Sodium-Dependent Movement of Covalently Bound FMN Residue(s) in Na+-Translocating NADH:Quinone Oxidoreductase

Na+-trans locating NADH:quinone oxidoreductase (Na+-NQR) is a component of respiratory electron-transport chain of various bacteria generating redox-driven transmembrane electrochemical Na+ potential. We found that the change in Na+ concentration in the reaction medium has no effect on the thermodynamic properties of prosthetic groups of Na+-NQR from Vibrio harveyi, as was revealed by the anaerobic equilibrium redox titration of the enzyme's EPR spectra. On the other hand, the change in Na+ concentration strongly alters the EPR spectral properties of the radical pair formed by the two anionic semiquinones of FMN residues bound to the NqrB and NqrC subunits (FMNNqrB and FMNNqrC). Using data obtained by pulse X- and Q:band EPR as well as by pulse ENDOR and ELDOR spectroscopy, the interspin distance between FMNNqrB and FMNNqrC was found to be 15.3 angstrom in the absence and 20.4 angstrom in the presence of Na+, respectively. Thus, the distance between the covalently bound FMN residues can vary by about 5 angstrom upon changes in Na+ concentration. Using these results, we propose a scheme of the sodium potential generation by Nat-NQR based on the redox- and sodium-dependent conformational changes in the enzyme.