Electron paramagnetic resonance studies of the iron-sulfur centers from complex I of Rhodothermus marinus

Rhodothermus marinus, a thermohalophilic gram negative bacterium, contains a type I NADH/quinone oxidoreductase (complex I). Its purification was optimized, yielding large amounts of pure and active protein. Furthermore, the stoichiometry of NADH oxidation and quinone reduction was shown to be 1:1. The large amounts of protein enabled a thorough characterization by electron paramagnetic resonance (EPR) spectroscopy at different temperatures and microwave powers, using NADH, NADPH, and dithionite as reducing agents. A minimum of two [2Fe-2S](2+/1+) and four [4Fe-4S](2+/1+) centers were observed in the purified complex. Redox titrations monitored by EPR spectroscopy made possible the determination of the reduction potentials of the iron-sulfur centers; with the exception of one of the [4Fe-4S](2+/1+) centers, which has a lower reduction potential, all the other centers have reduction potentials of -240 +/- 20 mV, pH 7.5.