Catalyzes the reversible stereospecific interconversion of fumarate to L-malate (PubMed:30761759). Experiments in other 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 (By similarity). 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) (PubMed:20231875, PubMed:26237645). 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) (PubMed:26237645). (S)-malate = fumarate + H2O (S)-malate = fumarate + H2O (S)-malate = fumarate + H2O kcat is 280 sec(-1) with L-malate (at pH 8.5) (PubMed:30761759). kcat is 170 sec(-1) with fumarate (at pH 8.5) (PubMed:30761759). Optimum pH is 7.5-8.0. Carbohydrate metabolism; tricarboxylic acid cycle; (S)-malate from fumarate: step 1/1. Homotetramer (PubMed:21445611, PubMed:30761759). Interacts with H2AZ1 (PubMed:26237645). Translocates to the nucleus in response to DNA damage: localizes to DNA double-strand breaks (DSBs) following phosphorylation by PRKDC. Expressed in red blood cells; underexpressed in red blood cells (cytoplasm) of patients with hereditary non-spherocytic hemolytic anemia of unknown etiology. Phosphorylation at Thr-236 by PRKDC in response to DNA damage promotes translocation to the nucleus and recruitment to DNA double-strand breaks (DSBs). The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Isoform Cytoplasmic: HLRCC is probably caused by an accumulation of fumarate (PubMed:30718813). Accumulation of fumarate coupled with protonation promotes the formation of non-enzymatic post-translational modification cysteine S-succination (S-(2-succinyl)cysteine) on proteins, such as SMARCC1 (PubMed:30718813). 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.