3.D.5.1.1
Na⁺-translocating NADH-quinone oxidoreductase, NqrABCDEF (Steuber et al. 2014).  Evolution of the Na⁺-NQR complex may have involved functional divergence from its RNF homologue (3.D.6) following duplication of the rnf operon, loss of the rnfB gene and the recruitment of the reductase subunit of an aromatic monooxygenase.  Two additional proteins, ApbE and NqrM (DUF539), are essential for activity. ApbE is responsible for covalent attachment of flavin mononucleotide (FMN) while NqrM (D0WX40) is necessary for biogenesis (Kostyrko et al. 2016).  NqrM has an N-terminal TMS and four cysteyl residues that are essential for full activity (Kostyrko et al. 2016). The 3D structure of NQR reveals a transmembrane channel in subunit NqrB. Toulouse et al. 2016; proposed that partial uncoupling of the Vibrio cholerae NQR observed with Li⁺, or with Na⁺ at pH 7.5 - 8.0, is caused by the backflow of the coupling cation through the channel in NqrB (Toulouse et al. 2016). A ubiquinone binding is present in subunit B at the interface between subunits B and D (Tuz et al. 2017). In Vibrio cholerae, the mechanism of the homologous system involves conformational coupling of redox-driven Na⁺-translocation has been studied (Hau et al. 2023).  Ion pumping in Na⁺-NQR is driven by large conformational changes coupling electron transfer to ion translocation. We have determined a series of cryo-EM and X-ray structures of the Na⁺-NQR that represent snapshots of the catalytic cycle. The six subunits NqrA, B, C, D, E, and F of Na⁺-NQR harbor a unique set of cofactors that shuttle the electrons from NADH twice across the membrane to quinone. The redox state of a unique intramembranous [2Fe-2S] cluster orchestrates the movements of subunit NqrC, which acts as an electron transfer switch. The authors propose that this switching movement controls the release of Na⁺ from a binding site localized in subunit NqrB (Hau et al. 2023).