Structure of fumarate reductase from Wolinella succinogenes at 2.2 angstrom resolution

Fumarate reductase couples the reduction of fumarate to succinate to the ox idation of quinol to quinone, in a reaction opposite to that catalysed by t he related complex II of the respiratory chain (succinate dehydrogenase). H ere we describe the crystal structure at 2.2 Angstrom resolution of the thr ee protein subunits containing fumarate reductase from the anaerobic bacter ium Wolinella succinogenes, Subunit A contains the site of fumarate reducti on and a covalently bound flavin adenine dinucleotide prosthetic group. Sub unit B contains three iron-sulphur centres, The menaquinol-oxidizing subuni t C consists of five membrane-spanning, primarily helical segments and bind s 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 oxidored uctases appear to be unique.