Catalyzes the conversion of dihydroorotate to orotate with fumarate as the electron acceptor. Molecular oxygen can replace fumarate in vitro. (S)-dihydroorotate + fumarate = orotate + succinate Binds 1 FMN per subunit. Optimum pH is 7. Optimum temperature is 37 degrees Celsius. Pyrimidine metabolism; UMP biosynthesis via de novo pathway. Homodimer. Belongs to the dihydroorotate dehydrogenase family. Type 1 subfamily.