Succinate:quinone oxidoreductase (SQR) Type C subfamily, Succinate dehydrogenase D (SdhD) subunit-like; composed of predominantly uncharacterized bacterial proteins with similarity to the E. coli SdhD subunit. One characterized protein is the respiratory Complex II SdhD subunit of the only eukaryotic member, Reclinomonas americana. SQR catalyzes the oxidation of succinate to fumarate coupled to the reduction of quinone to quinol. It 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. E. coli SQR is classified as Type C SQRs because it contains two transmembrane subunits and one heme group. The SdhD and SdhC subunits are membrane anchor subunits containing 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 proximal heme binding site of Escherichia coli Succinate dehydrogenase.
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.
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