Nitronate monooxygenase (NMO), formerly referred to as 2-nitropropane dioxygenase (NPD) (EC:188.8.131.52), is an FMN-dependent enzyme that uses molecular oxygen to oxidize (anionic) alkyl nitronates and, in the case of the enzyme from Neurospora crassa, (neutral) nitroalkanes to the corresponding carbonyl compounds and nitrite. Previously classified as 2-nitropropane dioxygenase, but it is now recognized that this was the result of the slow ionization of nitroalkanes to their nitronate (anionic) forms. The enzymes from the fungus Neurospora crassa and the yeast Williopsis saturnus var. mrakii (formerly classified as Hansenula mrakii) contain non-covalently bound FMN as the cofactor. Active towards linear alkyl nitronates of lengths between 2 and 6 carbon atoms and, with lower activity, towards propyl-2-nitronate. The enzyme from N. crassa can also utilize neutral nitroalkanes, but with lower activity. One atom of oxygen is incorporated into the carbonyl group of the aldehyde product. The reaction appears to involve the formation of an enzyme-bound nitronate radical and an a-peroxynitroethane species, which then decomposes, either in the active site of the enzyme or after release, to acetaldehyde and nitrite.