Plasmodium species mitochondrial electron transport chain cytochrome bc1 complex (cytochrome bc1 complex (sometimes abbreviated as "Complex III" in the context of the mitochondrial respiratory chain))

The Plasmodium species mitochondrial electron transport chain cytochrome bc₁ complex, also known as Complex III or ubiquinol-cytochrome c reductase, is a multi-subunit enzyme embedded in the inner membrane of mitochondria. It catalyzes electron transfer from ubiquinol to cytochrome c while pumping protons across the membrane—a process essential for maintaining ATP synthesis via oxidative phosphorylation. In Plasmodium parasites—the causative agents of malaria—this enzyme is vital not only for energy metabolism but also indirectly supports pyrimidine biosynthesis required for DNA replication. The central catalytic subunit, cytochrome b, contains two distinct quinone-binding sites targeted by several antimalarial drugs including atovaquone. Inhibition leads to collapse of mitochondrial function and death of the parasite. Drug resistance arises primarily through point mutations within these binding sites on Plasmodium’s mitochondrially encoded genes.[1][2][3]

Other names

Cytochrome bc1 complexComplex IIIUbiquinol-cytochrome c reductaseMitochondrial respiratory chain complex III

Drugs such as atovaquone inhibit the cytochrome bc1 complex by binding to its quinol oxidation site (Qo site) on cytochrome b. This blocks electron transfer from ubiquinol to cytochrome c, collapses the mitochondrial membrane potential, halts ATP production, and disrupts pyrimidine biosynthesis—ultimately killing the parasite[1][2]. CK‐2‐68 and related compounds act similarly by selectively inhibiting Plasmodium's version of this enzyme over mammalian forms[5]. Resistance can arise through mutations in key residues within these binding sites on cytochrome b[1].