Thermodynamic Properties of the Redox Centers of Na⁺-Translocating NADH:Quinone Oxidoreductase^†

Redox titration of all optically detectable prosthetic groups of Na⁺-translocating NADH:quinone oxidoreductase (Na⁺-NQR) at pH 7.5 showed that the functionally active enzyme possesses only three titratable flavin cofactors, one noncovalently bound FAD and two covalently bound FMN residues. All three flavins undergo different redox transitions during the function of the enzyme. The noncovalently bound FAD works as a “classical” two-electron carrier with a midpoint potential (E_m) of −200 mV. Each of the FMN residues is capable of only one-electron reduction:  one from neutral flavosemiquinone to fully reduced flavin (E_m = 20 mV) and the other from oxidized flavin to flavosemiquinone anion (E_m = −150 mV). The lacking second half of the redox transitions for the FMNs cannot be reached under our experimental conditions and is most likely not employed in the catalytic cycle. Besides the flavins, a [2Fe-2S] cluster was shown to function in the enzyme as a one-electron carrier with an E_m of −270 mV. The midpoint potentials of all the redox transitions determined in the enzyme were found to be independent of Na⁺ concentration. Even the components that exhibit very strong retardation in the rate of their reduction by NADH at low sodium concentrations experienced no change in the E_m values when the concentration of the coupling ion was changed 1000 times. On the basis of these data, plausible mechanisms for the translocation of transmembrane sodium ions by Na⁺-NQR are discussed.