TGRH88_085360

Toxoplasma gondii cytochrome b is a mitochondrially encoded subunit of the cytochrome bc1 complex (complex III) that carries out ubiquinol oxidation and quinone reduction, driving the respiratory electron transport chain and maintenance of the mitochondrial membrane potential essential for parasite survival (PMID:10802314, PMID:9813002). Biochemical respirometry localized the site of atovaquone inhibition to the segment between cytochrome b and cytochrome c1 within the bc1 complex, where nanomolar atovaquone collapses respiration and membrane potential (PMID:9813002). Two distinct catalytic sites of cytochrome b are pharmacologically targetable: the Qo (ubiquinol oxidation) pocket, where atovaquone binds — defined by resistance mutations M129L and I254L and confirmed by cryo-EM co-structure (PMID:10802314, PMID:40389671) — and the Qi (quinone reduction) site, where endochin-like quinolones such as ELQ-316 and ELQ-300 bind, defined by the T222P resistance mutation with cross-resistance to the reference Qi inhibitor antimycin and by a cryo-EM co-structure showing an apicomplexan-specific flipped ELQ-300 orientation (PMID:27919897, PMID:40389671). Cryo-EM further resolved cytochrome b within a III2-IV supercomplex with a uniquely kinked apicomplexan interface, and knockout of a supercomplex subunit disrupts III2-IV assembly without abolishing inhibitor binding to complex III2 (PMID:40389671). Cytochrome b is additionally the identified target of decoquinate through a resistance mutation (PMID:10519259), and simultaneous blockade of the Qo site with atovaquone and of an upstream ubiquinol-binding site with HDQ produces strong synergy, underscoring the centrality of the ubiquinone/ubiquinol cycle (PMID:17242151).

1. 1998 High

   Establishing whether apicomplexan parasites possess a functional respiratory chain and where atovaquone acts answered the basic question of the drug's molecular target within mitochondrial electron transport.

   Evidence In situ respirometry and safranin O membrane potential measurement in digitonin-permeabilized T. gondii tachyzoites with selective inhibitors and substrates

   PMID:9813002

   Open questions at the time

   • Did not identify the specific residues or binding pocket engaged by atovaquone
   • Did not distinguish Qo from Qi involvement structurally
2. 1999 Medium

   Genetic mapping of a decoquinate-resistance mutation to cytochrome b established cytochrome b as a drug target at the bc1 complex beyond atovaquone.

   Evidence Selection of decoquinate-resistant mutants and sequencing of the cytochrome b gene in T. gondii

   PMID:10519259

   Open questions at the time

   • Limited detail on the specific mutation and binding site
   • No biochemical confirmation of decoquinate-cytochrome b interaction
3. 2000 High

   Cloning the cytochrome b gene and mapping atovaquone-resistance mutations to the Qo domain pinpointed the ubiquinol oxidation site as the atovaquone binding region, moving from a functional segment to specific residues.

   Evidence Gene cloning and sequencing of wild-type and atovaquone-resistant T. gondii strains with structural modelling of the Qo domain (M129L, I254L)

   PMID:10802314

   Open questions at the time

   • Modelling only — no experimental structure of the atovaquone-bound complex
   • Did not address the Qi site or other inhibitors
4. 2003 High

   Surrogate biochemistry defined how atovaquone occupies the Qo pocket and interacts with the Rieske iron-sulfur protein, providing molecular mechanism informing the T. gondii Qo site.

   Evidence Inhibition kinetics of isolated bc1 complex, EPR of the Rieske center, and L275F mutagenesis in yeast cytochrome b

   PMID:12791689

   Open questions at the time

   • Derived from yeast/bovine surrogates, not T. gondii enzyme directly
   • Species-specific differences in binding not resolved
5. 2007 Medium

   Demonstrating synergy between atovaquone (Qo blockade) and HDQ (upstream ubiquinol-binding inhibition) confirmed the central role of the ubiquinone/ubiquinol cycle and validated combined-site targeting.

   Evidence In vitro T. gondii growth assay with fractional inhibitory concentration analysis of the atovaquone-HDQ combination

   PMID:17242151

   Open questions at the time

   • Cell-based only, no direct enzymatic measurement of dual inhibition
   • Single lab
6. 2016 High

   Identifying 4-(1H)-quinolone scaffolds that bind the Qi site opened a second, distinct pharmacological site on cytochrome b for anti-Toxoplasma action.

   Evidence Mutant yeast bc1 assays, co-crystallography, and in vitro/in vivo T. gondii infection models

   PMID:27412848

   Open questions at the time

   • Co-crystal structure was in surrogate rather than T. gondii enzyme
   • Did not map T. gondii-specific Qi resistance residues
7. 2017 High

   Mapping the T222P resistance mutation with antimycin cross-resistance provided direct genetic proof that ELQ-316 inhibits the Qi site of T. gondii cytochrome b, distinguishing it mechanistically from Qo-site atovaquone.

   Evidence Selection of ELQ-316-resistant clones, cytochrome b sequencing, and cross-resistance testing against the reference Qi inhibitor antimycin

   PMID:27919897

   Open questions at the time

   • No structural depiction of the ELQ-316-Qi complex at this stage
   • Fitness cost of T222P not addressed
8. 2017 Medium

   The TgMSH1 knockout showed that atovaquone resistance can arise via metabolic shutdown of respiration rather than cytochrome b Qo mutation, clarifying that Qo mutations are not the only resistance route.

   Evidence Tgmsh1 knockout with mtDNA sequencing, respiration measurements, glucose-dependence and atovaquone sensitivity assays

   PMID:29141004

   Open questions at the time

   • Accumulated cytochrome b missense mutations did not alter atovaquone-relevant residues
   • Metabolic bypass mechanism not fully dissected
9. 2019 Medium

   Amplification of the cytochrome b-containing mitochondrial genome in artemisinin-resistant lines and loss of membrane potential on treatment implicated complex III in artemisinin action, extending cytochrome b's relevance beyond bc1-specific inhibitors.

   Evidence In vitro resistance evolution, whole-genome sequencing, CRISPR/Cas9 point mutation, and membrane potential measurement

   PMID:31806760

   Open questions at the time

   • Cytochrome b involvement inferred from genome amplification, not a direct functional assay of cytochrome b
   • Precise artemisinin target within the mitochondrion unresolved
10. 2025 High

    Cryo-EM structures of the III2-IV supercomplex and complex III2 with bound inhibitors resolved atomic-level, species-specific atovaquone Qo binding and a flipped apicomplexan ELQ-300 Qi orientation, converting inferred mechanisms into direct structural models and enabling structure-guided drug design.

    Evidence Cryo-EM structure determination with inhibitor co-complexes, supercomplex-subunit knockout, and potency testing of new ELQs

    PMID:40389671

    Open questions at the time

    • In vivo efficacy and resistance profiles of the new subnanomolar ELQs not yet defined
    • Functional consequence of the unique III2-IV kinked interface not fully explained

• How cytochrome b sequence and structural divergence across apicomplexan species and strains shapes clinical resistance and selectivity of Qo/Qi inhibitors remains to be systematically resolved.
• No comprehensive map of Qo/Qi resistance-mutation fitness landscapes
• Selectivity margin versus mammalian cytochrome b not fully quantified