<p>In bacteria two distinct, membrane-bound, enzyme complexes are responsible forthe interconversion of fumarate and succinate (<db_xref db="EC" dbkey="1.3.99.1"/>): fumaratereductase (Frd) is used in anaerobic growth, and succinate dehydrogenase (Sdh)is used in aerobic growth. Both complexes consist of two main components: amembrane-extrinsic component composed of a FAD-binding flavoprotein and aniron-sulphur protein; and an hydrophobic component composed of a membraneanchor protein and/or a cytochrome B.</p><p>In eukaryotes mitochondrial succinate dehydrogenase (ubiquinone) (<db_xref db="EC" dbkey="1.3.5.1"/>)is an enzyme composed of two subunits: a FAD flavoprotein and and iron-sulphurprotein.</p><p>The flavoprotein subunit is a protein of about 60 to 70 Kd to which FAD iscovalently bound to a histidine residue which is located in the N-terminalsection of the protein [<cite idref="PUB00002503"/>]. The sequence around that histidine is wellconserved in Frd and Sdh from various bacterial and eukaryotic species [<cite idref="PUB00002711"/>].</p><p>This family includes members that bind FAD such as the flavoprotein subunits fromsuccinate and fumarate dehydrogenase, aspartate oxidase and the alpha subunit of adenylylsulphatereductase. </p>