The oxygenase reductase FAD/NADH binding domain acts as part of the multi-component bacterial oxygenases which oxidize hydrocarbons. Electron transfer is from NADH via FAD (in the oxygenase reductase) and an [2FE-2S] ferredoxin center (fused to the FAD/NADH domain and/or discrete) to the oxygenase. Dioxygenases add both atoms of oxygen to the substrate while mono-oxygenases add one atom to the substrate and one atom to water. In dioxygenases, Class I enzymes are 2 component, containing a reductase with Rieske type [2Fe-2S] redox centers and an oxygenase. Class II are 3 component, having discrete flavin and ferredoxin proteins and an oxygenase. Class III have 2 [2Fe-2S] centers, one fused to the flavin domain and the other separate.