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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).

Na+-NQR or NqrABCDEF of Vibrio alginolyticus