The Aerobic Mitochondrial Respiratory Chain

INTRODUCTION Fumarate reductase (FRD) catalyses the reduction of fumarate to succinate and is a key enzyme for the anaerobic functioning of many organisms respiring with fumarate as terminal electron acceptor. The ability to reduce fumarate is a common property among Gram-negative bacteria and some facultative anaerobic Gram-positive bacteria. The reduction of fumarate is also important in the metabolism of eukaryotes such as green algae, protozoa, parasitic helminths and some lower marine organisms. Fumarate reduction is the reverse reaction of the oxidation of succinate to fumarate, catalysed by succinate dehydrogenase (SDH), which occurs in aerobic cells. FRD and SDH are structurally similar and both are associated as multi-subunit complexes to the membrane; in eukaryotes this is the mitochondrial membrane. The catalytic domain of each type of complex is comprised of two polypeptides, one containing an FAD group and the other containing three iron-sulphur clusters. The catalytic domain is anchored to the membrane by one or two small hydrophobic polypeptides, the whole comprising complex