Metal reductase able to reduce Fe(III)-chelates to Fe(II)-chelates, as well as soluble Cr(VI) and U(VI), using NADH as electron donor. Cannot 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. Homotetramer. Has an FMN-binding N-terminal domain (NTD), an FAD-binding C-terminal domain (CTD), and a 4Fe-4S cluster between the two domains. CTD is the main contributor to the iron-reduction activity, and NTD and the 4Fe-4S cluster are not directly involved in such activity. In the N-terminal section; belongs to the NADH:flavin oxidoreductase/NADH oxidase family.