A flavoprotein (FAD) containing iron-sulfur centres. The complex, present in mitochondria, can be degraded to form EC 1.3.99.1, which no longer reacts with ubiquinone.

In bacteria two distinct, membrane-bound, enzyme complexes are responsible for the interconversion of fumarate and succinate (EC 1.3.99.1): fumarate reductase (Frd) is used in anaerobic growth, and succinate dehydrogenase (Sdh) is used in aerobic growth. Both complexes consist of two main components: a membrane-extrinsic component composed of a FAD-binding flavoprotein and an iron-sulfur protein; and an hydrophobic component composed of a membrane anchor protein and/or a cytochrome B.

In eukaryotes mitochondrial succinate dehydrogenase is an enzyme composed of two subunits: a FAD flavoprotein and and iron-sulfur protein.

The flavoprotein subunit is a protein of about 60 to 70 Kd to which FAD is covalently bound to a histidine residue which is located in the N-terminal section of the protein. The sequence around that histidine is well conserved in Frd and Sdh from various bacterial and eukaryotic species and can be used as a signature pattern.
Reference