Catalyzes the reversible stereospecific interconversion of fumarate to L-malate (PubMed:21498518). Experiments in different species have demonstrated that specific isoforms of this protein act in defined pathways and favor one direction over the other (Probable). 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. Catalyzes the dehydration of L-malate to fumarate. Fumarate metabolism in the cytosol plays a role during urea cycle and arginine metabolism; fumarate being a by-product of the urea cycle and amino-acid catabolism (By similarity). Also plays a role in DNA repair by promoting non-homologous end-joining (NHEJ). In response to DNA damage and phosphorylation by PRKDC, translocates to the nucleus and accumulates at DNA double-strand breaks (DSBs): acts by catalyzing formation of fumarate, an inhibitor of KDM2B histone demethylase activity, resulting in enhanced dimethylation of histone H3 'Lys-36' (H3K36me2) (By similarity). (S)-malate = fumarate + H2O (S)-malate = fumarate + H2O Carbohydrate metabolism; tricarboxylic acid cycle; (S)-malate from fumarate: step 1/1. Homotetramer (PubMed:21498518). Interacts with H2AZ1 (By similarity). Translocates to the nucleus in response to DNA damage: localizes to DNA double-strand breaks (DSBs) following phosphorylation by PRKDC. Phosphorylation at Thr-238 by PRKDC in response to DNA damage promotes translocation to the nucleus and recruitment to DNA double-strand breaks (DSBs). 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.