Catalyzes the conversion of dihydroorotate to orotate with fumarate as the electron acceptor. Molecular oxygen can replace fumarate in vitro. Does not use oxaloacetate or NAD or NADP as electron acceptors. (S)-dihydroorotate + fumarate = orotate + succinate Binds 1 FMN per subunit. The activity is independent of the presence of oxygen. Optimum pH is 8.5. Active from pH 7 to pH 10. Pyrimidine metabolism; UMP biosynthesis via de novo pathway. Homodimer. Present with 264000 molecules/cell in log phase SD medium. Belongs to the dihydroorotate dehydrogenase family. Type 1 subfamily.