Succinate:quinone oxidoreductase (SQR) Type C subfamily, Succinate dehydrogenase D (SdhD) subunit; SQR catalyzes the oxidation of succinate to fumarate coupled to the reduction of quinone to quinol. E. coli SQR, a member of this subfamily, reduces the high potential quinine, 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. SdhD and SdhC are the two transmembrane proteins of bacterial SQRs. They contain heme and quinone binding sites. The two-electron oxidation of succinate in the flavoprotein active site is coupled to the two-electron reduction of quinone in the membrane anchor 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.
Feature 1:proximal heme binding site [chemical binding site]
Evidence:
Structure:1NEK; Escherichia coli Succinate dehydrogenase binds heme; contacts at 3.5A
Structure:2ACZ; Escherichia coli Succinate dehydrogenase binds heme; contacts at 3.5A
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 transmembrane subunits.
Jiyao W et al.(2023). "The conserved domain database in 2023.",