Fumarate reductase

Fumarate reductase is the enzyme that converts fumarate to succinate, and is important in microbial metabolism as a part of anaerobic respiration.^[1] The catalyzed reaction is:

succinate + acceptor <=> fumarate + reduced acceptor

Fumarate reductases can be divided into two classes depending on the electron acceptor:

Fumarate reductase (NADH) (EC 1.3.1.6)
The enzyme is monomeric and soluble, and can reduce fumarate independently from the electron transport chain.^[2] Fumarate reductase is absent from all mammalian cells.
Fumarate reductase (quinol) (EC 1.3.5.4)
The membrane-bound enzyme covalently linked to flavin cofactors, which is composed of 3 or 4 subunits, transfers electrons from a quinol to fumarate. This class of enzyme is thus involved in the production of ATP by oxidative phosphorylation.^[1]

^ ^a ^b Tielens, A.G.; van Hellemond, J.J. (1998). "The electron transport chain in anearobically functioning eukaryotes". Biochim. Biophys. Acta. 1365: 71–78. doi:10.1016/s0005-2728(98)00045-0.
^ Camarasa; et al. (2007). "Role in anaerobiosis of the isoenzymes for Saccharomyces cerevisiae fumarate reductase encoded by OSM1 and FRDS1". Yeast. 24: 391–401. doi:10.1002/yea.1467.