500x500pxThe<unk>  catalytic cycle proceeds as follows: # The cofactor NADPH binds to the oxidized state of the FAD prosthetic group, reducing it to FADH2. # Molecular oxygen binds to the formed NADP+-FADH2-enzyme complex and is reduced, resulting in 4a-hydroperoxyflavin (4a-HPF or FADH-OOH). This species is stabilized by NADP+ in the catalytic site of the enzyme. These first two steps in the cycle are fast. # In the presence of a substrate (S), a nucleophilic attack occurs on the distal O-atom of the prosthetic group. The substrate is oxygenated to SO, forming the 4a-hydroxyflavin (FADH-OH). Only when the flavin is in the hydroperoxy form is when the xenobiotic substrate will react. # The flavin product then breaks down with release of water to reform FAD. # Due to the low dissociation constant of the NADP+-enzyme complex, NADP+ is released by the end of the cycle and the enzyme returns to its original state. The rate-limiting step involves either the breakdown of FADH-OH to water or the release of NADP+. # Quantum mechanics simulations showed the N-hydroxylation catalyzed by<unk> s initiated by homolysis of the O-O bond in the C4a-hydroperoxyflavin intermediate resulting in the formation of an internal hydrogen bonded hydroxyl radical.
Flavin-containing monooxygenase
Flavin containing monooxygenase 1
Biotinidase
Disaccharidase
Transfer factor
Proteases 
Amidase
Xanthine oxidase