Fumarate reductase is the enzyme that converts fumarate to succinate, and is important in microbial metabolism as a part of anaerobic respiration.
Succinate + acceptor <=> fumarate + reduced acceptor
In other words, 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).
Fumarate reductase complex includes three subunits. 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. The D subunit may be required to anchor the catalytic components of the fumarate reductase complex to the cytoplasmic membrane.