<p>Succinate dehydrogenase and fumarate reductase are homologous enzymes reversible in principle but favoured under different circumstances. This entry represents a narrowly defined clade of the succinate dehydrogenase flavoprotein subunit as found in mitochondria, in Rickettsia, in <taxon tax_id="562">Escherichia coli</taxon> and other proteobacteria, and in a few other lineages. However, excluded are all known fumarate reductases. It also excludes putative succinate dehydrogenases that appear to diverged before the split between E. coli succinate dehydrogenase and fumarate reductase.</p>