3.D.10.1.8
Quinol:fumarate reductase respiratory complex, FrdABC, where FrdA (627 aas) and FrdB (264 aas) are soluble proteins while FrdC has 218 aas and 5 TMSs.  The crystal structure has been determined at 3.6 Å resolution (Guan et al. 2018).  QFR in anaerobic bacteria catalyzes the reduction of fumarate to succinate by quinol in the anaerobic respiratory chain. The electron/proton-transfer pathway in the anaerobic sulphate-reducing bacterium Desulfovibrio gigas involves a homo-dimer, each protomer comprising two hydrophilic subunits, A and B, and one transmembrane subunit C, together with six redox cofactors including two b-hemes. One menaquinone molecule is bound near heme bL in the hydrophobic subunit C. This location of the menaquinone-binding site differs from the menaquinol-binding cavity proposed previously for QFR from Wolinella succinogenes (TC#3.D.10.1.3). The bound menaquinone may serve as an additional redox cofactor to mediate the proton-coupled electron transport across the membrane. Guan et al. 2018 proposed electron/proton-transfer pathways during the quinol-dependent reduction of fumarate to succinate in the D. gigas QFR.