Catalyzes the conversion of dihydroorotate to orotate with fumarate as the electron acceptor. (S)-dihydroorotate + fumarate = orotate + succinate Binds 1 FMN per subunit. Pyrimidine metabolism; UMP biosynthesis via de novo pathway. Homodimer. Belongs to the dihydroorotate dehydrogenase family. Type 1 subfamily.