Iron-sulfur subunit of 8-methylmenaquinol:fumarate reductase (MFR), that catalyzes the reduction of fumarate using 8-methylmenaquinol-6 as electron donor. The complex shows no succinate oxidation activity. Is involved in anaerobic metabolism. 8-methylmenaquinone-6 + succinate = 8-methylmenaquinol-6 + fumarate Binds 1 2Fe-2S cluster. Binds 2 [4Fe-4S] cluster. The MFR complex is composed of three subunits: a flavoprotein (SdhA), an iron-sulfur protein (SdhB), and one hydrophobic anchor protein (SdhE). Belongs to the succinate dehydrogenase/fumarate reductase iron-sulfur protein family.