Responsible for channeling the electrons from the oxidation of dihydroorotate from the FMN redox center in the PyrD type B subunit to the ultimate electron acceptor NAD(+). Binds 1 [2Fe-2S] cluster per subunit. Binds 1 FAD 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 PyrK family.