Succinate dehydrogenase and fumarate reductase are reverse directions of the same enzymatic interconversion, succinate + FAD+ = fumarate + FADH2 (<db_xref db="EC" dbkey="1.3.99.1"/>). In <taxon tax_id="562">Escherichia coli</taxon>, the forward and reverse reactions are catalyzed by distinct complexes: fumarate reductase operates under anaerobic conditions and succinate dehydrogenase operates under aerobic conditions. This group also includes a region of the B subunit of a cytosolic archaeal fumarate reductase.