FAD-linked dehydrogenase is

Succinate dehydrogenase or succinate-coenzyme Q reductase (SQR) or respiratory Complex II is an enzyme complex, found in many bacterial cells and in the inner mitochondrial membrane of mammalian mitochondria. It is the only enzyme that paicipates in both the citric acid cycle and the electron transpo together. In step 6 of the citric acid cycle, SQR catalyzes the oxidation of succinate to fumarate with the reduction of ubiquinone to ubiquinol. This occurs in the inner mitochondrial membrane by coupling the two reactions together. Little is known about the exact succinate oxidation mechanism. However, the crystal structure shows that FAD, Glu255, Arg286, and His242 of subunit A are good candidates for the initial deprotonation step. Thereafter, there are two possible elimination mechanisms: E2 or E1cb. In the E2 elimination, the mechanism is conceed. The basic residue or cofactor deprotonates the alpha carbon, and FAD accepts the hydride from the beta carbon, oxidizing the bound succinate to fumarate--refer to image 6. In E1cb, an enolate intermediate is formed, shown in image 7, before FAD accepts the hydride. Fuher research is required to determine which elimination mechanism succinate undergoes in Succinate Dehydrogenase. Oxidized fumarate, now loosely bound to the active site, is free to exit the protein.