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Correlating kinetic and structural data on ubiquinone binding and reduction by respiratory complex I

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Fedor, JG 
Jones, AJY 
Di Luca, A 
Kaila, VRI 


Respiratory complex I (NADH:ubiquinone oxidoreductase), one of the largest membrane-bound enzymes in mammalian cells, powers ATP synthesis by using the energy from electron transfer from NADH to ubiquinone-10 to drive protons across the energy-transducing mitochondrial inner membrane. Ubiquinone-10 is extremely hydrophobic, but in complex I the binding site for its redox-active quinone headgroup is ~20 Å above the membrane surface. Structural data suggest it accesses the site by a narrow channel long enough to accommodate almost all its ~50 Å isoprenoid chain. However, how ubiquinone/ol exchange occurs on catalytically-relevant timescales, and whether binding/dissociation events are involved in coupling electron transfer to proton translocation, are unknown. Here, we use proteoliposomes containing complex I, together with a quinol oxidase, to determine the kinetics of complex I catalysis with ubiquinones of varying isoprenoid chain length, from 1 to 10 units. We interpret our results using structural data, which show the hydrophobic channel is interrupted by a highly-charged region at isoprenoids 4 to 7. We demonstrate that ubiquinol-10 dissociation is not rate determining, and deduce that ubiquinone-10 has both the highest binding affinity and the fastest binding rate. We propose that the charged region and chain directionality assist product dissociation, and that isoprenoid stepping ensures short transit times. These properties of the channel do not benefit the exhange of short-chain quinones, for which product dissociation may become rate limiting. Thus, we discuss how the long channel does not hinder catalysis under physiological conditions, and the possible roles of ubiquinone/ubiquinol binding/dissociation in energy conversion.



NADH:ubiquinone oxidoreductase, bioenergetics, coenzyme Q10, electron transport chain, mitochondria, Amino Acid Motifs, Animals, Binding Sites, Biocatalysis, Cattle, Electron Transport Complex I, Gene Expression, Hydrophobic and Hydrophilic Interactions, Kinetics, Mitochondria, Heart, Models, Molecular, Oxidoreductases, Protein Binding, Protein Interaction Domains and Motifs, Protein Structure, Secondary, Proteolipids, Recombinant Proteins, Static Electricity, Substrate Specificity, Swine, Terpenes, Thermodynamics, Thermus thermophilus, Ubiquinone

Journal Title

Proceedings of the National Academy of Sciences of the United States of America

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National Academy of Sciences
Medical Research Council (MC_U105663141)
MRC (MC_UU_00015/2)
Medical Research Council (MC_UU_00015/7)
We thank C. Humphreys & Sons Abattoir (Chelmsford) for providing bovine hearts and Sotiria Tavoulari for help with the amido black assay. Computing time was provided by the SuperMuc at The Leibniz Rechenzentrum (Grant pr48de). This work was supported by the Medical Research Council (Grant U105663141 to J.H.) and by the German Research Foundation (V.R.I.K.).
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