirk Tischler

• Amino groups derived from naturally abundant amino acids or (di)amines can be used as "shuttles" in nature for oxygen transfer to provide intermediates or products comprising N-O functional groups such as \(\it N\)-hydroxy, oxazine, isoxazolidine, nitro, nitrone, oxime, \(\it C\)-, \(\it S\)-, or \(\it N\)-nitroso, and azoxy units. To this end, molecular oxygen is activated by flavin, heme, or metal cofactor-containing enzymes and transferred to initially obtain \(\it N\)-hydroxy compounds, which can be further functionalized. In this review, we focus on flavin-dependent \(\it N\)-hydroxylating enzymes, which play a major role in the production of secondary metabolites, such as siderophores or antimicrobial agents. Flavoprotein monooxygenases of higher organisms (among others, in humans) can interact with nitrogen-bearing secondary metabolites or are relevant with respect to detoxification metabolism and are thus of importance to understand potential medical applications. Many enzymes that catalyze N-hydroxylation reactions have specific substrate scopes and others are rather relaxed. The subsequent conversion towards various N-O or N-N comprising molecules is also described. Overall, flavin-dependent \(\it N\)-hydroxylating enzymes can accept amines, diamines, amino acids, amino sugars, and amino aromatic compounds and thus provide access to versatile families of compounds containing the N-O motif. Natural roles as well as synthetic applications are highlighted.