<P> Glucose oxidase (GOX) catalyzes the oxidation of β - D - glucose to D - glucono - δ - lactone with the simultaneous reduction of enzyme - bound flavin . GOX exists as a homodimer, with each subunit binding one FAD molecule . Crystal structures show that FAD binds in a deep pocket of the enzyme near the dimer interface . Studies showed that upon replacement of FAD with 8 - hydroxy - 5 - carba - 5 - deaza FAD, the stereochemistry of the reaction was determined by reacting with the re face of the flavin . During turnover, the neutral and anionic semiquinones are observed which indicates a radical mechanism . </P> <P> Prenylcysteine lyase (PCLase) catalyzes the cleavage of prenylcysteine (a protein modification) to form an isoprenoid aldehyde and the freed cysteine residue on the protein target . The FAD is non-covalently bound to PCLase . Not many mechanistic studies have been done looking at the reactions of the flavin, but the proposed mechanism is shown below . It is proposed that there is a hydride transfer from the C1 of the prenyl moiety to FAD that results in the reduction of the flavin to FADH and the formation of a carbocation that is stabilized by the neighboring sulfur atom . FADH then reacts with molecular oxygen to restore the oxidized enzyme . </P> <P> UDP - N - acetylenolpyruvylglucosamine Reductase (MurB) is an enzyme that catalyzes the NADPH - dependent reduction of enolpyruvyl - UDP - N - acetylglucosamine (substrate) to the corresponding D - lactyl compound UDP - N - acetylmuramic acid (product). MurB is a monomer and contains one FAD molecule . Before the substrate can be converted to product, NADPH must first reduce FAD . Once NADP dissociates, the substrate can bind and the reduced flavin can reduce the product . </P> <P> Glutathione reductase (GR) catalyzes the reduction of glutathione disulfide (GSSG) to glutathione (GSH). GR requires FAD and NADPH to facilitate this reaction; first a hydride must be transferred from NADPH to FAD . The reduced flavin can then act as a nucleophile to attack the disulfide, this forms the C4a - cysteine adduct . Elimination of this adduct results in a flavin - thiolate charge - transfer complex . </P>

What is the difference between fad and fadh2