Electron-transfer flavoprotein ETF

The oxidation of fatty acids is catalyzed by the FAD-containing acyl coenzyme A dehydrogenases which transfer reducing equivalents to the mitochondrial respiratory chain via a flavin-containing electron transfer flavoprotein (ETF) and subsequently via an ETF dehydrogenase (an Fe/S flavoprotein In addition to the mammalian... 

Fig. 5.2. Possible metabolic pathways in facultative anaerobic mitochondria. Shaded boxes show components of the electron-transport chain used during hypoxia, open boxes are components used during aerobiosis, and the hatched boxes (complex I and ATP-synthase) are components used under aerobic as well as anaerobic conditions. ASCT acetate succinate CoA-transferase, C cytochrome c, Cl, CIII and CIV complexes I, III and IV of the respiratory chain, CITR citrate, ECR enoyl-CoA reductase (such as present in Ascaris suum), ETF electron-transfer flavoprotein, ETF RQ OR electron-transfer flavoproteimrhodoquinone oxidoreductase, FRD fumarate reductase, FUM fumarate, MAE malate, OXAC oxaloacetate, PYR pyruvate, RQ rhodoquinone, SDH succinate dehydrogenase, SUCC succinate, Succ-CoA succinyl-CoA, TER trans-2-enoyl-CoA reductase (such as present in E. gracilis), UQ ubiquinone...

As described before, also the formation of branched-chain fatty acids by enoyl-CoA reductase activity is coupled to electron transport (Komuniecki and Harris 1995). In this case electrons are transported from NADH to rhodoquinone via complex I and subsequently to the electron-transfer flavoprotein (ETF) via ETF-reductase (Fig. 5.3). The soluble, non-membrane-bound ETF then transfers electrons to enoyl-CoA reductase, which uses the electrons for the condensation of two short-chain (C2-C3) acyl-CoA moieties for the formation of branched-chain fatty acids. 

A distinct electron transfer flavoprotein (ETF) is the single-electron acceptor for a variety of flavoprotein dehydrogenases, including acyl CoA, glutaryl CoA, sarcosine, and dimethylglycine dehydrogenases. It then transfers the electrons to ETF-ubiquinone reductase, the iron-sulfur flavoprotein that reduces ubiquinone in the mitochondrial electron transport chain. 

The mitochondrial respiratory chain, which contains at least 13 Fe-S clusters (Figure 6), perhaps best illustrates the importance of Fe-S clusters in membrane-bound electron transport. Electrons enter via three principal pathways, from the oxidation of NADH to NAD+ (NADH-ubiquinone oxidoreductase or Complex I) and succinate to fumarate (succinate ubiquinone oxidoreductase or Complex II), and from the /3-oxidation of fatty acids via the electron transferring flavoprotein (ETF-ubiquinone oxidoreductase). All three pathways involve a complex Fe S flavoprotein dehydrogenase, that is, NADH dehydrogenase, succinate dehydrogenase, and ETF dehydrogenase, and in each case the Fe-S clusters mediate electron transfer from the flavin active site to the ubiquinone pool via protein-associated ubiquinone. 

Fig. 3.1. A, The respiratory chain. Q and c stand for ubiquinone and cytochrome c, respectively. Auxiliary enzymes that reduce ubiquinone include succinate dehydrogenase (Complex II), a-glycerophosphate dehydrogenase and the electron-transferring flavoprotein (ETF) of fatty acid oxidation. Auxiliary enzymes that reduce cytochrome c include sulphite oxidase. B, Thermodynamic view of the respiratory chain in the resting state (State 4). Approximate values are calculated according to the Nernst equation using oxidoreduction states from work by Muraoka and Slater, (NAD, Q, cytochromes c c, and a oxidation of succinate [6]), and Wilson and Erecinska (b-562 and b-566 [7]). The NAD, Q, cytochrome b-562 and oxygen/water couples are assumed to equilibrate protonically with the M phase at pH 8 [7,8]. E j (A ,/ApH) for NAD, Q, 6-562, and oxygen/water are taken as —320 mV ( — 30 mV/pH), 66 mV (- 60 mV/pH), 40 mV (- 60 mV/pH), and 800 mV (- 60 mV/pH) [7-10]. FMN and the FeS centres of Complex I (except N-2) are assumed to be in redox equilibrium with the NAD/NADH couple, FeS(N-2) with ubiquinone [11], and cytochrome c, and the Rieske FeS centre with cytochrome c [10]. The position of cytochrome a in the figure stems from its redox state [6] and its apparent effective E -, 285 mV in...

Figure 8. ESR (upper) and ENDOR (lower) spectra from flavoproteins at — 160°C. a, Semiquinone radicals from the one-electron reduction of electron-transfer flavoprotein (ETF) b, semiquinone radicals from the one-electron reduction of sarcosine dehydrogenase. From [144], with permission.

Fig. 23.8. Transfer of electrons from acyl CoA dehydrogenase to the electron transport chain. Abbreviations ETF, electron-transferring flavoprotein ETF-QO, electron-transferring flavoprotein-Coenzyme Q oxidoreductase.

Fig. 13.1.1. Schematic overview of mitochondrial oxidative phosphorylation. A part of the mitochondrion is represented, showing the outer mitochondrial membrane (OMM), inner mitochondrial membrane (IMM) and crista (an invagination of the inner membrane). Substrates for oxidation enter the mitochondrion through specific carrier proteins, e.g., the pyruvate transporter, (PyrT). Reducing equivalents from fatty acyl CoA dehydrogenases, pyruvate dehydrogenase and the TCA cycle are delivered to the electron transport chain through NADH, succinate ubiquinol oxidoreductase (SQO), electron transfer flavoprotein (ETF) and its ubiquinol-...

Figure 37.4 Three-dimensional structure of electron-transfer flavoprotein (ETF) alone and in complex with medium-chain acyl-CoA dehydrogenase (MCAD). (A) Human ETF is a dimer of two distinct polypeptide chains, and harbours FAD and AMP cofactors (white sticks). Structure is divided in three sub-domains that are shown in roman numerals. (B) Crystallographic structure of ETF MCAD complex. ETF domain III is responsible for establishing protein-protein specific interactions. ETF domain II undergoes a dramatic conformational change upon complex formation (compare flavin position in panel A) in order to allow efleetive electron transfer to the flavin of MCAD. Structures of ETF and ETF MCAD complexes were obtained from Protein Data Bank (PDB lefv and 2A1T, respectively).

Figure 37.6 Effect of flavin cofactor binding on the stability of the human electron-transfer flavoprotein (ETF) mutant variant Aspl28Asn. (A) Activity of the protein is affected by incubation at 39 °C (open circles) however, in the presence of 2.5-fold excess FAD the activity is preserved (black circles). (B) The stability of ETF Aspl28Asn to urea-induced chemical denaturation is higher when the flavin is bound to the protein (black circles) than in flavin-depleted ETF (open circles). (C) The presence of flavin cofactor affects the proteolytic susceptibility of ETF Aspl28Asn. Upon incubation with trypsin protease ETF Aspl28Asn is rapidly degraded (top panel), whereas in the presence of excess flavin, the protein is more resistance to proteolysis.

Fig. 15.7 The respiratory chain a simplified scheme showing the inter-relationships of dehydrogenases, electron-transfer flavoprotein (ETF) (Chapter 14), other flavoproteins (Fp) and components of the respiratory chain.

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