Reduction of FAD

FIGURE 20.14 The succinate dehydrogenase reaction. Oxidation of succinate occurs with reduction of [FAD]. Reoxidation of [FADH9] transfers electrons to coenzyme Q. 

Note that flavin coenzymes can carry out either one-electron or two-electron transfers. The succinate dehydrogenase reaction represents a net two-electron reduction of FAD. 

FIGURE 21.33 The glycerophosphate shuttle (also known as the glycerol phosphate shuttle) couples the cytosolic oxidation of NADH with mitochondrial reduction of [FAD]. 

Redox pairs Oxidation (loss of electrons) of one compound is always accompanied by reduction (gain of electrons) of a second substance. For example, Figure 6.11 shows the oxidation of NADH to NAD+ accompanied by the reduction of FAD to FADH2. Such oxidation-reduction reactions can be written as the sum of two halfreactions an isolated oxidation reaction and a separate reduction reaction (see Figure 6.11). NAD+ and NADH form a redox pair, as do FAD and FADH2. Redox pairs differ in their tendency to lose electrons. This tendency is a characteristic of a particular redox pair, and can be quantitatively specified by a constant, E (the standard reduction potential), with units in volts. 

The first steps, not shown, involve the reduction of FAD to FADH2 by NADPH and the binding of glutathione (glutathione is a sulfhydryl compound, see figure 22.15). The mechanism by which oxidized glutathione is reduced by the E FADH2 is shown. 

How eould this eatalytie bias be controlled One possibility is that the proton transfer pathway eould eontribute to specifieity (Peters et al., 1998). Another possibility is that differences in midpoint potential of the FeS clusters (or other redox sites) that constitute the intramolecular wire could be tuned to facilitate one of the two directions of the reaction. For example, these redox sites could best match the midpoint potentials of a particular oxidized or reduced electron carrier (Holm and Sander, 1999). Apparently, a conformational change in succinate dehydrogenase, coupled to the reduction of FAD, is responsible for its catalytic bias for fumarate reduction (Hirst et al., 1996). 

FAD is the coenzyme of a class dehydrogenases called jlavoproteins. The flavin moiety of the molecule is derived from riboflavin (vitamin B2). Reduction of FAD involves the two unsubstituted N atoms of the isoalloxazine structure. 

Fig. 19.10. Reduction of FAD. FAD accepts two electrons as two hydrogen atoms and is reduced. The reduced coenzyme is denoted in this text as FAD(2H) because it often accepts a total of two electrons one at a time, never going to the fully reduced form, FADH2. FMN (flavin mononucleotide) consists of riboflavin with one phosphate group attached.

F. 20.4. One-electron steps in the reduction of FAD. When FAD and FMN accept single electrons, they are converted to the half-reduced semi-quinone, a semistable free radical form. They can also accept two electrons to form the fuUy reduced form, FADH2. However, in most dehydrogenases, FADH2 is never formed. Instead, the first electron is shared with a group on the protein as the next electron is transferred. Therefore, in this text, overall acceptance of two electrons by FAD has been denoted by the more general abbreviation, FAD(2H). 

Imidazoleacetate and i.-lysine monooxygenases from pseudomonads were obtained in crystalline form. Both enzymes were established to contain FAD probably as a sole cofactor. Available evidence indicates that the reaction catalyzed by these enzymes involves the reduction of FAD, the monooxygenation of substrate, and the reoxidation of reduced FAD. The mechanism of activating molecular oxygen by these FAD-containing monooxygenases is discussed. 

When an equimolar amount of NADH was anaerobically added to the enzyme in the presence of imidazoleacetate, the reduction of FAD... 

Although the identification of the C-labeled product as piperidine 2-carboxyUc acid has not been established and the mechanism of its formation has not been clearly elucidated, the formation of the a-keto acid may suggest the intermediate formation of dehydrogenated L-lysine concomitant with the reduction of FAD. 

Flavin nucleotides—flavin adenine dinucleotide (FAD) and flavin mononucleotide (riboflavin-5 -phosphoric acid) (FMN)—as prosthetic groups can either comprise a metal or serve as cofactors without a metal. Electrochemical transformations of FAD and FMN are characterized by strong adsorption. The area occupied by a FAD molecule on a mercury electrode is 280 A, i.e., close to the geometrical dimensions corresponding to the molecular model. Reduction of FAD and FMN proceeds in two reversible... 

Sucdnaterubiquinone Reductase— Reduction of FAD by Succinate to Produce FADH2, Which Ultimately Results in Reduction of Ubiquinone to Produce Ubiquinol for Subsequent Reactions of the Electron Transport Chain... 

Step 1 Succinate Reduction of FAD to Produce FADH2 and Fumarate... 

Two main families of histone demethylases have been identified. The LSD family (EC number 1.14.1 l.Bl) employs a monoamine oxidase mechanism to oxidise methyllysine to an imine (with concomitant reduction of FAD to FADH2), followed by addition of water to give a hemiaminal intermediate, which fragments to achieve demethylation. The JmjC family (EC number... 

The free energy change in the oxidation of succinate to fumarate is about — 150kJ (-36 kcal) moP. This is considerably less than the energy required to reduce NAD to NADH (-220 kJ(-52-6 kcal) moP ), but it is adequate for the reduction of FAD. This explains why NAD is not used as the hydrogen acceptor in this reaction. In general, reactions that involve the introduction of a double bond into a saturated chain require the participation of a flavoprotein. 

Oxidation to yield a carbon—carbon double bond. Although this reaction is shown in Figure 5.18 as being linked to reduction of FAD, the coenzyme is tightly enzyme bound, and succinate dehydrogenase reacts directly with ubiquinone in the electron transport chain (section 3.3.1.2). 

The direct incomplete oxidation of sugars without phosphorylation leads to the formation of the corresponding ketones. The aldoses are oxidized into aldonic acids. The aldehydic function of this sugar is transformed into a carboxylic acid function. Glucose is oxidized into gluconic acid in this manner. The glucose oxidase catalyzes the reaction, which is coupled with the reduction of FAD. In acetic acid bacteria, electrons and protons are transported by the cytochrome chain to oxygen, which is the final acceptor. 

Figure 2 illustrates the bleaching of the 1345 cm SERRS band, associated with reduction of adsorbed FAD and GO on Ag, as a function of applied potential. The midpoints for the bleaching of FAD and GO occur at approximately equal potentials (-285 5 mV and -275 10 mV vs Ag/AgCl respectively) and are coincident with reduction of FAD observed by differential pulse voltammetry. This similarity is consistent with other results that suggest that we are actually observing free FAD reduction in the adsorbed GO. 

---