In the de novo pyrimidine biosynthesis pathway, catalyzes the stereospecific oxidation of (S)-dihydroorotate to orotate with reduction of flavin and the transfer of electrons to ubiquinone, which is part of the repiratory chain. Does not use fumarate and NAD as electron acceptors. (S)-dihydroorotate + a quinone = a quinol + orotate Binds 1 FMN per subunit. Pyrimidine metabolism; UMP biosynthesis via de novo pathway; orotate from (S)-dihydroorotate (quinone route): step 1/1. Belongs to the dihydroorotate dehydrogenase family. Type 2 subfamily.