Abstract
The Na+-translocating nicotinamide adenine dinucleotide (NADH):quinine oxidoreductase (Na+–NQR) is a component of respiratory chain of various bacteria and it generates a redox-driven transmembrane electrochemical Na+ potential. It contains four different flavin prosthetic groups, including two flavin mononucleotide (FMN) residues covalently bound to the subunits NqrB and NqrC. Na+–NQR from Vibrio harveyi was poised at different redox potentials to prepare two samples, containing either both FMNNqrB and FMNNqrC or only FMNNqrB in a paramagnetic state. These two samples were comparatively studied using pulse electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and electron-electron double resonance (ELDOR) spectroscopy. The echo-detected EPR spectra and electron spin relaxation properties were very similar for flavin radicals in both samples. The splitting of the outer peaks in the proton ENDOR spectra, assigned to the C(8α) methyl protons, allows to identify both radicals as anionic flavosemiquinones. The mean interspin distance of 20.7 A between these radicals was determined by pulse ELDOR experiment, which allows to estimate the edge-to-edge distance (re) between these flavin centers as: 11.7 A < re < 20.7 A. The direct electron transfer between FMNNqrB and FMNNqrC during the physiological turnover of the Na+–NQR complex is suggested.
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