Catalyzes the anaerobic reduction of fumarate to succinate (PubMed:24361839, PubMed:33107907). Uses NADH as the inherent electron donor in this process (PubMed:33107907). Is involved in anaerobic fumarate respiration in K.pneumoniae (PubMed:33107907). NAD(+) + succinate = fumarate + H(+) + NADH Binds 1 FAD per subunit. Binds 2 FMN prosthetic groups per subunit. 1 FMN is bound covalently, while the other is non-covalent. kcat is about 10 sec(-1). Optimum pH is 6.0-6.5. Monomer. Induced under anaerobic conditions in the presence of fumarate or malate, but not tartrate. Is repressed under aerobic conditions. Consists of three domains. The first domain of FRD (OYE-like, PF00724) is homologous to Old Yellow Enzyme (OYE). It carries a non-covalently bound FMN and is used for NADH oxidation. The second, FMN_bind domain (PF04205) contains a covalently bound FMN, that acts as an electron carrier between the two other domains of FRD. The C-terminal FAD_binding_2 domain contains a non-covalently bound FAD and forms a site for fumarate reduction. Is flavinylated on Thr-447 by ApbE2, encoded in a neighboring gene (PubMed:24361839). Flavinylation is essential for catalytic activity (PubMed:31834358). Cells lacking this gene lack cytoplasmic fumarate reductase activity, while retaining this activity in the membrane fraction. Belongs to the FAD-dependent oxidoreductase 2 family. FRD/SDH subfamily.