The Na+-Translocating NADH: Ubiquinone Oxidoreductase (Na+-NQR)
The Na+-translocating NADH:quinone oxidoreductase (Na+-NQR) is a unique prokaryotic respiratory enzyme that couples the oxidation of NADH and the reduction of quinone to the selective pumping of Na+ across the cell membrane. Na+-NQR plays a key role in the energetics of marine and pathogenic bacteria, where it functions as the entrance of the respiratory chain. Na+-NQR is the only respiratory enzyme that creates a Na+ gradient, used by the cells to transport nutrients, secrete toxins and antibiotics, and to maintain ion homeostasis. In this chapter, we review the unique properties of Na+-NQR in terms of its redox cofactors and substrates, the electron transfer reactions, the translocation of Na+ and the coupling between these two reactions. We also compare biochemical data with the recently published structure of the enzyme. Finally, we summarize the questions that are of the greatest current interest in studying this fascinating complex.