Succinate:quinone oxidoreductase (SQR)-like Type B subfamily 2, transmembrane subunit; composed of proteins with similarity to the SQRs of Geobacter metallireducens and Corynebacterium glutamicum. SQR catalyzes the oxidation of succinate to fumarate coupled to the reduction of quinone to quinol. C. glutamicum 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 in the transmembrane subunit. The transmembrane subunit of members of this subfamily is also called Sdh cytochrome b558 subunit based on the Bacillus subtilis protein. 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. Proteins in this subfamily from G. metallireducens and G. sulfurreducens are bifunctional enzymes with SQR and QFR activities.
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:Wolinella succinogenes fumarate reductase 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.
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