Energy‐dependent Complex I‐associated ubisemiquinones in submitochondrial particles

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Abstract
Two distinct species of Complex I‐associated ubisemiquinones (SQNf and SQNs) were detected by cryogenic EPR analysis of tightly coupled submitochondrial particles oxidizing NADH or succinate under steady‐state conditions. The g = 2.00 signals from both fast‐relaxing at 40 K) and slow‐relaxing ) are sensitive to uncouplers, rotenone and thermally induced deactivation of Complex I. At higher temperatures the SQNf signal is broadened and only the SQNs, signal is seen (). The spin‐spin interaction between SQNf and the iron‐sulfur cluster N2 was detected as split peaks of the 2.05 signal with a coupling constant of 1.65 mT, revealing their mutual distance of 8–11 Å. The data obtained are consistent with a model in which N2 and two interacting bound ubisemiquinone species are spatially arranged within the hydrophobic domain of Complex I, participating in the vectorial proton translocation.

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