Mitochondrial cytochrome bc1 complex, Qi site of cytochrome b (Complex III Qi site)

The Mitochondrial cytochrome bc1 complex, specifically the Qi site of cytochrome b, is a vital component of the mitochondrial respiratory chain, also known as Complex III. It is located on the matrix-facing side of the inner mitochondrial membrane and is responsible for the reduction of ubiquinone to ubiquinol as part of the Q-cycle mechanism. This site is a validated therapeutic target for various anti-infective agents, including antimalarials, fungicides, and treatments for neglected tropical diseases like Chagas disease and leishmaniasis. Inhibitors binding to the Qi site, such as antimycin A and certain 4(1H)-quinolones, disrupt electron flow and proton translocation, leading to a collapse of the mitochondrial membrane potential and subsequent cell death. In parasites like Plasmodium falciparum, this inhibition also halts de novo pyrimidine biosynthesis, which is dependent on the pool of oxidized ubiquinone. A significant challenge in drug development for this target is achieving high selectivity for the pathogen's complex over the human version to minimize host cytotoxicity. Furthermore, the rapid emergence of resistance through point mutations in the cytochrome b gene remains a critical concern for long-term therapeutic efficacy.

Other names

Ubiquinol-cytochrome c reductase Qi siteCytochrome b Qi siteQN siteInner quinone binding siteComplex III QN siteUbiquinone reduction site

Inhibition of the Qi site blocks the reduction of ubiquinone to ubiquinol within the Q-cycle of the mitochondrial electron transport chain. This disruption halts electron flow from cytochrome b to the quinone pool, prevents proton translocation across the inner mitochondrial membrane, and leads to the collapse of the mitochondrial membrane potential. Consequently, ATP synthesis is inhibited, and in certain pathogens like Plasmodium, the lack of oxidized ubiquinone also disrupts essential metabolic pathways such as de novo pyrimidine biosynthesis, ultimately leading to cell death.