FAD-FADH

Due to the variety of enzymatic environments in which FAD/ FADH /FADHZ can be held, enzymes that contain them exhibit redox potentials ranging from -0.45 to + 0.15 V at pH 7 (Bugg, 1997). Xenobiotic compounds whose structures suggest they can be reduced via free radical mechanisms or via hydride transfers may be reduced by such flavoproteins. 

For example, acyl-CoA dehydrogenases are thought to form an anionic species of reduced FAD (FADH ) which is tightly bound to the protein.149... 

The oxidation reduction potentials of MMOR have been measured for two MMO systems by monitoring the changes in the optical and EPR spectra. In MMO Bath the potentials for the FAD/FADH [Fe2S2] / [Fe2S2] and FADH/FADH2 couples were found to be nl50, n220, and... 

An average of one mediator was boimd per f 2-75 kDa of enzyme. The mediators used have redox potentials 0.07 V to 0.55 V positive of the FAD/FADH enzyme s redox potential (—0.05 V relative to the NHE at pH 7), but those which have a redox potential more negative than glucose oxidase enzyme do not mediate electron transfer. Electrons can be relayed both by tunneling and by motion of the mediator in and out of the protein chains. For distances >8 A, tunneling rates decrease ex-... 

The vaiue given is for free FAD/FADH. The Ei of the protein-bound coenzyme varies. 

FAD (FADH) flavin adenine dinucleotide (reduced form)... 

The nitroreductase reactions of NQOl have been studied more thoroughly [29,38,65,66]. Rat NQOl contains FAD with the standard (two-electron reduction) potential at pH 7.0, E°7, of-0.159 V [67], The ability of NQOl to perform two-electron transfer is most probably determined by the instability of its anionic FAD semiqui-none, since the E 7 of FAD/FAD and FAD/FADH- couples are equal to -0.200 V and -0.118 V, respectively [67], The majority of nitroaromatic compounds including TNT are very slow NQOl substrates (kcJKm = 102—104 M 1s 1, kcat = 0.1—1.0 s ), with the exception of TNBO, tetryl, and pentryl, whose reactivities are intermediate (kcJKm > 105 M s kcat > 10 s ). Possibly, NQOl reduces TNT to NHOH-DNTs, which is further reduced to dihydroxylamino-NT at a similar rate. NQOl performs reductive N-denitration of tetryl and pentryl (Equation 9.6) with the formation of picramides, other unidentified products, and 02-, which points to the involvement of single-electron transfer steps [29,66], The relationship between the reactivity of nitroaromatics toward NQOl and their reduction potential is absent the structural parameters determining their selectivity are unclear. 

Interestingly, when the dehydrogenation of coenzyme-bound thioester (Scheme 8.70, et se )—in the growing chain of acetyl CoA thioesters of long-, medium-, and short-chain fatty acids generated in the polyketide synthase cassette of reactions— occurs, it still appears that the FAD. FADH cofactor is involved, but there are... 

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