SummaryThe NADH:ubiquinone oxidoreductase, respiratory complex I, couples electron transfer from NADH to ubiquinone with the translocation of protons across the membrane. The complex consists of a peripheral arm catalyzing the redox reaction and a membrane arm catalyzing proton translocation. The membrane arm is almost completely aligned by a 110 Å unique horizontal helix that is discussed to transmit conformational changes induced by the redox reaction in a piston-like movement to the membrane arm driving proton translocation. Here, we analyzed such a proposed movement by cysteine-scanning of the helix of the Escherichia coli complex I. The accessibility of engineered cysteine residues and the flexibility of individual positions were determined by labeling the preparations with a fluorescent marker and a spinprobe, respectively, in the oxidized and reduced states. The differences in fluorescence labeling and the rotational flexibility of the spin probe between both redox states indicate only slight conformational changes at distinct positions of the helix but not a large movement.

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