Succinate:quinone oxidoreductase (SQR) Type A subfamily, Succinate dehydrogenase C (SdhC)-like subunit; SQR catalyzes the oxidation of succinate to fumarate coupled to the reduction of quinone to quinol. Members of this subfamily reduce low potential quinones such as menaquinone and thermoplasmaquinone. 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 similar to the Thermoplasma acidophilum SQR and are classified as Type A because they contain two transmembrane subunits as well as two heme groups. Although there are no structures available for this subfamily, the presence of two hemes has been proven spectroscopically for T. acidophilum. The two membrane anchor subunits are similar to the SdhD and SdhC subunits of bacterial SQRs, which 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:putative proximal heme binding site [chemical binding site]
Evidence:
Comment:Based on similarity to Type B SQRs, and the proximal heme binding site of Wolinella succinogenes fumarate reductase transmembrane subunit.
Comment:Members of this subfamily may contain two heme groups, one proximal and the other distal to the [3Fe-4S] cluster of the iron-sulfur subunit. Two histidines, one from each membrane anchor subunit, coordinate with iron in each heme. The proximal heme is also referred to as the high-potential 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.",