Catalyzes the conversion of dihydroorotate to orotate with NAD(+) as electron acceptor. (S)-dihydroorotate + NAD(+) = H(+) + NADH + orotate Binds 1 FMN per subunit. Pyrimidine metabolism; UMP biosynthesis via de novo pathway; orotate from (S)-dihydroorotate (NAD(+) route): step 1/1. Heterotetramer of 2 PyrK and 2 PyrD type B subunits. Belongs to the dihydroorotate dehydrogenase family. Type 1 subfamily. Extended N-terminus.