The structure of Na⁺-translocating of NADH:ubiquinone oxidoreductase of Vibrio cholerae: implications on coupling between electron transfer and Na⁺ transport.

The Na⁺-translocating NADH:ubiquinone oxidoreductase (Na⁺-NQR) of Vibrio cholerae is a respiratory complex that couples the exergonic oxidation of NADH to the transport of Na⁺ across the cytoplasmic membrane. It is composed of six different subunits, NqrA, NqrB, NqrC, NqrD, NqrE, and NqrF, which harbor FAD, FMN, riboflavin, quinone, and two FeS centers as redox co-factors. We recently determined the X-ray structure of the entire Na⁺-NQR complex at 3.5-Å resolution and complemented the analysis by high-resolution structures of NqrA, NqrC, and NqrF. The position of flavin and FeS co-factors both at the cytoplasmic and the periplasmic side revealed an electron transfer pathway from cytoplasmic subunit NqrF across the membrane to the periplasmic NqrC, and via NqrB back to the quinone reduction site on cytoplasmic NqrA. A so far unknown Fe site located in the midst of membrane-embedded subunits NqrD and NqrE shuttles the electrons over the membrane. Some distances observed between redox centers appear to be too large for effective electron transfer and require conformational changes that are most likely involved in Na⁺ transport. Based on the structure, we propose a mechanism where redox induced conformational changes critically couple electron transfer to Na⁺ translocation from the cytoplasm to the periplasm through a channel in subunit NqrB.