TGRH88_085370

Cytochrome b is the mitochondrially encoded core catalytic subunit of the Toxoplasma gondii cytochrome bc1 complex (complex III), the electron-transfer step that couples ubiquinol oxidation to maintenance of mitochondrial membrane potential in the respiratory chain (PMID:9813002, PMID:10802314). The protein carries two distinct catalytic sites that are the targets of antiparasitic drugs: a Qo (ubiquinol-oxidation) site, where atovaquone acts to block electron transport between cytochrome b and c1 and collapse the membrane potential, with resistance conferred by M129L/I254L substitutions (PMID:10802314, PMID:9813002); and a Qi (quinone-reduction) site, where endochin-like quinolones and antimycin act, with the T222P substitution conferring cross-resistance to both (PMID:27919897, PMID:27412848). Structure-activity work shows that quinolone substituents determine which site a compound engages, and that naturally occurring residue differences in T. gondii cytochrome b set baseline drug susceptibility relative to Plasmodium falciparum (PMID:30117728). Cryo-EM structures of the complex III2 and a clade-specific III2-IV supercomplex define the atomic basis of species-selective atovaquone binding at the Qo site and a flipped Qi-site orientation of ELQ-300, and establish that the apicomplexan supercomplex architecture is critical for parasite fitness (PMID:40389671). The parasite retains metabolic plasticity that permits survival when oxidative phosphorylation is shut down under atovaquone pressure, without requiring canonical Qo resistance mutations (PMID:29141004).

1. 1998 High

   Establishing where atovaquone acts in the parasite respiratory chain was needed to define cytochrome b's functional role; direct assays localized inhibition to the cytochrome b-to-c1 electron-transfer step.

   Evidence Digitonin-permeabilized tachyzoites with respiration and membrane-potential (safranin O) assays under inhibitor titration

   PMID:9813002

   Open questions at the time

   • Did not identify the specific residues of the binding site
   • No structural model of the inhibited complex
2. 1999 Medium

   Identifying the molecular target of decoquinate tested whether cytochrome b is a drug target; a resistance mutation in cytochrome b implicated the bc1 complex.

   Evidence Drug-resistant mutant generation and gene sequencing

   PMID:10519259

   Open questions at the time

   • Single lab, no in vitro reconstitution
   • Exact binding site within cytochrome b not mapped
3. 2000 High

   Mapping the atovaquone-binding site required cloning cytochrome b and locating resistance mutations; M129L and I254L in the Qo domain established the ubiquinol-binding pocket as the target.

   Evidence Atovaquone-resistant mutant generation, gene cloning/sequencing, structural modeling of the Qo domain

   PMID:10802314

   Open questions at the time

   • Structural model only, no experimental structure of bound drug
   • Qi site not characterized
4. 2016 High

   Whether the second catalytic site could be drugged in T. gondii was addressed by developing quinolones that bind the Qi site, confirmed by structure and surrogate biochemistry.

   Evidence Co-crystallography with bc1 complex, mutant yeast surrogate system, in vitro parasite growth inhibition

   PMID:27412848

   Open questions at the time

   • Qi-site binding confirmed in surrogate systems rather than native T. gondii complex
   • In vivo efficacy not established here
5. 2017 High

   Genetic evidence was needed to assign endochin-like quinolones to the Qi site; a T222P substitution conferring cross-resistance to ELQ-316 and antimycin placed both inhibitors at the Qi site.

   Evidence Drug-resistant mutant selection, gene sequencing, genetic characterization of resistant clones

   PMID:27919897

   Open questions at the time

   • Structural basis of the T222P effect not defined
   • Effect on native electron transfer kinetics not measured
6. 2017 Medium

   Whether cytochrome b Qo mutations are the only route to atovaquone survival was tested via a mutator background; parasites shut down respiration entirely, revealing metabolic plasticity as an escape route independent of canonical Qo resistance.

   Evidence TgMSH1 knockout, mitochondrial DNA sequencing, electron-transport inhibition assay, atovaquone resistance selection

   PMID:29141004

   Open questions at the time

   • Metabolic pathway sustaining survival without OXPHOS not defined
   • Single lab
7. 2018 Medium

   How to distinguish Qo- versus Qi-targeting compounds and explain species-specific susceptibility was addressed by structure-activity and comparative sequence analysis of cytochrome b.

   Evidence Comparative sequence analysis, testing against wild-type and resistant T. gondii and P. falciparum, in vitro growth inhibition

   PMID:30117728

   Open questions at the time

   • No structural confirmation of T. gondii-specific residue effects
   • Single lab
8. 2019 Medium

   Whether cytochrome b and the mitochondrion contribute to artemisinin resistance was probed; resistant lines amplified the mitochondrial genome including cytochrome b and altered membrane potential.

   Evidence Whole-genome sequencing, mitochondrial copy-number analysis, membrane-potential assay, CRISPR/Cas9 point mutation

   PMID:31806760

   Open questions at the time

   • Cytochrome b amplification is correlative; direct mechanistic link is indirect
   • Causal contribution of cytochrome b versus other amplified genes unresolved
9. 2025 High

   The atomic basis of species-selective inhibitor binding and the parasite-specific complex architecture was resolved by cryo-EM, defining Qo atovaquone contacts, a flipped Qi ELQ-300 pose, and a fitness-critical III2-IV supercomplex.

   Evidence Cryo-EM of complex III2 and III2-IV supercomplex, inhibitor soaking, supercomplex-disassembly knockout, structure-guided chemistry

   PMID:40389671

   Open questions at the time

   • Precise role of III2-IV in electron-transfer efficiency remained challenged by the disassembly knockout
   • Functional consequences of clade-specific subunits not fully defined

• The metabolic pathways that allow T. gondii to survive respiratory shutdown, and the precise physiological function of the apicomplexan III2-IV supercomplex, remain to be defined.
• Mechanism of OXPHOS-independent survival unresolved
• Role of the supercomplex in electron-transfer efficiency unsettled