II- Succinate:quinone oxidoreductase

Complex II, Succinate:Quinone Oxidoreductase (SQR), catalyzes the oxidation of succinate to fumarate, donating electrons to quinones, and until now it was not shown to contribute to the establishment of the electrochemical membrane potential. The enzyme is composed by a cytoplasmatic and an anchor domain. The cytoplasmatic domain is built of two subunits, a flavoprotein (SdhA), harbouring a covalently bound FAD, and an iron–sulfur protein (SdhB), containing one [2Fe-2S] 2+/1+ (S1), one [4Fe-4S] 2+/1+ (S2), and one [3Fe-4S] 1+/0 (S3) (or a second [4Fe-4S] 2+/1+ ) clusters. Depending on the anchor nature and on the FeS cluster composition, the enzymes can be divided into 5 types, A to E (Lancaster et al. 2000; Lemos et al. 2002) (Figure 4). The anchor domain provides the binding site for the quinone and can be composed by transmembrane (types A to D) or by “putative” monotopic polypeptides (type E).