Linking NrfD / PsrC ‐like architecture to energy conservation: Functional residues in the quinone reactive QrcABCD complex of sulfate‐reducing bacteria

The QrcABCD quinone reductase complex is an electrogenic complex present in sulfate-reducing bacteria of the Desulfobacterota phylum. It operates as a cytochrome c3:menaquinone oxidoreductase involved in electron transfer from periplasmic hydrogen or formate oxidation to the menaquinone (MK) pool. Two proteins in this complex, QrcC and QrcD, form a redox dimer (QrcCD) responsible for MK reduction coupled to proton uptake from the cytoplasm. QrcD belongs to the NrfD/PsrC family, and homologs are found in many bacterial redox complexes in different bioenergetic contexts. In this work a homologous overexpression system for QrcABCD was established in Nitratidesulfovibrio vulgaris Hildenborough and used to produce variants with changes in key amino acids proposed to be involved in energy conservation. Growth studies of the modified strains combined with activity assays with isolated protein variants reconstituted in proteoliposomes revealed the essential role of key amino acids involved in the MK-binding site on the P-side of the membrane, and as part of a proposed proton uptake channel from the cytoplasm to the MK-binding site. The results support the proposed model for energy conservation where, upon formate or hydrogen oxidation, QrcABCD is involved in a redox-loop mechanism with another membrane complex, generating pmf by proton and electron uptake from different sides of the membrane, without active proton pumping.