Structure of fumarate reductase from Wolinella succinogenes at 2.2 Å resolution

Fumarate reductase couples the reduction of fumarate to succinate to the oxidation of quinol to quinone, in a reaction opposite to that catalysed by the related complex II of the respiratory chain (succinate dehydrogenase). Here we describe the crystal structure at 2.2 Angstrom resolution of the three protein subunits containing fumarate reductase from the anaerobic bacterium Wolinella succinogenes, Subunit A contains the site of fumarate reduction and a covalently bound flavin adenine dinucleotide prosthetic group. Subunit B contains three iron-sulphur centres, The menaquinol-oxidizing subunit C consists of five membrane-spanning, primarily helical segments and binds two haem b molecules. On the basis of the structure, we propose a pathway of electron transfer from the dihaem cytochrome b to the site of fumarate reduction and a mechanism of fumarate reduction. The relative orientations of the soluble and membrane-embedded subunits of succinate:quinone oxidoreductases appear to be unique.