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(+). Together with PyrD, also forms a metal reductase complex able to reduce Fe(III)-chelates to Fe(II)-chelates, as well as soluble Cr(VI) and U(VI), using NADH as electron donor. To a lesser extent, can also use NADPH as an electron donor. Is unable to reduce riboflavin and FMN with NADH as electron donor. May have an in vivo role in metal reduction in D.reducens, which is an organism capable of reducing contaminant heavy metals and radionuclides. Binds 1 FAD per subunit. Binds 1 [2Fe-2S] cluster 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 (By similarity). However, the metal reductase complex seems to be composed of a heterooctamer of 4 PyrK and 4 PyrD subunits (PubMed:25389064). Was recovered from both the soluble as well as the insoluble (presumably membrane) protein fraction, and thus may be in some way also associated with the membrane. Belongs to the PyrK family.