NAD-dependent formate dehydrogenase (FDH1) was isolated from the a-proteobacterium Methylobacterium extorquensAM1 under oxic conditions. The enzyme was found to be a heterodimer of two subunits (a1b1)of107 and 61 kDa, respectively. The purified enzyme contained per mol enzyme 5 mol nonheme iron and acid-labile sulfur, mol noncovalently bound FMN, and mol tungsten.