Sequencing and preliminary characterization of the Na⁺-translocating NADH:ubiquinone oxidoreductase from Vibrio harveyi

The Na⁺-translocating NADH: ubiquinone oxidoreductase (Na⁺-NQR) generates an electrochemical Na⁺ potential driven by aerobic respiration. Previous studies on the enzyme from Vibrio alginolyticus have shown that the Na⁺-NQR has six subunits, and it is known to contain FAD and an FeS center as redox cofactors. In the current work, the enzyme from the marine bacterium Vibrio harveyi has been purified and characterized. In addition to FAD, a second flavin, tentatively identified as FMN, was discovered to be covalently attached to the NqrC subunit. The purified V. harveyi Na⁺-NQR was reconstituted into proteoliposomes. The generation of a transmembrane electric potential by the enzyme upon NADH:Q₁ oxidoreduction was strictly dependent on Na⁺, resistant to the protonophore CCCP, and sensitive to the sodium ionophore ETH-157, showing that the enzyme operates as a primary electrogenic sodium pump. Interior alkalinization of the inside-out proteoliposomes due to the operation of the Na⁺NQR was accelerated by CCCP, inhibited by valinomycin, and completely arrested by ETH-157. Hence, the protons required for ubiquinol formation must be taken up from the outside of the liposomes, which corresponds to the bacterial cytoplasm. The Na⁺-NQR operon from this bacterium was sequenced, and the sequence shows strong homology to the previously reported Na⁺-NQR operons from V. alginolyticus and Haemophilus influenzae. Homology studies show that a number of other bacteria, including a number of pathogenic species, also have an Na⁺-NQR operon.