Catalyzes the reversible stereospecific interconversion of fumarate to L-malate (PubMed:23643539). 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 (PubMed:23643539). 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 (PubMed:23643539). Also plays a role in DNA repair by promoting non-homologous end-joining (NHEJ) (By similarity). 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. Interacts with H2AZ1. Translocates to the nucleus in response to DNA damage: localizes to DNA double-strand breaks (DSBs) following phosphorylation by PRKDC. Phosphorylation at Thr-233 by PRKDC in response to DNA damage promotes translocation to the nucleus and recruitment to DNA double-strand breaks (DSBs). Embryonic lethality (PubMed:17418408). Conditional deletion in the kidney leads to the development of renal cysts, reminiscent of hereditary leiomyomatosis and renal cell cancer (HLRCC) phenotype in human (PubMed:17418408). Renal cysts are caused by accumulation of fumarate that promotes the formation of non-enzymatic post-translational modification cysteine S-succination (S-(2-succinyl)cysteine) on proteins, such as Keap1 (PubMed:22014577). 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.