Flavin adenine dinucleotide radical

FIGURE 32-7 Sources of free radical formation which may contribute to injury during ischemia-reperfusion. Nitric oxide synthase, the mitochondrial electron-transport chain and metabolism of arachidonic acid are among the likely contributors. CaM, calcium/calmodulin FAD, flavin adenine dinucleotide FMN, flavin mononucleotide HtT, tetrahydrobiopterin HETES, hydroxyeicosatetraenoic acids L, lipid alkoxyl radical LOO, lipid peroxyl radical NO, nitric oxide 0 "2, superoxide radical. 

CoQ Coenzyme Q FADH- Radical form of reduced flavin adenine dinucleotide... 

Xanthine oxidase, which requires Fe, Mo and flavin adenine dinucleotide (FAD) as co-factors, is capable of oxidizing lipids via the production of superoxide radicals. It represents about 20% of the MFGM protein and part is readily lost from the membrane, e.g. on cooling isoelectric focusing... 

Flavin adenine dinucleotide. See FAD Flavin adenine diphosphate. See FAD Flavin coenzymes 766,780 - 795 modified 788, 789 reduced 794 Flavin radicals 792 color of 794 formation constant 794 Flavocytochrome b2 782, 794, 847 Flavodoxins 793, 799, 800 Flavoprotein(s) 513, 788... 

Unusual features of riboflavin as recorded by some researchers include (1) High levels in liver inhibit tumor formation by azo compounds in animals (2) free radicals are formed by light or dehydrogenation flavine semiquinone dihydroflavin+ (3) free vitamin is found only in retina, urine, milk, and semen (4) substitution of adenine by other purines and pyrimidines destroys activity of flavin adenine dinucleotide (FAD) ... 

Fig. 3. Reduced forms of flavin adenine dinucleotide, (a) FADH2. (b) Neutral semiquinone radical.

The transient absorption spectra similar to that of the ion-pair state of indole cation radical and flavin anion radical were also observed in D-amino acid oxidase (5), although the spectra were not so clear as those of flavodoxin. In D-amino acid oxidase, the coenzyme, flavin adenine dinucleotide (FAD), is wealtly fluorescent. The fluorescence lifetime was reported to be 40 ps (16), which became drastically shorter (less than 5 ps) when benzoate, a competitive inhibitor, was combined with the enzyme at FAD binding site (17). The dissociation constant of FAD was also marlcedly decreased by the binding of benzoate (17). These results suggest that interaction between isoalloxazine and the quencher became stronger as the inhibitor combined with the enzyme. Absorbance of the transient spectra of D-amino acid oxidase-benzoate complex was remarkably decreased. In this case both rate constants of formation and decay of the CT state could become much faster than those in the case of D-amino acid oxidase free from benzoate. 

As for reduction processes, C02 free radicals were shown to react specifically with disulfide bonds (122). They were extensively used to study the redox properties of disulfide bonds, thiyl and disulfide free radicals in proteins. This is discussed in paragraph 5. However, they do react with thiol functions also (37). For proteins containing a prosthetic group, the reduction concerns also oxidized valencies of metals and flavins. Flavin adenine dinucleotide (FAD) or Flavin Mononucleotide (FMN). The proportion of reduced disulfide/reduced prosthetic group varies considerably with the protein. For instance, lipoamide dehydrogenase contains one disulfide bond close to a flavin (FAD). Free radicals can reduce only the flavin, although both are in the active site (123). In chicken egg white riboflavin binding protein, competitive formation of both disulfide and semireduced flavin is observed (124). 

The flavin enzymes contain the riboflavin derivatives flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) (D 10.4.3) as prosthetic groups. They act as the carrier of hydrogen or electrons and participate in a large number of oxido-reductions. Flavin enzymes may be oxidized or reduced in a one-step (Fig. 13, pathway A) or a two-step reaction (Fig. 13, pathway B). In the case of the one-step mechanism the addition and elimination of a hydride ion plays an important role. In the two-step transformation a flavin radical appears as an intermediate. 

The Use of an Inverse Isotope Effect to Delineate an Enzyme Mechanism D-Amino acid oxidase catalyzes the oxidation of amino acids to imino acids via the transfer of a hydride to the coenzyme flavin adenine dinucleotide (FAD). The mechanism first involves deprotonation of the amino acid to create a carbanion (see below). This carbanion can then undergo either a nucleophilic addition to the flavin of FAD (Path A) or an electron transfer to the flavin, creating radicals that combine to give the same product as the nucleophilic addition (Path B). Expulsion of the flavin as a leaving group, concomitant with some proton transfers, gives the oxidized imino acid product. 

FADH% flavin adenine dinucleotide, neutral semiquinone radical FAD" , flavin adenine dinucleotide, anion semiquinone radical FMN, flavin mononucleotide ... 

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