TGRH88_085470

Toxoplasma gondii cytochrome b is the mitochondrially encoded catalytic subunit of the cytochrome bc1 complex (complex III), where it drives respiratory electron transport between cytochromes b and c1 and maintains mitochondrial membrane potential (PMID:9813002, PMID:10802314). It functions through the canonical two-site ubiquinone/ubiquinol cycle: a ubiquinol oxidation (Qo) pocket that cooperates with the Rieske iron-sulfur protein, and a ubiquinone reduction (Qi) pocket (PMID:12791689, PMID:27919897). Cytochrome b operates in the same ubiquinone/ubiquinol redox cycle as the alternative NADH dehydrogenase, as revealed by synergistic inhibition when both entry points are blocked (PMID:17242151). Cryo-EM of the T. gondii complex III2 and III2-IV supercomplex resolved atovaquone bound in the Qo pocket and ELQ-300 bound at the Qi site in an orientation flipped relative to the mammalian enzyme, defining the structural basis of species-selective inhibition, and knockout of supercomplex assembly established that the complex is critical for parasite fitness (PMID:40389671). The Qo site is the target of atovaquone, whose resistance mutations (M129L, I254L) and the species-specificity residue 275 map to this pocket (PMID:10802314, PMID:12791689), whereas ELQ-class quinolones can act at either the Qo or the Qi site, the latter confirmed by a Thr222Pro mutation cross-resistant to the Qi inhibitor antimycin (PMID:27919897, PMID:27412848, PMID:30117728).

1. 1998 High

   Establishing whether apicomplexans run a functional respiratory chain and where atovaquone acts placed cytochrome b at the heart of parasite mitochondrial respiration.

   Evidence In situ respiration and membrane-potential assays in digitonin-permeabilized tachyzoites with inhibitor pharmacology

   PMID:9813002

   Open questions at the time

   • Localized inhibition between cytochrome b and c1 pharmacologically but did not resolve the specific binding site
   • No structural detail of the target pocket
2. 1999 Medium

   Sequencing a decoquinate-resistant mutant identified cytochrome b/complex III as the drug target, extending the target range beyond atovaquone.

   Evidence Drug-resistant mutant selection and cytochrome b gene sequencing

   PMID:10519259

   Open questions at the time

   • Single study without full mechanistic detail
   • Binding site of decoquinate not mapped
3. 2000 High

   Cloning the gene and mapping atovaquone-resistance mutations (M129L, I254L) to the Qo pocket defined the ubiquinol oxidation site as the atovaquone-binding target.

   Evidence Gene cloning, sequencing of wild-type and resistant strains, structural modeling of the Qo domain

   PMID:10802314

   Open questions at the time

   • Resistance mapping is correlative; no in vitro binding measurement in T. gondii
   • Qi site not addressed
4. 2003 High

   Using the yeast bc1 complex as a surrogate resolved that atovaquone binds competitively at the Qo pocket via the Rieske center and that residue 275 governs species-differential sensitivity.

   Evidence Enzyme inhibition kinetics, EPR of the Rieske center, site-directed mutagenesis, structural modeling in isolated yeast bc1

   PMID:12791689

   Open questions at the time

   • Mechanism inferred in yeast, not T. gondii enzyme directly
   • Does not address Qi-site pharmacology
5. 2007 Medium

   Synergy between an alternative NADH dehydrogenase inhibitor and atovaquone placed cytochrome b in a shared ubiquinone/ubiquinol electron-relay cycle.

   Evidence Drug combination synergy assay with fractional inhibitory concentration calculation in infected cells

   PMID:17242151

   Open questions at the time

   • Pharmacological inference, not direct biochemical measurement of electron flux
   • Single lab
6. 2016 High

   Co-crystallography and mutant yeast showed 4-(1H)-quinolones bind the Qi site, establishing a second druggable pocket on cytochrome b active against both tachyzoites and cysts.

   Evidence Mutant yeast surrogate, co-crystal structure at the bc1 Qi site, in vitro inhibition of tachyzoites and cysts

   PMID:27412848

   Open questions at the time

   • Structure obtained in surrogate enzyme, not T. gondii cytochrome b
   • In vivo efficacy not established here
7. 2017 High

   A Thr222Pro mutation conferring cross-resistance to the Qi inhibitor antimycin genetically confirmed that ELQ-316 acts at the Qi site in T. gondii.

   Evidence Drug-pressure selection, cytochrome b sequencing, cross-resistance analysis with antimycin

   PMID:27919897

   Open questions at the time

   • Site assignment inferred from cross-resistance, not direct structural binding in T. gondii
   • Single resistance mutation
8. 2017 Medium

   Knockout of the mitochondrial MutS homolog TgMSH1 revealed cytochrome b mutation accumulation and, under atovaquone pressure, a resistance mechanism operating through complete respiration shutdown independent of Qo mutations.

   Evidence Genetic knockout of TgMSH1, cytochrome b sequencing, electron transport assay, atovaquone resistance selection

   PMID:29141004

   Open questions at the time

   • Molecular basis of the Qo-independent resistance mechanism not defined
   • Single lab
9. 2018 Medium

   Structure-activity profiling across wild-type and resistant strains established Qo inhibitors as the most potent and linked T. gondii/P. falciparum cytochrome b sequence differences to differential drug susceptibility.

   Evidence Inhibition assays against wild-type and resistant strains, comparative cytochrome b sequence analysis, SAR studies

   PMID:30117728

   Open questions at the time

   • No direct structural confirmation in T. gondii cytochrome b
   • Sequence-susceptibility link is correlative
10. 2019 Medium

    Artemisinin-resistant lines showing mitochondrial genome amplification of cytochrome b and altered membrane potential implicated complex III as part of the artemisinin-affected mitochondrial target.

    Evidence In vitro resistance selection, whole-genome sequencing, CRISPR/Cas9 editing, membrane potential measurement

    PMID:31806760

    Open questions at the time

    • Cytochrome b amplification is correlative rather than a direct target demonstration
    • Whether cytochrome b itself mediates artemisinin action is unresolved
11. 2025 High

    Cryo-EM of the T. gondii complex III2 and III2-IV supercomplex directly visualized atovaquone in the Qo pocket and ELQ-300 in a flipped Qi orientation, defining the structural basis of species-selective inhibition and establishing the supercomplex as critical for parasite fitness.

    Evidence Cryo-EM structure determination with bound inhibitors, genetic knockout of a supercomplex assembly factor, fitness assays

    PMID:40389671

    Open questions at the time

    • Challenges but does not resolve the role of supercomplexes in electron transfer efficiency
    • Catalytic turnover kinetics of the T. gondii enzyme not measured

• The molecular basis of cytochrome b-independent atovaquone resistance and the functional consequence of mitochondrial genome amplification remain unresolved.
• Mechanism of respiration-shutdown resistance in TgMSH1 mutants unknown
• Whether cytochrome b amplification directly modulates drug sensitivity untested
• Electron-transfer role of the III2-IV supercomplex undefined