Flavoprotein dehydrogenases free radicals

Similarly, the reduction of riboflavin prosthetic groups (FP) of some flavoprotein dehydrogenases may also involve a free radical intermediate. 

However, these experiments may not have established a mechanism for natural flavoprotein catalysis because the properties of 5-deazaflavins resemble those of NAD+ more than of flavins.239 Their oxidation-reduction potentials are low, they do not form stable free radicals, and their reduced forms don t react readily with 02. Nevertheless, for an acyl-CoA dehydrogenase the rate of reaction of the deazaflavin is almost as fast as that of natural FAD.238 For these enzymes a hydride ion transfer from the (3 CH (reaction type D of Table 15-1) is made easy by removal of the a-H of the acyl-CoA to form an enolate anion intermediate. 

Riboflavin and flavoprotein pass through semiquinonoid states during two-equivalent reduction (303,456,457,547,548,699,704 compare 393). Furthermore, radicals and semiquinones appear to be stabilized by adsorption (69). Beinert has observed the spectrum of an intermediate reduction stage of FAD and of a flavoprotein enzyme, fatty acyl CoA dehydrogenase (36,37). This intermediate form (at pH 7.4 and 4.3) shows a band at 580 m which can be produced (1) by reduction of enzyme by dithionite in absence of substrate or by substrate in the absence of dithionite, ( ) by reduction of free flavin by dithionite or by sodium amalgam, and (S) by leoxidation of either reduced enzyme or reduced free flavin with oxygen. The intermediate is stable when formed in the presence of substrate, although it is unstable when produced by dithionite in the absence of substrate. The.se relationships can be interpreted in terms of an enzyme-sub-... 

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