FMN oxidation

Fig. 12. Alternative routes of electron transfer from the substrate carbanion to the FMN. Oxidation of substrate can occur by a two-electron transfer via a covalent intermediate (A -I- E), by formation of a radical pair followed by a second one-electron transfer to give the covalent intermediate (B + C + E), and by formation of a radical pair directly followed by a second one-electron transfer to give reduced FMN (B + D).

ELAVOPROTEINS. Flavin is an essential substance for the activity of a number of important oxidoreductases. We discuss the chemistry of flavin and its derivatives, FMN and FAD, in the chapter on electron transport and oxidative phosphorylation (Chapter 21). 

FIGURE 18.22 The redox states of FAD and FMN. The boxes correspond to the colors of each of these forms. The atoms primarily involved in electron transfer are indicated by red shading in the oxidized form, white in the semiqninone form, and bine in the reduced form. 

The second step involves the transfer of electrons from the reduced [FMNHg] to a series of Fe-S proteins, including both 2Fe-2S and 4Fe-4S clusters (see Figures 20.8 and 20.16). The unique redox properties of the flavin group of FMN are probably important here. NADH is a two-electron donor, whereas the Fe-S proteins are one-electron transfer agents. The flavin of FMN has three redox states—the oxidized, semiquinone, and reduced states. It can act as either a one-electron or a two-electron transfer agent and may serve as a critical link between NADH and the Fe-S proteins. 

Moreover, an electron transfer chain could be reconstituted in vitro that is able to oxidize aldehydes to carboxylic acids with concomitant reduction of protons and net production of dihydrogen (213, 243). The first enzyme in this chain is an aldehyde oxidoreductase (AOR), a homodimer (100 kDa) containing one Mo cofactor (MOD) and two [2Fe—2S] centers per subunit (199). The enzyme catalytic cycle can be regenerated by transferring electrons to flavodoxin, an FMN-con-taining protein of 16 kDa (and afterwards to a multiheme cytochrome and then to hydrogenase) ... 

The reduced NADH of the tespitatoty chain is in turn oxidized by a metalloflavoptotein enzyme—NADH dehydrogenase. This enzyme contains FeS and FMN, is tighdy bound to the tespitatoty chain, and passes te-ducing equivalents on to Q. 

With these results, we named the new enzyme as phenylacetaldoxime dehydratase (EC 4.99.1.7). It was also suggested that the enzyme utilizes FMN as an electron acceptor, because the value was increased about five times under anaerobic condition and the sulfite ion could replace FMN, although the enzyme requires oxidized form of FMN. It was revealed that the enzyme is a quite unique enzyme whose apparent function is to catalyze a dehydration reaction. The reaction mechanism is of much interest. 

It is well known that the selective transport of ions through a mitochondrial inner membrane is attained when the oxygen supplied by the respiration oxidizes glycolysis products in mitochondria with the aid of such substances as flavin mononucleotide (FMN), fi-nicotinamide adenine dinucleotide (NADH), and quinone (Q) derivatives [1-3]. The energy that enables ion transport has been attributed to that supplied by electron transport through the membrane due to a redox reaction occurring at the aqueous-membrane interface accompanied by respiration [1-5],... 

Summarizing the results obtained by controlled potential electrolysis and polarography, the reaction process for the electrolytic evolution of CO2 was estimated to be as follows the first step was one electron transfer from DMFC in NB to FMN in W as in Eq. (7). The second step was the catalytic reduction of O2 by FMNH as in Eq. (8). The final step was the oxidation of pyruvic acid by the reduction product of O2, H2O2, in W as in Eq. (9), well-known as an oxidative decarboxylation of a-keto acids [43] ... 

Fig. 6.27 An example of Mitchell s loops . The substrate SH2 is oxidized by NAD+ (nicotinamideadeninedinucleotide) and the reduced form transports two protons and two electrons, of which two protons remain in the intracristal space and two electrons are transported back by the Fe-S protein to reduce FMN (flavinmononucleotide), the reduced form of which transports two protons and two electrons in the opposite direction...

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