Flavoprotein

Two and twelve moles of ATP are produced, respectively, per mole of glucose consumed in the glycolytic pathway and each turn of the Krebs (citrate) cycle. In fat metaboHsm, many high energy bonds are produced per mole of fatty ester oxidized. Eor example, 129 high energy phosphate bonds are produced per mole of palmitate. Oxidative phosphorylation has a remarkable 75% efficiency. Three moles of ATP are utilized per transfer of two electrons, compared to the theoretical four. The process occurs via a series of reactions involving flavoproteins, quinones such as coenzyme Q, and cytochromes. 

In contrast to the nicotinamide nucleotide dehydrogenases, the prosthetic groups FMN and FAD are firmly associated with the proteins, and the flavin groups are usually only separated from the apoen2yme (protein) by acid treatment in water. However, in several covalently bound flavoproteins, the enzyme and flavin coen2ymes are covalently affixed. In these cases, the flavin groups are isolated after the proteolytic digestion of the flavoproteins. 

Lim, L.W., et al. Three-dimensional structure of the iron-sulfur flavoprotein trimethylamine dehydrogenase at 2.4 A resolution. J. Biol. Chem. 261 15140-15146, 1986. 

Access to three different redox states allows flavin coenzymes to participate in one-electron transfer and two-electron transfer reactions. Partly because of this, flavoproteins catalyze many different reactions in biological systems and work together with many different electron acceptors and donors. These include two-electron acceptor/donors, such as NAD and NADP, one- or two-elec-... 

Typical values for reduction of bound FAD in flavoproteins such as succinate dehydrogenase (see Bonomi, F., Pagani, S., Cerletti, P, and Giori, C., 1983. European Journal of Biochemistry 134 439-445). 

Reduction potentials can also be quite sensitive to molecular environment. The influence of environment is especially important for flavins, such as FAD/FADHg and FMN/FMNHg. These species are normally bound to their respective flavoproteins the reduction potential of bound FAD, for example, can be very different from the value shown in Table 21.1 for the free FAD-FADHg couple of —0.219 V. A problem at the end of the chapter addresses this... 

As we have seen, the metabolic energy from oxidation of food materials—sugars, fats, and amino acids—is funneled into formation of reduced coenzymes (NADH) and reduced flavoproteins ([FADHg]). The electron transport chain reoxidizes the coenzymes, and channels the free energy obtained from these reactions into the synthesis of ATP. This reoxidation process involves the removal of both protons and electrons from the coenzymes. Electrons move from NADH and [FADHg] to molecular oxygen, Og, which is the terminal acceptor of electrons in the chain. The reoxidation of NADH,... 

All these intermediates except for cytochrome c are membrane-associated (either in the mitochondrial inner membrane of eukaryotes or in the plasma membrane of prokaryotes). All three types of proteins involved in this chain— flavoproteins, cytochromes, and iron-sulfur proteins—possess electron-transferring prosthetic groups. 

Three protein complexes have been isolated, including the flavoprotein (FP), iron-sulfur protein (IP), and hydrophobic protein (HP). FP contains three peptides (of mass 51, 24, and 10 kD) and bound FMN and has 2 Fe-S centers (a 2Fe-2S center and a 4Fe-4S center). IP contains six peptides and at least 3 Fe-S centers. HP contains at least seven peptides and one Fe-S center. 

Complex II is perhaps better known by its other name—succinate dehydrogenase, the only TCA cycle enzyme that is an integral membrane protein in the inner mitochondrial membrane. This enzyme has a mass of approximately 100 to 140 kD and is composed of four subunits two Fe-S proteins of masses 70 kD and 27 kD, and two other peptides of masses 15 kD and 13 kD. Also known as flavoprotein 2 (FP2), it contains an FAD covalently bound to a histidine residue (see Figure 20.15), and three Fe-S centers a 4Fe-4S cluster, a 3Fe-4S cluster, and a 2Fe-2S cluster. When succinate is converted to fumarate in the TCA cycle, concomitant reduction of bound FAD to FADHg occurs in succinate dehydrogenase. This FADHg transfers its electrons immediately to Fe-S centers, which pass them on to UQ. Electron flow from succinate to UQ,... 

Formation of tire active coenzyme 5 -deoxyadeno.sylcobalamin from inactive vitamin Bjg i-s initiated by die action of flavoprotein reducta.se.s. The re.sulting Co. specie.s, dubbed a. supernucleophile, attacks the 5 -carbon of ATP in an unusual adeno.syl transfer. 

To summarize, squalene epoxidase is a flavoprotein capable of catalyzing the insertion of oxygen into the 2,3-double bond of squalene to give 2,3-oxidosqualene, with the second oxygen atom from 02 being reduced to water. The reducing equivalents necessary for this transformation are relayed from NADPH through NADPH-cytochrome c reductase to the flavin cofactor of the epoxidase. 

Fe-2S]-containing flavoprotein from the 3,6-dideoxyhexose biosynthetic pathway of Yersinia pseudotuberculosis [106], has been successful as a substitute for the as yet unidentified endogenous reductase [105]. 

Figure 5.5 Respiratory chains in A. niger. SHAM = salicyl-hydroxamic acid Fp = flavoprotein -= inhibition x and y are unidentified components.

AbouKhair, N. K., Ziegler, M. M., and Baldwin, T. O. (1984). The catalytic turnover of bacterial luciferase produces a quasi-stable species of altered conformation. In Bray, R. C., et al. (eds.), Flavins Flavoproteins, Proc. Int. Symp., 8th, pp. 371-374. de Gruyter, Berlin. 

Eckstein, J. W., and Ghisla, S. (1991). On the mechanism of bacterial luciferase. 4a,5-Dihydroflavins as model compounds for reaction intermediates. In Flavins Flavoproteins, Proc. Int. Symp., 10th, 1990, 269-272. 

Tu, S.-C. (1991). Oxygenated flavin intermediates of bacterial luciferase and flavoprotein aromatic hydroxylases enzymology and chemical models. Adv. Oxygenated Processes 3 115-140. 

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