Succinate:quinone oxidoreductase (SQR) Type B subfamily 1, transmembrane subunit; composed of proteins similar to Bacillus subtilis SQR. SQR catalyzes the oxidation of succinate to fumarate coupled to the reduction of quinone to quinol. Bacillus subtilis SQR reduces low potential quinones such as menaquinone. SQR is also called succinate dehydrogenase (Sdh) 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 B as they contain one transmembrane subunit and two heme groups. The heme and quinone binding sites reside on the transmembrane subunit. The transmembrane subunit of Bacillus subtilis SQR is also called Sdh cytochrome b558 subunit. The structural arrangement allows efficient electron transfer between the catalytic subunit, through iron-sulfur centers, and the transmembrane subunit containing the electron acceptor (quinone). 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:
Comment:Based on the proximal heme binding site of Wolinella succinogenes fumarate reductase transmembrane subunit.
Comment:Members of this subfamily contain two heme groups, one proximal and the other distal to the [3Fe-4S] cluster of the iron-sulfur subunit. Two histidines 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 subunit.
Jiyao W et al.(2023). "The conserved domain database in 2023.",