Fumarate reductase electronic coupling

Fumarate is able to serve as an electron acceptor in anaerobic respiration, as it may be reduced reversibly to succinate in a two-electron process. The succinate-fumarate couple may therefore be utilized as an oxidant or reductant in the respiratory chain, and so differs from the other examples given in this section. These two reactions are catalyzed by succinate dehydrogenase and fumarate reductase, which have many similarities in subunit structure. These are shown in Table 29. Although they are different enzymes, the fumarate reductase can substitute for succinate dehydrogenase under certain conditions. The synthesis of succinate dehydrogenase is induced... 

Mitochondria from adult H. diminuta exhibit an NADH-coupled fumarate reductase (Table 5.11). This presents a potential dilemma with respect to the utilisation of intramitochondrial reducing equivalents by this worm. As reducing equivalents are generated by the malic enzyme in the form of NADP, a mechanism for the transfer of hydride ions from NADPH to NAD to produce NADH is required so that electron-transport-associated activities can proceed and terminate with the reduction of fumarate to succinate. Such a mechanism does exist in H. diminuta as there is a non-energy-linked, membrane-associated transhydrogenase (214, 217, 221, 476). This transhydrogenase, which also occurs in H. microstoma (216) and Spirometra mansonoides (220) catalyses the reaction ... 

Proton-coupled electron transfer (PCET) is known to play an important role in a variety of biological processes, including microbial iron transport by ferric enterobactin, enzyme catalysis in systems such as fumarate reductase and nitrate reducatase, and dioxygen binding by the non-heme iron protein hemerythrin. " As such, pH-dependent electrochemical studies can play an important role in unraveling these mechanisms. The most heavily studied biological system known to involve PCET is cytochrome c oxidase, the terminal electron-transfer complex of the mitochondrial respiratory chain, which catalyzes the reduction of molecular oxygen to water. ... 

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