TGRH88_085380

Toxoplasma gondii cytochrome b is a mitochondrially encoded, ubiquinol-oxidizing subunit of the cytochrome bc1 complex (complex III) that carries respiratory electron transport between cytochrome b and c1; its inhibition disrupts mitochondrial membrane potential and blocks oxidative phosphorylation in tachyzoites (PMID:9813002). It is the molecular target of atovaquone, which binds the ubiquinol-oxidation (Qo) pocket in cooperation with the Rieske iron-sulfur protein, and atovaquone-resistant parasites carry Qo-domain point mutations (M129L, I254L) (PMID:10802314, PMID:12791689). A structurally distinct class of 4(1H)-quinolone diaryl ethers (endochin-like quinolones, ELQs) instead engages the quinone-reduction (Qi) site: a Thr222Pro substitution confers resistance to ELQ-316 with cross-resistance to the Qi inhibitor antimycin, and Qi binding was confirmed co-crystallographically (PMID:27919897, PMID:27412848). Cryo-EM structures of the T. gondii III2-IV supercomplex and isolated complex III2 resolved atovaquone in the Qo site and ELQ-300 in the Qi site with a flipped, parasite-specific binding geometry that explains species-selective inhibition and enabled design of subnanomolar ELQs (PMID:40389671). Cytochrome b is highly expressed in bradyzoite cysts, extending its relevance to the latent stage (PMID:27412848), and it also functions as a node for combination therapy, acting synergistically with type II NADH dehydrogenase inhibition (PMID:17242151). Resistance is not solely genetic: parasites can survive atovaquone by shutting down oxidative phosphorylation entirely, independent of cytochrome b mutation (PMID:29141004).

1. 1998 High

   Establishing what respiratory step atovaquone blocks was needed to define its antiparasitic mechanism; direct assay placed the block within complex III and linked it to loss of membrane potential and ATP synthesis.

   Evidence In situ respiration and safranin O membrane-potential assays in digitonin-permeabilized tachyzoites with inhibitor profiling

   PMID:9813002

   Open questions at the time

   • Did not identify the specific cytochrome b residues or subsite engaged
   • No structural resolution of the binding pocket
2. 1999 Medium

   Identifying the target of decoquinate addressed whether multiple quinolone-class drugs converge on the same protein; a resistance mutation implicated cytochrome b.

   Evidence Cytochrome b gene sequencing from a decoquinate-resistant mutant

   PMID:10519259

   Open questions at the time

   • Single method (sequencing) without functional reconstitution
   • Subsite (Qo vs Qi) not defined
3. 2000 High

   Cloning the gene and mapping atovaquone-resistance mutations to a functional domain established cytochrome b as the atovaquone target and localized the drug to the Qo site.

   Evidence Cloning/sequencing of wild-type and resistant strains with Qo-domain structural modeling

   PMID:10802314

   Open questions at the time

   • No experimental structure of the T. gondii Qo pocket
   • Mechanism of species selectivity not resolved
4. 2003 High

   Determining how atovaquone selectivity is encoded clarified species-specific inhibition; the drug binds the Qo pocket contacting the Rieske protein, with residue 275 identity governing sensitivity.

   Evidence Purified bc1 kinetics, EPR of the Rieske center, redox measurement, and L275F mutagenesis in yeast

   PMID:12791689

   Open questions at the time

   • Performed in yeast, not T. gondii cytochrome b directly
   • Does not address Qi-site inhibitors
5. 2007 Medium

   Testing combination pharmacology showed cytochrome b inhibition can be potentiated by blocking an upstream ubiquinone step, demonstrating synergistic vulnerability of the respiratory chain.

   Evidence In vitro growth inhibition with HDQ + atovaquone and fractional inhibitory concentration analysis

   PMID:17242151

   Open questions at the time

   • Synergy inferred from known targets, not direct biochemical dissection of cytochrome b
   • No structural correlate
6. 2016 High

   Defining a second druggable subsite and stage-specific expression established the Qi site as a target active against latent cysts, broadening therapeutic reach.

   Evidence In vitro cyst model (EGS strain), inhibition assays, mutant yeast complementation, and co-crystallography at the bc1 Qi site

   PMID:27412848

   Open questions at the time

   • Cyst-stage IC50 remains high (4 uM)
   • Qi structural data from surrogate systems, not T. gondii enzyme
7. 2017 High

   Genetically localizing ELQ-316 action distinguished Qi- from Qo-site inhibitors; a Thr222Pro mutation conferring ELQ and antimycin cross-resistance placed ELQs at the Qi site.

   Evidence Drug-pressure selection, cytochrome b sequencing, and cross-resistance quantitation

   PMID:27919897

   Open questions at the time

   • No direct structure of the T. gondii Qi pocket in this study
   • Fitness cost of T222P not addressed
8. 2017 Medium

   Uncovering a non-genetic resistance route revealed that cytochrome b inhibition can be bypassed metabolically; TgMSH1-null parasites survive atovaquone by shutting down oxidative phosphorylation rather than by Qo mutation.

   Evidence TgMSH1 knockout, mitochondrial DNA sequencing, electron transport and multi-inhibitor growth assays, glucose deprivation

   PMID:29141004

   Open questions at the time

   • Molecular basis of the OXPHOS-shutdown state undefined
   • Single lab
9. 2018 Medium

   Cross-species sequence and SAR comparison clarified how quinolone substituents partition between Qo and Qi inhibition, guiding selectivity engineering.

   Evidence T. gondii vs P. falciparum cytochrome b sequence comparison and SAR profiling on wild-type and resistant strains

   PMID:30117728

   Open questions at the time

   • No T. gondii-specific structural data
   • Selectivity determinants inferred pharmacologically
10. 2019 Medium

    Probing artemisinin action implicated the cytochrome b-containing mitochondrion; resistant lines amplified the mitochondrial genome and showed altered membrane potential.

    Evidence In vitro resistance selection, whole-genome sequencing, CRISPR mutation introduction, and membrane-potential assays

    PMID:31806760

    Open questions at the time

    • Cytochrome b involvement inferred from genome amplification, not direct protein assay
    • No structural or biochemical link to complex III activity
11. 2025 High

    Solving the parasite respiratory supercomplex structures resolved the long-standing gap in T. gondii-specific inhibitor geometry, showing atovaquone at Qo and ELQ-300 in a flipped Qi pose that explains selectivity and enabled potency optimization.

    Evidence Cryo-EM of III2-IV supercomplex and complex III2, inhibitor co-structures, knockout assembly analysis, and structure-guided medicinal chemistry

    PMID:40389671

    Open questions at the time

    • In vivo efficacy of optimized ELQs not established here
    • Resistance liabilities of new compounds unknown

• How cytochrome b mutation-independent atovaquone resistance is enacted metabolically, and whether structure-guided ELQs retain activity against Qi-mutant and latent-stage parasites in vivo, remain open.
• Molecular mechanism of OXPHOS-shutdown survival undefined
• In vivo efficacy against cysts and resistant mutants untested
• No T. gondii-specific atovaquone/ELQ cross-resistance structural map beyond the reported poses