The NADH:ubiquinol oxidoreductase, or Complex I, is a central enzyme in respiratory chains that generates the proton motive force (pmf) driving ATP synthesis. Complex I catalyzes electron transfer from NADH to quinone (Q), triggering proton translocation across the membrane up to 200 Å from the quinone binding site. Despite decades of biochemical, biophysical, and structural studies, the long-range coupling mechanism remains poorly understood and highly debated. Moreover, Complex I dysfunction is responsible for half of mitochondrial disorders. To elucidate the coupling between quinone catalysis and proton pumping, we combine site-directed mutagenesis with biochemical and biophysical characterization, structural determination, and multiscale simulations. Our results reveal that key residues from the switching network, which bridge the Q tunnel and the antiporter-like subunits in Complex I, serve as critical coupling elements linking quinone reduction to proton translocation. Our findings provide mechanistic insight into the proton pumping process as well, which is gated by the opening and closing of an ion pair. More broadly, our integrated approach offers new functional understanding of long-range proton-coupled electron transfer reactions in biology.
Adel Beghiah (Sun,) studied this question.