The coupling mechanism of mammalian respiratory complex I

Mitochondrial complex I couples NADH:ubiquinone oxidoreduction to proton pumping by an unknown mechanism. Here we present cryo-EM structures of ovine complex I in five different conditions, including turnover, at resolutions up to 2.3-2.5 Å. Resolved water molecules allowed us to experimentally define the proton translocation pathways. Quinone binds at three positions along the quinone cavity, as does the inhibitor rotenone that also binds within subunit ND4. Dramatic conformational changes around the quinone cavity couple the redox reaction to proton translocation during "open" to "closed" state transitions of the enzyme. In the induced deactive state, the "open" conformation is arrested by the ND6 subunit. We propose a detailed molecular coupling mechanism of complex I, which is an unexpected combination of conformational changes and electrostatic interactions.