Fumarate Reductase FUM

The 2-eleetron midpoint potential of the FAD in FUM is n55mV (Aekrell et al., 1989). The 1-eleetron potentials for the iron-sulfur centers are -nSOrnV (2Fe-2S), n300 (4Fe-4S) and n70mV (3Fe-4S) (Kowal et al., 1995). The eleetron transfer rates between the individual clusters or from the 2Fe-2S eluster to FAD are not known. 

FIGURE 12. Stereo diagram of the complete fimiarate reductase complex. The FAD-binding subunit is at the top, the iron-sulfur subunit is in the center and die two membrane anchoring subunits that provide die binding sites for two molecules of menaquinone are at the bottom. In this molecule electron h ansfer occiffs from menaquinone at die bottom to FAD at die top during reduction of fumarate by menaquinone. Skeletal models of two molecules of menaquinone, a 3Fe-4S, a 4Fe-4S, a 2Fe-2S, an FAD molecule and one molecule of oxalate are included. 

FIGURE 13. Stereo view of the FAD-binding subunit (top) and the 2Fe-2S domain (residues lnl06) of the iron-sulfur subunit (bottom) of fumarate reductase. 

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Fig. 3 Speculative metabolic schemes of the main pathways in carbohydrate metabolism in N. ovalis Abbreviations AcCoA, acetyl-CoA, Cl, complex I, Citr, citrate, FRD, fumarate reductase, FUM, fumarate, Hyd, hydrogenase, a-KG, a-ketoglutarate, MAL, malate, OXAC, oxaloacetate, PDH, pyruvate dehydrogenase, PEP, phosphoenolpyruvate carboxyk-inase, PYR, pyruvate, RQ, rhodoquinone, SUCC, succinate, SUCC-CoA, succinyl-CoA...

Fig. 20.1. Generalized scheme of the main pathways of aerobic and anaerobic carbohydrate degradation in parasitic flatworms. The aerobic pathway is indicated by open arrows, whereas the anaerobic pathway (malate dismutation) is indicated by solid arrows. Abbreviations AcCoA, acetyl-CoA ASCT, acetateisuccinate CoA-transferase C, cytochrome c CI-CIV, complexes I—IV of the respiratory chain CITR, citrate FRD, fumarate reductase FUM, fumarate MAL, malate Methylmal-CoA, methylmalonyl-CoA OXAC, oxaloacetate PEP, phosphoenolpyruvate PROP, propionate Prop-CoA, propionyl-CoA PYR, pyruvate RQ, rhodoquinone SDH, succinate dehydrogenase SUCC, succinate Succ CoA, succinyl CoA UQ, ubiquinone.

Boxes indicate electron-transport chain complexes, whereas ovals represent the electron transporters UQ, RQ and cytochrome c. The open boxes represent complexes involved in the classical aerobic respiratory chain, whereas grey boxes represent complexes involved in malate dismutation. The vertical bar represents a scale for the standard redox potentials in mV. Translocation of protons by the complexes is indicated by H+ +. Abbreviations Cl, Clll and CIV, complexes I, III and IV of the respiratory chain cyt c, cytochrome c FRD, fumarate reductase Fum, fumarate SDH, succinate dehydrogenase Succ, succinate RQ, rhodoquinone UQ, ubiquinone. 

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...

AcCoA, acetyl-CoA pta, phosphate acetyl-transferase ackA, acetate kinase ppc, PEP carboxykinase mdh, malate dehydrogenase fum, fumarase frd, fumarate reductase pyc, pyruvate carboxylase and pykF, pyruvate kinase. This figure was redrawn based on [36]. With permission 2009 Federation of European Microbiological Societies published by Blackwell Publishing Ltd. 

MAL, malate FUM, fumarate. ppc, PEP car-boxykinase pekA, PEP carboxykinase pykF, pyruvate kinase maeB, malic enzyme mdh, malate dehydrogenase fumC, fumarase frdABCD, fumarate reductase IdhA, lactate dehydrogenase pHB, pyruvate fbrmatelyase pta, phosphate acetyl-transferase ackA, acetate kinase adhE, alcohol dehydrogenase and MQ, menaquinone. 

Fig. 5.3. The major components involved in mitochondrial NADH oxidation in facultative anaerobic mitochondria. In anaerobically functioning mitochondria, NADH is oxidized either by soluble enzymes (left) or by membrane-bound complexes of the electron-transport chain (middle). Under aerobic conditions, a classic respiratory chain is used to oxidize NADH (right). Proton translocation is indicated by H with arrows. Ovals represent the electron transporters RQ, UQ and cytochrome c (cyt. c), and electron transport is indicated by dashed arrows. The vertical bar represents a scale for the standard redox potentials in millivolts. Fum fumarate, NADH-DH NADH dehydrogenase, NADH-ECR soluble NADH enoyl-CoA reductase, RQH2 rhodoquinol, Succ succinate, UQH2 ubiquinol...

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