SQR catalyzes the oxidation of succinate to fumarate coupled to the reduction of quinone to quinol. Eukaryotic SQRs reduce high potential quinones such as ubiquinone. SQR is also called succinate dehydrogenase or Complex II, and is part of the citric acid cycle and the aerobic respiratory chain. SQR is composed of a flavoprotein catalytic subunit, an iron-sulfur protein and one or two hydrophobic transmembrane subunits. Members of this subfamily are classified as Type C SQRs because they contain two transmembrane subunits and one heme group. CybS and CybL are the two transmembrane proteins of eukaryotic SQRs. They contain heme and quinone binding sites. CybS is the eukaryotic homolog of the bacterial SdhD subunit. The two-electron oxidation of succinate in the flavoprotein active site is coupled to the two-electron reduction of quinone in the transmembrane subunits via electron transport through FAD and three iron-sulfur centers. The reversible reduction of quinone is an essential feature of respiration, allowing transfer of electrons between respiratory complexes. Mutations in human Complex II result in various physiological disorders including hereditary paraganglioma and pheochromocytoma tumors. The gene encoding for the SdhD subunit is classified as a tumor suppressor gene.
Feature 1:proximal heme binding site [chemical binding site]
Evidence:
Structure:1ZP0; Sus scrofa mitochondrial Respiratory Complex II membrane anchor subunits bind heme; contacts at 3.5A - View structure with Cn3D
Structure:2FBW; Gallus gallus Respiratory Complex II membrane anchor subunits bind heme; contacts at 3.5A - View structure with Cn3D
Comment:Members of this subfamily contain one heme group, proximal to the [3Fe-4S] cluster of the iron-sulfur subunit. Two histidines from different membrane anchor subunits coordinate with iron in heme.
Comment:Heme is essential for functional assembly and structural stability of the membrane anchor subunits.
Jiyao W et al.(2023). "The conserved domain database in 2023.",