The iron-sulfur clusters 2 and ubisemiquinone radicals of NADH:ubiquinone oxidoreductase are involved in energy coupling in submitochondrial particles

The behavior of ubisemiquinone radicals and the iron-sulfur clusters 2 of NADH:ubiquinone oxidoreductase (Complex I) in coupled and uncoupled submitochondrial particles (SMP), oxidizing either NADH or succinate under steady-state conditions, was studied. Multifrequency EPR spectra revealed that the two new g(z) lines of the clusters 2, only observed during coupled electron transfer under conditions where energy dissipation is rate-limiting [De Jong, A. M. Ph., Kotlyar, A. B., & Albracht, S. P. J. (1994) Biochim. Biophys. Acta 1186, 163-171], are the result of a spin-spin interaction of 2.8 mT. Investigation of the radical signals present in coupled SMP indicated that more than 90% of the radicals can be ascribed to two types of semiquinones which are bound to Complex I (Q(I)-radicals) or ubiquinol:cytochrome c oxidoreductase (Complex III; Q(III)-radicals). The presence of Q(III)-radicals, but not that of Q(I)-radicals, was completely abolished by uncoupler. Part of the Q(I)-radicals weakly interact with the clusters 2 of Complex I. This uncoupler-sensitive interaction can amount to a splitting of the radical EPR signal of at most 1 mT, considerably weaker than the 2.8 mT splitting of the g(z) lines of the clusters 2. We propose that the 2.8 mT splitting of these g(z) lines results from an energy-induced spin-spin interaction between the two clusters 2 within the TYKY subunit of Complex I. The two clusters 2 show no interaction during electron transfer in uncoupled SMP or in fully-reduced anaerobic-coupled SMP. The results point to a direct role of the Fe-S clusters 2 and the Q(I)-radicals in the mechanism of coupled electron transfer catalyzed by Complex I.