Flavoprotein 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. SdhA contains the dicarboxylate reduction site. 8-methylmenaquinone-6 + succinate = 8-methylmenaquinol-6 + fumarate The MFR complex is composed of three subunits: a flavoprotein (SdhA), an iron-sulfur protein (SdhB), and one hydrophobic anchor protein (SdhE). Is exported to the periplasm via the Tat pathway. Predicted to be exported by the Tat system. The position of the signal peptide cleavage has not been experimentally proven. Belongs to the FAD-dependent oxidoreductase 2 family. FRD/SDH subfamily.