Localization and function of ubiquinone-8 in the Na+ -pumping NADH:Quinone oxidoreductase from Vibrio cholerae

Na⁺ is the second major coupling ion at membranes after protons, and many pathogenic bacteria use the sodium-motive force to their advantage. A prominent example is Vibrio cholerae which relies on the Na⁺ -pumping NADH:quinone oxidoreductase (Na⁺ -NQR) as the first complex in its respiratory chain. The Na⁺ -NQR is a multisubunit, membrane-embedded NADH dehydrogenase which oxidizes NADH and reduces quinone to quinol (Casutt et al. 2010). Existing models describing redox-driven Na⁺ -translocation by the Na⁺ -NQR are based on the assumption that the pump contains four flavins and one FeS cluster (Juarez et al. 2010). Here we show that the large, peripheral NqrA subunit of the Na⁺ -NQR binds one molecule of ubiquinone-8 (Q₈). Investigations of the dynamic interaction of NqrA with quinones by surface plasmon resonance and saturation transfer difference NMR reveal a high affinity which is determined by the methoxy groups at the C-2 and C-3 positions of the quinone headgroup. A novel scheme of electron transfer in Na⁺ -NQR is proposed which is initiated by NADH oxidation on subunit NqrF and leads to quinol formation on subunit NqrA.