Flavin radicals formation constant

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... 

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. 

Obviously, more work is required to further substantiate the presence of the proposed radical intermediates in the p-hydroxybenzoate hydroxylase reaction, possibly via EPR and spin-trapping studies. Studies by Detmer and Massey 247) on phenol hydroxylase have indicated that the reaction rate constants for the conversion of meta-substituted substrates plotted versus the Hammett parameters yield a straight line of slope equal to 0.5. This is consistent with the mechanism proposed by Anderson, as the negative slope is expected for an electrophilic aromatic substitution reaction, while the small magnitude of the slope may be indicative of a radical mechanism. Furthermore, recent work by Massey and co-workers on p-hydroxybenzoate hydroxylase utilizing 6-hydroxy-FAD as cofactor and p-aminobenzoate as substrate indicated that the absorption spectrum of intermediate 67 exhibited a satellite band at 440 nm 248). Anderson et al. suggest that the satellite band may result from the formation of an aromatic phenoxyl radical at the C-6 position of the isoalloxazine ring of the flavin 244). This species would result from a shift of the initial peroxyl radical center from C(4a) to C-6 via N(5) 245). 

Reduction to H2O takes place when electron transfer in the respiratory chain is completed by terminal oxidases. Reduction to H2O2 is associated with flavoproteins in flavin respiration, and superoxide radicals (O2 ) arise constantly in the respiratory chain and some other biological systems. Hydroperoxide is toxic to the cell and is degraded by catalase and/or peroxidase. Superoxide radicals are highly toxic to the cell, in addition, they promote the formation of more toxic products, namely, singlet oxygen and hydroxyl radicals ... 

---