Quinol:fumarate reductase (QFR) Type D subfamily, 13kD hydrophobic subunit D; 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:quinine oxidoreductase (SQR). QFRs oxidize low potential quinols such as menaquinol 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 D as they contain two transmembrane subunits (C and D) and no heme groups. 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 quinone binding site resides in the transmembrane subunits.
Feature 1:proximal quinone binding site [chemical binding site]
Evidence:
Structure:1L0V; Escherichia coli Quinol-Fumarate Reductase binds the proximal menaquinone; contacts at 3.5A
Comment:Escherichia coli QFR binds two menaquinone molecules, one proximal and the other distal to the [3Fe-4S] cluster of the iron-sulfur protein.
Comment:Known inhibitors, such as HQNO, block the binding of menaquinone at the proximal binding site. A reduced quinol bound at this site is thought to be the immediate electron donor to the redox cofactors in the soluble subunits.
Jiyao W et al.(2023). "The conserved domain database in 2023.",