Pulsed EPR Study of the Interaction Between 23Na+ and Flavin in the Sodium-Pumping NADH:Ubiquinone Oxidoreductase (NQR) from Vibrio cholerae

Sodium-pumping NADH: ubiquinone oxidoreductase (Na+-NQR) is an important component of the aerobic respiratory chain of Vibrio cholerae. It oxidizes NADH, reduces ubiquinone, and uses the free energy of this redox reaction to move sodium across the cell membrane. The enzyme is a membrane complex of six subunits, two 2Fe−2S centers, and four flavins. Both the oxidized and reduced forms of Na+-NQR exhibit EPR signals due to flavin semiquinone radicals. It has been shown that in the oxidized form of the enzyme, the radical is a neutral flavin, while in the NADH-reduced form, the radical is an anionic flavin. Electron Spin Echo Envelope Modulation Spectroscopy (ESEEM) was used to probe the presence of the magnetic nucleus 23Na in the immediate vicinity of the paramagnetic centers. The contribution of the 23Na nucleus was observed only in the ESEEM spectra of the anionic flavin semiquinone previously assigned to FMNNqrB. Analysis shows that the Na+ ion is within ~3–4 Å of the flavin radical. This distance is consistent with two models: (i) complexation of the Na+ ion with the carbonyl group of CO4; or alternatively, (ii) a “cation-π interaction,” between Na+ and the electron-rich π-system of the flavin aromatic rings.