Flavin‐N5‐oxide: A new redox state in flavin enzymology

Flavoproteins comprise of one of the most studied family of enzymes and catalyze a number of redox reactions. The flavin prosthetic group can exist either in oxidized or one or two electron reduced form during the redox chemistry. All previously studied flavin‐dependent monooxygenase have been shown to catalyze oxygenation by a flavin‐C4a‐hydroperoxide, which transfer a single oxygen atom to its organic substrate. However, recent study have shown an unprecedented oxygenating species in the flavoprotein EncM that is involved in the biosynthetic pathway of the antibiotic enterocin [1]. EncM catalyzes the oxygenation of a polyketide substrate using a stable flavin‐N5 oxide species. This leads to a rare Favoriskii‐type oxidative rearrangement of the carbon skeleton chain of its substrate. The enzyme gets reduced during substrate turnover, followed by its reaction with molecular O 2 to converts it back to the flavin N5‐oxide species via an unknown pathway. The N5‐oxide represents a new redox state for flavin chemistry. It can transfer an oxygen atom obtained from the reduction of O 2 . While the flavin‐C4a‐peroxyflavin species is highly unstable, the flavin‐N5 oxide is a stable redox intermediate, and its identification has been confirmed by absorbance spectroscopy, mass spectroscopy, isotope labelling[2]. EncM exits as a dimer with distinct flavin and substrate binding domains and a a8‐histidyl‐FAD covalent linkage with positively charges residues surrounding it: a structure common in many oxidases. This study sheds light on the mechanism, potential substrates and the structural basis that leads to this unique flavin redox state.