Irreversibly catalyzes the reduction of fumarate to succinate. Together with the second isozyme of soluble fumarate reductase (OSM1), essential for anaerobic growth. Involved in maintaining redox balance. Reduction of fumarate is the main source of succinate during fermentation, and under anaerobic conditions, the formation of succinate is strictly required for the reoxidation of FADH(2). NAD(+) + succinate = fumarate + H(+) + NADH Binds 1 FAD per monomer. During anaerobic growth. The N-terminus is blocked. Present with 7620 molecules/cell in log phase SD medium. Belongs to the FAD-dependent oxidoreductase 2 family. FRD/SDH subfamily.