Fumarate anaerobic metabolic pathways

Figure 17.1 Anaerobic metabolic pathways involved in SA production in wild-type Actinobacillus succinogenes. PEP, phos-phoenolpyruvate OAA, oxaloacetate MAL, malate FUM, fumarate ICT, IsocItrate CIT, citrate acetyl-P, acetyl-phosphate pck, PEP carboxykinase QOH, menaquinol Idh, lactate dehydrogenase pfi, pyruvate formatelyase ...

Figure 17.2 Anaerobic metabolic pathways involved in SA production in wild-type Anaerobiospirillum succiniciproducens. PEP, phosphoenolpyruvate OAA, oxaloacetate MAE, malate FUM, fumarate El, nonspecific protein of the phosphotransferase system (PTS) HPr, non-specific phosphoryl carrier protein of PTS pyk, pyruvate kinase pfo.

All parasitic flatworms capable of anaerobic metabolism favour malate as the primary mitochondrial substrate and the oxidative decarboxylations of first malate and then pyruvate generate intramitochondrial reducing power in the form of NADH (Fig. 20.1). In contrast, the pathways used to reoxidize intramitochondrial NADH are quite diverse and depend on the stage or species of parasite under examination, but in all cases, redox balance is maintained and electron-transport associated ATP is generated by the NADH-reduction of fumarate to succinate. In the cestode, hi. diminuta, succinate and acetate are the major end products of anaerobic malate dismutation and are excreted in the predicted 2 1 ratio. In the trematode F. hepatica, succinate is then further decarboxylated to propionate with an additional substrate level phosphorylation coupled to the decarboxylation of methylmalonyl CoA. F. hepatica forms primarily propionate and acetate as end products, again in a ratio of 2 1 to maintain redox balance. 

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...

The only known function of PhQ in cyanobacteria and plants is to function as an electron transfer cofactor in PS I. In spite of its importance in cyanobacteria, the biosynthetic route of PhQ was not previously elucidated. Many prokaryotes contain the metabolic pathway for the biosynthesis of menaquinone (MQ), a PhQ-Hke molecule (Figure 119.1). In certain bacteria, MQ is used during fumarate reduction in anaerobic respiration. - In green sulfur bacteria and in heliobacteria, MQ may function as a loosely bound secondary electron acceptor in the photosynthetic reaction center. The genes encoding enzymes involved in the conversion of chorismate to MQ were cloned in a variety of organisms. MQ differs from PhQ only in the tail portion of the molecule an unsaturated C-40 side chain is present, rather than a mostly saturated C-20 phytyl side chain. Therefore, the synthesis of the naphthalene rings in PhQ and MQ involves similar steps in both pathways. 

Fig. 17.11 Summary of carbohydrate metabolism in yeast growing anaerobically, i.e. in fermentation. The dotted lines represent the oxidative pathway for the formation of succinate (and possibly fumarate and malate) from pyruvate. The alternative reductive pathway is indicated by solid lines. 

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