Fumarate reduction is the most commonly used electron-sink for the reoxidation of reduced cofactors in anaerobically functioning mitochondria (see before). Fumarate reduction is catalysed in these mitochondria by a FRD complex, which is structurally very similar to the SDH complexes present in aerobically functioning mitochondria. Both complexes usually consist of four nonidentical subunits: a flavin-containing A subunit (Fp subunit), a B subunit that contains three iron-sulphur clusters (Ip subunit) and two hydrophobic, cytochrome b containing subunits C and D that are essential for the attachment of the catalytic subunits A and B to the membrane and for the interaction of the catalytic subunits with the quinones (Ackrell et al. 1992; Hederstedt and Ohnishi 1992). The Fp and Ip subunits of SDH are highly conserved in different species and are also closely related to the Fp and Ip sub-units of FRD. Analyses of enzyme kinetics, as well as the known differences in primary structures of prokaryotic and eukaryotic complexes that reduce fumarate, led to the suggestion that fumarate-reducing eukaryotes possess an enzyme complex for the reduction of fumarate that is structurally related to SDH-type complex II, but that has the functional characteristics of the FRD complexes of prokaryotes (Van Hellemond et al. 1995). Despite the limited amount of data available so far, the sequences of eukaryotic FRDs are clearly more closely related to those of SDHs than to those of bacterial FRDs, indicating that during the evolution of anaerobic mitochondria, their SDH is modified to preferentially catalyse the reverse reaction and use lower-potential quinones (Tielens and Van Hellemond 1998; Tielens et al. 2002).
Most prokaryotes are known to use menaquinone for electron transport to FRD, whereas the anaerobically functioning mitochondria of eukaryotes use rhodoquinone. Although menaquinone and rhodoquinone are functional equivalents, because they both have a low standard electron potential, their structure differs significantly. The prokaryotic electron transporter menaquinone is a naphthoquinone, whereas rhodoquinone and ubiquinone are benzoquinones; therefore, the quinone involved in electron transport to fumarate in eukaryotes (rhodoquinone) is structurally more similar to the quinone involved in aerobic mitochondrial electron transport (ubiquinone) than to its functional equivalent in prokaryotes, menaquinone. The quinone structure thus also suggests that electron transport in anaerobically functioning mitochondria is more closely related to that in aerobically functioning mitochondria than to its functional homologue in prokaryotes (Van Hellemond et al. 1995; Tielens and Van Hellemond 1998). Furthermore, the specific enzyme enoyl-CoA reductase, involved in the biosynthesis of branched-chain fatty acids in anaerobically functioning mitochondria of Ascaris, shows sequence similarities to mitochondrial acyl-CoA dehydrogenases found in many aerobic functioning eukaryotes (Duran et al. 1993). This is further evidence that the mechanisms to reoxidize reduced cofactors in the absence of oxygen evolved from components already present in aerobically functioning mitochondria.
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