Catalytic relevance of a quinol anion in biological energy conversion by respiratory complex I

Redox chemistry of quinones is an essential component of life on Earth. In the mitochondrial electron transport chain, the ubiquinone molecule is reduced to ubiquinol by respiratory complex I to drive the synthesis of ATP. By performing both classical and hybrid QM/MM simulations on high-resolution cryo-EM structures, including quantitative free energy calculations, we show that the semiquinone species in complex I is anionic in nature and can be trapped in the active site chamber for its subsequent reduction. Two-electron reduction of ubiquinone yields a metastable ubiquinol anion, which is electrostatically pushed by 15-20 Å towards the exit of the ubiquinone binding chamber to drive the proton pump of complex I. As part of the two-electron reduction of ubiquinone, protonic rearrangements take place in the active site in which a highly conserved histidine converts from its one tautomeric state to another. The combined findings challenge the currently held views on quinone redox chemistry of respiratory complex I and provide a detailed and testable mechanistic picture of the proton-coupled electron transfer reaction at its active site under wild-type and mutant conditions.