Succinate:quinone oxidoreductase (SQR) and Quinol:fumarate reductase (QFR) Type B subfamily, transmembrane subunit; SQR catalyzes the oxidation of succinate to fumarate coupled to the reduction of quinone to quinol, while QFR catalyzes the reverse reaction. SQR, also called succinate dehydrogenase or Complex II, is part of the citric acid cycle and the aerobic respiratory chain, while QFR is involved in anaerobic respiration with fumarate as the terminal electron acceptor. SQR and QFR share a common subunit arrangement, composed of a flavoprotein catalytic subunit, an iron-sulfur protein and one or two hydrophobic transmembrane subunits. Type B proteins contain one transmembrane subunit and two heme groups. The heme and quinone binding sites reside in the transmembrane subunits. The structural arrangement allows efficient electron transfer between the catalytic subunit, through iron-sulfur centers, and the transmembrane subunit containing the electron donor/acceptor (quinol or quinone). The reversible reduction of quinone is an essential feature of respiration, allowing the transfer of electrons between respiratory complexes.
Feature 1:proximal heme binding site [chemical binding site]
Evidence:
Structure:1QLA; Wolinella succinogenes Fumarate Reductase transmembrane subunit binds the proximal heme; contacts at 3.5A
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.
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.",