TGRH88_085460

Toxoplasma gondii cytochrome b is a mitochondrially encoded subunit of the cytochrome bc1 complex (complex III) that functions in the parasite's respiratory electron transport chain, where inhibition disrupts respiration and collapses mitochondrial membrane potential (PMID:9813002, PMID:40389671). Cryo-EM structures of the T. gondii complex III2 and the III2-IV supercomplex show that cytochrome b provides two catalytically distinct drug-binding sites: the ubiquinol-oxidation Qo site, where atovaquone binds through species-specific interactions, and the Qi site, where endochin-like quinolones such as ELQ-300 bind in an orientation distinct from that in the mammalian enzyme (PMID:40389671). Atovaquone acts as a competitive inhibitor at the Qo pocket in association with the Rieske iron-sulfur protein, and drug resistance arises through point mutations in the Qo domain (e.g. M129L, I254L), residues lining the ubiquinol pocket governing species-differential inhibition (PMID:10802314, PMID:12791689, PMID:21251932). Resistance to Qi-site inhibitors similarly maps to cytochrome b, with a Thr222Pro substitution conferring cross-resistance to ELQ-316 and the classical Qi inhibitor antimycin A (PMID:27919897). Cytochrome b is also the target of decoquinate (PMID:10519259), and structure-guided design against these sites has produced quinolones with subnanomolar activity (PMID:40389671). Beyond drug-target characterization, the corpus does not detail the protein's biogenesis, assembly, or regulation independent of complex III.

1. 1998 High

   Establishing that T. gondii possesses a functional respiratory chain answered whether the parasite's mitochondrion sustains electron transport that could be drug-targeted, localizing the inhibitory site to the bc1 complex.

   Evidence In situ respiration and safranin O membrane-potential assays in digitonin-permeabilized tachyzoites with atovaquone, antimycin A, and cyanide

   PMID:9813002

   Open questions at the time

   • Did not resolve which cytochrome b residues bind atovaquone
   • Did not distinguish Qo from Qi contributions
2. 1999 Medium

   Identifying a cytochrome b mutation in a decoquinate-resistant mutant showed that cytochrome b is a drug target beyond atovaquone.

   Evidence Selection of decoquinate-resistant T. gondii mutants and gene sequencing

   PMID:10519259

   Open questions at the time

   • Abstract-only report
   • Binding site of decoquinate on cytochrome b not defined
3. 2000 High

   Cloning cytochrome b and mapping atovaquone-resistance mutations to the ubiquinol pocket answered where atovaquone acts, establishing the Qo site of cytochrome b as its molecular target.

   Evidence Gene cloning/sequencing of wild-type and atovaquone-resistant strains with Qo domain structural modeling

   PMID:10802314

   Open questions at the time

   • No experimentally resolved structure
   • Mechanism of species-differential susceptibility not addressed
4. 2003 High

   Reconstitution in a yeast surrogate defined atovaquone as a competitive Qo-site inhibitor coupled to the Rieske protein and pinpointed residue 275 as a determinant of species-differential inhibition.

   Evidence Enzymatic Ki assays, EPR of Rieske iron-sulfur protein, and L275F mutagenesis of yeast cytochrome b

   PMID:12791689

   Open questions at the time

   • Performed in yeast, not T. gondii enzyme
   • Does not address Qi-site inhibitors
5. 2011 Medium

   Functional testing of pathogen-equivalent mutations in isolated yeast complexes established residue-level requirements for Qo-site inhibition across Plasmodium, Pneumocystis, and Toxoplasma.

   Evidence Enzymatic assays with isolated yeast bc1 complexes bearing equivalent cytochrome b mutations plus novel hydroxy-naphthoquinones

   PMID:21251932

   Open questions at the time

   • Surrogate yeast system rather than native T. gondii complex
   • Qi site not addressed
6. 2016 Medium

   Co-crystallographic and mutant-yeast work established that 4-(1H)-quinolone scaffolds engage the Qi site, extending the druggable geometry of cytochrome b beyond the Qo pocket.

   Evidence Mutant yeast bc1 assay, co-crystallography, and activity assays against T. gondii tachyzoites and cysts

   PMID:27412848

   Open questions at the time

   • T. gondii cytochrome b not structurally resolved here
   • Bradyzoite expression finding is observational
7. 2017 High

   A Thr222Pro cytochrome b substitution conferring cross-resistance to ELQ-316 and antimycin A provided genetic evidence that ELQ-316 acts at the Qi site in T. gondii.

   Evidence Drug-pressure selection of resistant clones, sequencing, and cross-resistance profiling

   PMID:27919897

   Open questions at the time

   • No structural confirmation of ELQ-316 binding pose
   • Fitness cost of Thr222Pro not assessed
8. 2017 Medium

   Analysis of Tgmsh1-null parasites revealed an atovaquone-resistance route independent of cytochrome b Qo mutations, via complete respiratory shutdown.

   Evidence Tgmsh1 knockout, mitochondrial DNA sequencing, and electron transport inhibition/resistance assays

   PMID:29141004

   Open questions at the time

   • Molecular basis of respiratory shutdown unresolved
   • Single lab
9. 2018 Medium

   Structure-activity analysis with resistant strains showed quinolones can target either the Qo or Qi site and that species-specific cytochrome b sequence differences shape susceptibility.

   Evidence Proliferation assays with wild-type and resistant T. gondii and P. falciparum plus cytochrome b sequence comparison

   PMID:30117728

   Open questions at the time

   • No direct mutagenesis of T. gondii cytochrome b
   • Binding sites inferred pharmacologically
10. 2019 Medium

    Amplification of the mitochondrial genome including cytochrome b in artemisinin-resistant lines, alongside altered membrane potential, implicated mitochondrial function as an artemisinin target.

    Evidence Whole-genome sequencing of resistant lines, CRISPR/Cas9 mutagenesis, and membrane potential measurement

    PMID:31806760

    Open questions at the time

    • Cytochrome b not directly manipulated; implicated only via copy-number change
    • Direct target of artemisinin not proven
11. 2025 High

    Cryo-EM structures of the native T. gondii complex III2 and III2-IV supercomplex resolved atovaquone at the Qo site and ELQ-300 at the Qi site, revealing species-specific and flipped binding poses and enabling structure-guided design of subnanomolar inhibitors.

    Evidence Cryo-EM of drug-bound complexes, supercomplex knockout, and structure-guided compound synthesis with proliferation assays

    PMID:40389671

    Open questions at the time

    • Catalytic turnover kinetics of the native enzyme not detailed
    • Regulation and assembly pathway of cytochrome b not resolved

• How cytochrome b is transcribed, translated, and assembled into complex III, and how this is regulated across tachyzoite and bradyzoite stages, remains uncharacterized in the corpus.
• No biogenesis or assembly mechanism described
• Stage-specific regulation beyond observational bradyzoite expression unknown