Cytochrome bc1 complex quinone binding site (Q-site)

The Cytochrome bc1 complex quinone binding site, often called the Mitochondrial complex III Q-site, comprises two distinct catalytic pockets—the ubiquinol-oxidation (Qo) site and the ubiquinone-reduction (Qi) site—located within the cytochrome b subunit of the mitochondrial electron transport chain (NIH, 2023). These sites are the functional heart of the "Q-cycle," which couples the transfer of electrons from ubiquinol to cytochrome c with the translocation of protons across the inner mitochondrial membrane to generate the proton motive force necessary for ATP synthesis (Wikipedia, 2024). In addition to its role in bioenergetics, the Qo site is a major source of mitochondrial reactive oxygen species (ROS), which function as signaling molecules in the cellular response to hypoxia (Bell et al., 2007). In apicomplexan parasites like Plasmodium falciparum, the Q-site is vital for regenerating the ubiquinone pool required for de novo pyrimidine biosynthesis, making it a critical target for antimalarial drugs such as atovaquone (NIH, 2022). Atovaquone and other inhibitors competitively bind to these sites to disrupt parasitic respiration, though their efficacy is often challenged by the emergence of resistance-conferring mutations in the cytochrome b gene (ResearchGate, 2022). Furthermore, achieving selectivity to avoid off-target inhibition of human Complex III is a primary safety consideration in the development of new therapeutics targeting this site (Gosset.ai, 2026).

Other names

Mitochondrial complex III Q-siteQo siteQi siteUbiquinol oxidation siteUbiquinone reduction siteP-siteN-siteCytochrome b active site

Inhibition of the Q-cycle by competitive binding to the ubiquinol-oxidation (Qo) or ubiquinone-reduction (Qi) sites, preventing electron transfer to cytochrome c and disrupting the mitochondrial proton gradient and ATP synthesis.