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

Na⁺-translocating 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 <i>Vibrio harveyi</i>, 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 (FMN_NqrB and FMN_NqrC). Using data obtained by pulse X- and Q-band EPR as well as by pulse ENDOR and ELDOR spectroscopy, the interspin distance between FMN_NqrB and FMN_NqrC was found to be 15.3 Å in the absence and 20.4 Å in the presence of Na⁺, respectively. Thus, the distance between the covalently bound FMN residues can vary by about 5 Å upon changes in Na⁺ concentration. Using these results, we propose a scheme of the sodium potential generation by Na⁺-NQR based on the redox- and sodium-dependent conformational changes in the enzyme