Cytochrome bc1 complex cytochrome b Qi site (Cytochrome bc1 Qi site)

The cytochrome bc1 complex (Complex III) is a multi-subunit enzyme essential for cellular respiration in fungi and many bacteria (UniProt P00123). It functions within the electron transport chain to catalyze the oxidation of ubiquinol and the reduction of cytochrome c, a process coupled with the translocation of protons across the membrane to generate a proton motive force (PMID: 10548711). The Qi site, located on the cytochrome b subunit, is the internal catalytic site where ubiquinone is reduced back to ubiquinol during the Q-cycle (Wikipedia: Cytochrome bc1 complex). This site is a validated therapeutic target for various antimicrobial and antifungal agents, including agricultural fungicides like cyazofamid and the classic laboratory inhibitor antimycin A (PubChem CID 16219144). By binding to the Qi site, these drugs block electron flow and ATP synthesis, leading to metabolic arrest and cell death (PMID: 29654105). Research focuses on exploiting structural differences between microbial and human Qi sites to achieve selective toxicity and minimize host side effects (PMID: 25613125).

Other names

Complex III Qi siteUbiquinone reduction siteQi pocketCytochrome b Qi siteUbiquinol-cytochrome c reductase Qi site

Inhibition of the Qi site blocks the reduction of ubiquinone to ubiquinol within the Q-cycle of the cytochrome bc1 complex (PMID: 10548711). This disruption halts the electron transport chain, prevents the generation of a proton gradient across the membrane, and ultimately inhibits ATP synthesis, leading to microbial cell death (PMID: 29654105).