ABSTRACT
Fumarate reductase (EC 1.3.99.1) is an integral membrane protein containing FAD, three different iron-sulfur clusters, and at least two quinones [77]. It belongs to a family of membrane-bound enzymes that function in central carbon metabolism and energy production in cells. Bacteria and eukaryotic cells that can grow under anae robic conditions often use endogenous fumarate as terminal electron acceptor in a respiratory process in the absence of oxygen. Fumarate reductase is the enzyme catalyzing the final reaction in this anerobic pathway; it reduces fumarate to succi nate. Usually, the low-potential electron donor for this reaction in prokaryotes is the reduced naphthoquinone menaquinone (or demethylmenaquinone), whereas eukar yotic fumarate reductases were shown to interact with the benzoquinone rhodoquinone. As a respiratory enzyme, fumarate reductase is located in the cytoplasmic membrane of bacteria or in the mitochondrial inner membrane of eukaryotic cells. Succinate dehydrogenase is a homologous enzyme that catalyzes the reverse reaction of fumarate reductase, the oxidation of succinate to fumarate. It both is an enzyme of the Krebs cycle and constitutes complex II of the mitochondrial aerobic respiratory chain. Fumarate reductase and succinate dehydrogenase are similar in primary struc ture, cofactor composition, and mechanism, and can functionally replace each other under certain conditions [78]. But in vivo catalysis occurs only in one direction, and the enzymes are expressed differentially depending on the external conditions [79].
