Flavoprotein dehydrogenases

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

Flavoprotein dehydrogenases usually accept electrons from reduced pyridine nucleotides and donate them to a suitable electron acceptor. The oxidation-reduction midpoint potential of the FAD of the oxidase has been determined by ESR spectroscopy and shown to be -280 mV. The NADP+/ NADPH redox potential is -320 mV and that of the cytochrome b is -245 mV hence, the flavin is thermodynamically capable of accepting electrons from NADPH and transferring them to cytochrome b. As two electrons are transferred from NADPH, although O2 reduction requires only one electron, the scheme of electron transfer shown in Figure 5.8 has been proposed by Cross and Jones (1991). 

Flavoproteins form either 15 or 16 in high yield, if they are formed at all. A helpful, general rule is that flavoprotein oxidases (see below) form 16, flavoprotein dehydrogenases and flavodoxins form 15 whereas flavoprotein mono-... 

The oxygen reactivity of flavohydroquinone bound to apoflavoprotein dehydrogenases can vary considerably from fast (flavodoxins), moderate (xanthine oxidase) to nil (succinate dehydrogenase) Most, but not all, flavoprotein dehydrogenases contain one or more types of metal prosthetic groups, e.g. xanthine oxidase contains also Fe and Mo. Since these metal ions are involved in electron flux, their possible participation in the reaction with O2 cannot be excluded. Much evidence, however, indicates that the flavin is involved in the one-electron reduction of Oj, as shown in Equation (5). 

The chemistry of flavins is complex, a fact that is reflected in the uncertainity that has accompanied efforts to understand mechanisms. For flavoproteins at least four mechanistic possibilities must be considered.1533 233 (a) A reasonable hydride-transfer mechanism can be written for flavoprotein dehydrogenases (Eq. 15-23). The hydride ion is donated at N-5 and a proton is accepted at N-l. The oxidation of alcohols, amines, ketones, and reduced pyridine nucleotides can all be visualized in this way. Support for such a mechanism came from study of the nonenzymatic oxidation of NADH by flavins, a reaction that occurs at moderate speed in water at room temperature. A variety of flavins and dihydropyridine derivatives have been studied, and the electronic effects observed for the reaction are compatible with the hydride ion mecha-nism.234 236... 

The central ring of 1-deazaflavins remains a pyrazine in X, a di-hydropyrazine in the two-electron-reduced form, XI, and continues to dominate the chemistry with oxygen. Like the parent riboflavins, and unlike the 5-deazaflavins, the dihydro- 1-deaza system, XI, is reoxidized by 02 in a fraction of a second in air-saturated solutions (Table II) the semiquinone is accessible and 1-deazaFAD enzymes show full catalytic competence with flavoprotein dehydrogenases and oxidases (24). Turnover numbers vary from about 1% to 100% that of cognate FAD-enzymes but this variation reflects the -280 mV vs. —200 mV E° values, respectively, for 1-deazariboflavin vs. riboflavin. The redox steps may or may not limit Vmax with a given enzyme (15, 24). 

The flavodoxins, which resemble the larger flavoprotein dehydrogenases [24], are well-characterized flavoproteins [25] in which oxidation involves formation of a semiquinone radical (Fig. 3b) [26],... 

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