Catalyzes the reversible stereospecific interconversion of fumarate to L-malate (PubMed:29688630). Catalyzes the hydration of fumarate to L-malate in the tricarboxylic acid (TCA) cycle to facilitate a transition step in the production of energy in the form of NADH (By similarity). (S)-malate = fumarate + H2O Fumarate hydratase activity (fumarate to L-malate) is strongly inhibited by phosphoenolpyruvate, citrate, oxaloacetate, ATP and ADP (PubMed:29688630). Malate dehydratase activity (malate to fumarate) is activated by oxaloacetate, pyruvate, Asn and Gln (PubMed:29688630). Malate dehydratase activity (malate to fumarate) is inhibited by citrate, succinate, ADP, ATP, glucose-6P and phosphoenolpyruvate (PubMed:29688630). kcat is 21 sec(-1) for fumarate (at pH 8.0) (PubMed:29688630). kcat is 15.3 sec(-1) for (S)-malate (at pH 8.0) (PubMed:29688630). Carbohydrate metabolism; tricarboxylic acid cycle; (S)-malate from fumarate: step 1/1. Homotetramer. Embryonic lethality. There are 2 substrate-binding sites: the catalytic A site, and the non-catalytic B site that may play a role in the transfer of substrate or product between the active site and the solvent. Alternatively, the B site may bind allosteric effectors. Belongs to the class-II fumarase/aspartase family. Fumarase subfamily.