It is a membrane bound protein capable of oxidizing a quinone and passing the released electrons to an awaiting fumarate to be reduced.

[1] Subunit A contains the site of fumarate reduction and a covalently bound flavin adenine dinucleotide (FAD) prosthetic group.

In some fumarate reductase structures, one or more heme groups are additionally bound to the C subunit and participate in the electron transfer.

The edge-to-edge distances between the quinol, the iron sulfur clusters, and the FAD in this enzyme do not exceed 12.5 Angstroms and can be seen on the image below.

Succinate dehydrogenase (SQR) is a key enzyme in both the citric acid cycle and the electron transport chain in the mitochondria of eukaryotes and single celled organisms.

[10] It is a key enzyme in aerobic respiration and it performs the opposite reaction of QFR, by coupling the reduction of a quinone to the formation of succinate for use in the citric acid cycle.

[18] Other man-made antibiotics, including Chalcones have also been proven to successfully inhibit fumarate reductase in addition to other cellular enzymes in order to cripple bacterial growth.

However, FAD has been shown to be the most significant cause of superoxide and peroxide formation in fumarate reductase, due to higher solvent accessibility in the active site than in the locations of the quinone and iron-sulfur clusters.