This family consists of several bacterial fumarate hydratase proteins FumA and FumB. Fumarase, or fumarate hydratase (EC 220.127.116.11), is a component of the citric acid cycle. In facultative anaerobes such as Escherichia coli, fumarase also engages in the reductive pathway from oxaloacetate to succinate during anaerobic growth. Three fumarases, FumA, FumB, and FumC, have been reported in E. coli. fumA and fumB genes are homologous and encode products of identical sizes which form thermolabile dimers of Mr 120,000. FumA and FumB are class I enzymes and are members of the iron-dependent hydrolases, which include aconitase and malate hydratase. The active FumA contains a 4Fe-4S centre, and it can be inactivated upon oxidation to give a 3Fe-4S centre.