Quinol:fumarate reductase (QFR) Type B subfamily, transmembrane subunit; QFR couples the reduction of fumarate to succinate to the oxidation of quinol to quinone, the opposite reaction to that catalyzed by the related protein, succinate:quinone oxidoreductase (SQR). QFRs oxidize low potential quinols such as menaquinol and rhodoquinol and are 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. 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 structural arrangement allows efficient electron transfer between the catalytic subunit, through iron-sulfur centers, and the transmembrane subunit containing the electron donor (quinol). The Type B enzyme from Desulfovibrio gigas is capable of fumarate reduction and succinate oxidation.
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.",