{"gene":"RH88_085370","gene_symbol":"","run_date":"2026-09-07T10:25:42","timeline":{"discoveries":[{"year":2000,"finding":"Cytochrome b of T. gondii was cloned and sequenced; atovaquone-resistant mutants harbor mutations M129L and I254L within the Qo domain (ubiquinol-binding pocket) of cytochrome b, establishing that atovaquone inhibits electron transport at the cytochrome bc1 complex by targeting this domain. Additionally, the protein may differ from other homologues by terminating within the mitochondrial membrane.","method":"Drug-resistant mutant generation, gene cloning and sequencing, structural modeling of Qo domain","journal":"Molecular and biochemical parasitology","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct sequencing of resistant mutants, structural modeling, first complete characterization of the T. gondii cytochrome b gene with functional mutation mapping","pmids":["10802314"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1999,"finding":"A mutation in cytochrome b was identified in a decoquinate-resistant mutant of T. gondii, indicating that cytochrome b is the target of decoquinate at the cytochrome bc1 complex.","method":"Drug-resistant mutant generation and gene sequencing","journal":"The Journal of eukaryotic microbiology","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — single lab, gene sequencing of resistant mutant, no in vitro reconstitution","pmids":["10519259"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1998,"finding":"Atovaquone at nanomolar concentrations inhibits the respiratory chain of T. gondii between cytochrome b and c1 (analogous to antimycin A1 action), disrupting mitochondrial membrane potential; this was shown using digitonin-permeabilized tachyzoites with direct respiration and membrane potential assays.","method":"Digitonin permeabilization, respiration assay (succinate/ADP), mitochondrial membrane potential measurement (safranin O), inhibitor titration","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct biochemical assay in permeabilized parasites with multiple substrates and inhibitors, single lab but multiple orthogonal methods","pmids":["9813002"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2017,"finding":"A T222P substitution in cytochrome b confers 49-fold resistance to ELQ-316 and 19-fold resistance to antimycin in T. gondii, providing genetic evidence that ELQ-316 and antimycin act at the Qi site of the cytochrome bc1 complex.","method":"Drug-resistant mutant selection, gene sequencing, genetic characterization of resistant clones","journal":"Antimicrobial agents and chemotherapy","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic epistasis via resistance mutation, cross-resistance with known Qi inhibitor antimycin, single lab with two orthogonal lines of evidence","pmids":["27919897"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2016,"finding":"Novel 4-(1H)-quinolone scaffolds targeting the cytochrome bc1 complex Qi site inhibit T. gondii active infection and cysts; binding to the Qi site was confirmed by mutant yeast and co-crystallographic studies.","method":"Co-crystallography with bc1 complex, mutant yeast surrogate system, in vitro parasite growth inhibition assay","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 1 / Moderate — co-crystal structure and mutant yeast biochemical confirmation, multiple orthogonal methods in single study","pmids":["27412848"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":"EGS"}]},{"year":2018,"finding":"Structure-activity analysis of 4(1H)-quinolones revealed that Qo site and Qi site inhibitors of the cytochrome bc1 complex can be distinguished by substituents on the quinolone ring, and that naturally occurring amino acid differences in wild-type T. gondii cytochrome b at positions equivalent to P. falciparum resistance-associated residues may influence baseline drug susceptibility.","method":"Comparative sequence analysis of cytochrome b, testing against wild-type and drug-resistant T. gondii and P. falciparum strains, in vitro growth inhibition","journal":"ACS infectious diseases","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — pharmacological dissection of binding sites using resistant strains, single lab, no structural confirmation for T. gondii-specific residues","pmids":["30117728"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2019,"finding":"Artemisinin-resistant T. gondii lines showed dramatic amplification of the mitochondrial genome including cytochrome b and cytochrome c oxidase I genes; treatment of wild-type parasites with high artemisinin concentrations decreased mitochondrial membrane potential, and this phenotype was stably altered in resistant parasites, implicating the mitochondrion (and cytochrome b-containing complex) as a target.","method":"Whole-genome sequencing of resistant lines, mitochondrial genome copy number analysis, mitochondrial membrane potential measurement, CRISPR/Cas9 point mutation introduction","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (WGS, CRISPR, functional assay), single lab; cytochrome b amplification is correlative with resistance but mechanistic link to cytochrome b specifically is indirect","pmids":["31806760"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2017,"finding":"Parasites lacking TgMSH1 accumulate missense mutations in cytochrome b but none at known atovaquone-sensitivity determinants; atovaquone still inhibits electron transport in these mutants. However, culture under atovaquone pressure leads to complete shutdown of respiration, revealing that T. gondii can survive without oxidative phosphorylation through metabolic plasticity rather than canonical cytochrome b Qo mutations.","method":"TgMSH1 knockout, mitochondrial DNA sequencing, electron transport inhibition assay, atovaquone resistance selection","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic knockout with biochemical functional assay, single lab, multiple orthogonal methods; cytochrome b mutations explicitly tested as negative for known resistance determinants","pmids":["29141004"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2025,"finding":"Cryo-electron microscopy structures of the T. gondii cytochrome bc1 complex (complex III2) and III2-IV supercomplex were resolved, revealing clade-specific subunits forming an apicomplexan-conserved III2-IV interface with a unique kinked architecture. Atovaquone binding interactions in the Qo site underpinning species specificity were structurally defined. ELQ-300 binds the apicomplexan Qi site in a flipped orientation compared with related compounds in the mammalian enzyme. Supercomplex disassembly knockout challenged the proposed role of III2-IV in electron transfer efficiency, though III2-IV was confirmed critical for parasite fitness.","method":"Cryo-electron microscopy structure determination, knockout analysis (supercomplex disassembly), inhibitor soaking (atovaquone, ELQ-300), structure-guided medicinal chemistry","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structures with functional validation by knockout and inhibitor binding, multiple orthogonal methods including structure, genetic KO, and biochemical inhibition in single rigorous study","pmids":["40389671"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]}],"current_model":"Toxoplasma gondii cytochrome b is the mitochondrially encoded core subunit of the cytochrome bc1 complex (complex III), where it contains a Qo (ubiquinol oxidation) site and a Qi (quinone reduction) site; atovaquone competitively inhibits ubiquinol oxidation at the Qo domain (with resistance conferred by M129L/I254L mutations), endochin-like quinolones inhibit the Qi site (with resistance conferred by a T222P substitution), and cryo-EM structures have now defined the molecular basis of species-selective inhibitor binding and revealed a unique apicomplexan III2-IV supercomplex architecture critical for parasite fitness."},"narrative":{"mechanistic_narrative":"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].","teleology":[{"year":1998,"claim":"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","pmids":["9813002"],"confidence":"High","gaps":["Did not identify the specific residues of the binding site","No structural model of the inhibited complex"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1999,"claim":"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","pmids":["10519259"],"confidence":"Medium","gaps":["Single lab, no in vitro reconstitution","Exact binding site within cytochrome b not mapped"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2000,"claim":"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","pmids":["10802314"],"confidence":"High","gaps":["Structural model only, no experimental structure of bound drug","Qi site not characterized"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2016,"claim":"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","pmids":["27412848"],"confidence":"High","gaps":["Qi-site binding confirmed in surrogate systems rather than native T. gondii complex","In vivo efficacy not established here"],"attribution":[{"species":"Toxoplasma gondii","strain":"EGS"}]},{"year":2017,"claim":"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","pmids":["27919897"],"confidence":"High","gaps":["Structural basis of the T222P effect not defined","Effect on native electron transfer kinetics not measured"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2017,"claim":"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","pmids":["29141004"],"confidence":"Medium","gaps":["Metabolic pathway sustaining survival without OXPHOS not defined","Single lab"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2018,"claim":"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","pmids":["30117728"],"confidence":"Medium","gaps":["No structural confirmation of T. gondii-specific residue effects","Single lab"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2019,"claim":"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","pmids":["31806760"],"confidence":"Medium","gaps":["Cytochrome b amplification is correlative; direct mechanistic link is indirect","Causal contribution of cytochrome b versus other amplified genes unresolved"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2025,"claim":"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","pmids":["40389671"],"confidence":"High","gaps":["Precise role of III2-IV in electron-transfer efficiency remained challenged by the disassembly knockout","Functional consequences of clade-specific subunits not fully defined"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":null,"claim":"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.","evidence":"","pmids":[],"confidence":"Medium","gaps":["Mechanism of OXPHOS-independent survival unresolved","Role of the supercomplex in electron-transfer efficiency unsettled"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]}],"mechanism_profile":{"molecular_activity":[],"localization":[],"pathway":[],"complexes":["Cytochrome bc1 complex (complex III2)","Apicomplexan III2-IV supercomplex"],"partners":["CYTOCHROME C1 (ELECTRON TRANSFER TO C1)","CYTOCHROME C OXIDASE / COMPLEX IV (WITHIN III2-IV SUPERCOMPLEX)"],"other_free_text":["Qo (ubiquinol oxidation) site targeted by atovaquone; resistance via M129L/I254L","Qi (quinone reduction) site targeted by endochin-like quinolones (ELQ-316/ELQ-300) and antimycin; resistance via T222P","Mitochondrially encoded; gene amplified in artemisinin-resistant lines","Parasite retains metabolic plasticity permitting survival without oxidative phosphorylation under atovaquone pressure"]},"strain_conflicts":[]},"prefetch_data":{},"corpus_meta":[{"pmid":"10802314","id":"PMC_10802314","title":"Characterization of cytochrome b from Toxoplasma gondii and Q(o) domain mutations as a mechanism of atovaquone-resistance.","date":"2000","source":"Molecular and biochemical parasitology","url":"https://pubmed.ncbi.nlm.nih.gov/10802314","citation_count":124,"is_preprint":false},{"pmid":"12791689","id":"PMC_12791689","title":"Molecular basis for atovaquone binding to the cytochrome bc1 complex.","date":"2003","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12791689","citation_count":113,"is_preprint":false},{"pmid":"18212105","id":"PMC_18212105","title":"In vitro susceptibility of various genotypic strains of Toxoplasma gondii to pyrimethamine, sulfadiazine, and atovaquone.","date":"2008","source":"Antimicrobial agents and chemotherapy","url":"https://pubmed.ncbi.nlm.nih.gov/18212105","citation_count":106,"is_preprint":false},{"pmid":"9813002","id":"PMC_9813002","title":"Respiration and oxidative phosphorylation in the apicomplexan parasite Toxoplasma gondii.","date":"1998","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/9813002","citation_count":91,"is_preprint":false},{"pmid":"17242151","id":"PMC_17242151","title":"Growth inhibition of Toxoplasma gondii and Plasmodium falciparum by nanomolar concentrations of 1-hydroxy-2-dodecyl-4(1H)quinolone, a high-affinity inhibitor of alternative (type II) NADH dehydrogenases.","date":"2007","source":"Antimicrobial agents and chemotherapy","url":"https://pubmed.ncbi.nlm.nih.gov/17242151","citation_count":66,"is_preprint":false},{"pmid":"25512210","id":"PMC_25512210","title":"Molecular characterisation of Sarcocystis lutrae n. sp. and Toxoplasma gondii from the musculature of two Eurasian otters (Lutra lutra) in Norway.","date":"2014","source":"Parasitology research","url":"https://pubmed.ncbi.nlm.nih.gov/25512210","citation_count":60,"is_preprint":false},{"pmid":"17826334","id":"PMC_17826334","title":"Modeling the molecular basis of atovaquone resistance in parasites and pathogenic fungi.","date":"2007","source":"Trends in parasitology","url":"https://pubmed.ncbi.nlm.nih.gov/17826334","citation_count":52,"is_preprint":false},{"pmid":"27919897","id":"PMC_27919897","title":"Genetic Evidence for Cytochrome b Qi Site Inhibition by 4(1H)-Quinolone-3-Diarylethers and Antimycin in Toxoplasma gondii.","date":"2017","source":"Antimicrobial agents and chemotherapy","url":"https://pubmed.ncbi.nlm.nih.gov/27919897","citation_count":49,"is_preprint":false},{"pmid":"30117728","id":"PMC_30117728","title":"Targeted Structure-Activity Analysis of Endochin-like Quinolones Reveals Potent Qi and Qo Site Inhibitors of Toxoplasma gondii and Plasmodium falciparum Cytochrome bc1 and Identifies ELQ-400 as a Remarkably Effective Compound against Acute Experimental Toxoplasmosis.","date":"2018","source":"ACS infectious diseases","url":"https://pubmed.ncbi.nlm.nih.gov/30117728","citation_count":40,"is_preprint":false},{"pmid":"27412848","id":"PMC_27412848","title":"New paradigms for understanding and step changes in treating active and chronic, persistent apicomplexan infections.","date":"2016","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/27412848","citation_count":37,"is_preprint":false},{"pmid":"31806760","id":"PMC_31806760","title":"Evolution of resistance in vitro reveals mechanisms of artemisinin activity in Toxoplasma gondii.","date":"2019","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/31806760","citation_count":35,"is_preprint":false},{"pmid":"21251932","id":"PMC_21251932","title":"Design of anti-parasitic and anti-fungal hydroxy-naphthoquinones that are less susceptible to drug resistance.","date":"2011","source":"Molecular and biochemical parasitology","url":"https://pubmed.ncbi.nlm.nih.gov/21251932","citation_count":31,"is_preprint":false},{"pmid":"23358734","id":"PMC_23358734","title":"Characterisation of full-length mitochondrial copies and partial nuclear copies (numts) of the cytochrome b and cytochrome c oxidase subunit I genes of Toxoplasma gondii, Neospora caninum, Hammondia heydorni and Hammondia triffittae (Apicomplexa: Sarcocystidae).","date":"2013","source":"Parasitology research","url":"https://pubmed.ncbi.nlm.nih.gov/23358734","citation_count":30,"is_preprint":false},{"pmid":"32626661","id":"PMC_32626661","title":"Potent Tetrahydroquinolone Eliminates Apicomplexan Parasites.","date":"2020","source":"Frontiers in cellular and infection microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/32626661","citation_count":26,"is_preprint":false},{"pmid":"32540978","id":"PMC_32540978","title":"Orally Bioavailable Endochin-Like Quinolone Carbonate Ester Prodrug Reduces Toxoplasma gondii Brain Cysts.","date":"2020","source":"Antimicrobial agents and chemotherapy","url":"https://pubmed.ncbi.nlm.nih.gov/32540978","citation_count":26,"is_preprint":false},{"pmid":"32161765","id":"PMC_32161765","title":"Activities of Endochin-Like Quinolones Against in vitro Cultured Besnoitia besnoiti Tachyzoites.","date":"2020","source":"Frontiers in veterinary science","url":"https://pubmed.ncbi.nlm.nih.gov/32161765","citation_count":16,"is_preprint":false},{"pmid":"10519259","id":"PMC_10519259","title":"Cytochrome b mutation identified in a decoquinate-resistant mutant of Toxoplasma gondii.","date":"1999","source":"The Journal of eukaryotic microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/10519259","citation_count":12,"is_preprint":false},{"pmid":"40389671","id":"PMC_40389671","title":"Structure, assembly and inhibition of the Toxoplasma gondii respiratory chain supercomplex.","date":"2025","source":"Nature structural & molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/40389671","citation_count":10,"is_preprint":false},{"pmid":"38820542","id":"PMC_38820542","title":"Molecular prevalence, phylogeny and hematological impact of Toxoplasma gondii and Plasmodium spp. in common quails from Punjab, Pakistan.","date":"2024","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/38820542","citation_count":10,"is_preprint":false},{"pmid":"29141004","id":"PMC_29141004","title":"Lack of mitochondrial MutS homolog 1 in Toxoplasma gondii disrupts maintenance and fidelity of mitochondrial DNA and reveals metabolic plasticity.","date":"2017","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/29141004","citation_count":9,"is_preprint":false},{"pmid":"27007245","id":"PMC_27007245","title":"Molecular phylogeny of Toxoplasmatinae: comparison between inferences based on mitochondrial and apicoplast genetic sequences.","date":"2016","source":"Revista brasileira de parasitologia veterinaria = Brazilian journal of veterinary parasitology : Orgao Oficial do Colegio Brasileiro de Parasitologia Veterinaria","url":"https://pubmed.ncbi.nlm.nih.gov/27007245","citation_count":6,"is_preprint":false},{"pmid":"42058244","id":"PMC_42058244","title":"Phylogenetic study of the protozoan Toxoplasma gondii using cytochrome b and GRA20 genes.","date":"2025","source":"Archives of Razi Institute","url":"https://pubmed.ncbi.nlm.nih.gov/42058244","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":14412,"output_tokens":2541,"usd":0.040676,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":8279,"output_tokens":3772,"usd":0.067848,"stage2_stop_reason":"end_turn"},"total_usd":0.108524,"stage1_batch_id":"msgbatch_018xTxyBPcQFK9jdgubtxgsj","stage2_batch_id":"msgbatch_01UiG5BTaJP7BvKKL6RLeyv2","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2000,\n      \"finding\": \"Cytochrome b of T. gondii was cloned and sequenced; atovaquone-resistant mutants harbor mutations M129L and I254L within the Qo domain (ubiquinol-binding pocket) of cytochrome b, establishing that atovaquone inhibits electron transport at the cytochrome bc1 complex by targeting this domain. Additionally, the protein may differ from other homologues by terminating within the mitochondrial membrane.\",\n      \"method\": \"Drug-resistant mutant generation, gene cloning and sequencing, structural modeling of Qo domain\",\n      \"journal\": \"Molecular and biochemical parasitology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct sequencing of resistant mutants, structural modeling, first complete characterization of the T. gondii cytochrome b gene with functional mutation mapping\",\n      \"pmids\": [\"10802314\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"A mutation in cytochrome b was identified in a decoquinate-resistant mutant of T. gondii, indicating that cytochrome b is the target of decoquinate at the cytochrome bc1 complex.\",\n      \"method\": \"Drug-resistant mutant generation and gene sequencing\",\n      \"journal\": \"The Journal of eukaryotic microbiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — single lab, gene sequencing of resistant mutant, no in vitro reconstitution\",\n      \"pmids\": [\"10519259\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Atovaquone at nanomolar concentrations inhibits the respiratory chain of T. gondii between cytochrome b and c1 (analogous to antimycin A1 action), disrupting mitochondrial membrane potential; this was shown using digitonin-permeabilized tachyzoites with direct respiration and membrane potential assays.\",\n      \"method\": \"Digitonin permeabilization, respiration assay (succinate/ADP), mitochondrial membrane potential measurement (safranin O), inhibitor titration\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct biochemical assay in permeabilized parasites with multiple substrates and inhibitors, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"9813002\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"A T222P substitution in cytochrome b confers 49-fold resistance to ELQ-316 and 19-fold resistance to antimycin in T. gondii, providing genetic evidence that ELQ-316 and antimycin act at the Qi site of the cytochrome bc1 complex.\",\n      \"method\": \"Drug-resistant mutant selection, gene sequencing, genetic characterization of resistant clones\",\n      \"journal\": \"Antimicrobial agents and chemotherapy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis via resistance mutation, cross-resistance with known Qi inhibitor antimycin, single lab with two orthogonal lines of evidence\",\n      \"pmids\": [\"27919897\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Novel 4-(1H)-quinolone scaffolds targeting the cytochrome bc1 complex Qi site inhibit T. gondii active infection and cysts; binding to the Qi site was confirmed by mutant yeast and co-crystallographic studies.\",\n      \"method\": \"Co-crystallography with bc1 complex, mutant yeast surrogate system, in vitro parasite growth inhibition assay\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — co-crystal structure and mutant yeast biochemical confirmation, multiple orthogonal methods in single study\",\n      \"pmids\": [\"27412848\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"EGS\"}]\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Structure-activity analysis of 4(1H)-quinolones revealed that Qo site and Qi site inhibitors of the cytochrome bc1 complex can be distinguished by substituents on the quinolone ring, and that naturally occurring amino acid differences in wild-type T. gondii cytochrome b at positions equivalent to P. falciparum resistance-associated residues may influence baseline drug susceptibility.\",\n      \"method\": \"Comparative sequence analysis of cytochrome b, testing against wild-type and drug-resistant T. gondii and P. falciparum strains, in vitro growth inhibition\",\n      \"journal\": \"ACS infectious diseases\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — pharmacological dissection of binding sites using resistant strains, single lab, no structural confirmation for T. gondii-specific residues\",\n      \"pmids\": [\"30117728\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Artemisinin-resistant T. gondii lines showed dramatic amplification of the mitochondrial genome including cytochrome b and cytochrome c oxidase I genes; treatment of wild-type parasites with high artemisinin concentrations decreased mitochondrial membrane potential, and this phenotype was stably altered in resistant parasites, implicating the mitochondrion (and cytochrome b-containing complex) as a target.\",\n      \"method\": \"Whole-genome sequencing of resistant lines, mitochondrial genome copy number analysis, mitochondrial membrane potential measurement, CRISPR/Cas9 point mutation introduction\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (WGS, CRISPR, functional assay), single lab; cytochrome b amplification is correlative with resistance but mechanistic link to cytochrome b specifically is indirect\",\n      \"pmids\": [\"31806760\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Parasites lacking TgMSH1 accumulate missense mutations in cytochrome b but none at known atovaquone-sensitivity determinants; atovaquone still inhibits electron transport in these mutants. However, culture under atovaquone pressure leads to complete shutdown of respiration, revealing that T. gondii can survive without oxidative phosphorylation through metabolic plasticity rather than canonical cytochrome b Qo mutations.\",\n      \"method\": \"TgMSH1 knockout, mitochondrial DNA sequencing, electron transport inhibition assay, atovaquone resistance selection\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic knockout with biochemical functional assay, single lab, multiple orthogonal methods; cytochrome b mutations explicitly tested as negative for known resistance determinants\",\n      \"pmids\": [\"29141004\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cryo-electron microscopy structures of the T. gondii cytochrome bc1 complex (complex III2) and III2-IV supercomplex were resolved, revealing clade-specific subunits forming an apicomplexan-conserved III2-IV interface with a unique kinked architecture. Atovaquone binding interactions in the Qo site underpinning species specificity were structurally defined. ELQ-300 binds the apicomplexan Qi site in a flipped orientation compared with related compounds in the mammalian enzyme. Supercomplex disassembly knockout challenged the proposed role of III2-IV in electron transfer efficiency, though III2-IV was confirmed critical for parasite fitness.\",\n      \"method\": \"Cryo-electron microscopy structure determination, knockout analysis (supercomplex disassembly), inhibitor soaking (atovaquone, ELQ-300), structure-guided medicinal chemistry\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structures with functional validation by knockout and inhibitor binding, multiple orthogonal methods including structure, genetic KO, and biochemical inhibition in single rigorous study\",\n      \"pmids\": [\"40389671\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    }\n  ],\n  \"current_model\": \"Toxoplasma gondii cytochrome b is the mitochondrially encoded core subunit of the cytochrome bc1 complex (complex III), where it contains a Qo (ubiquinol oxidation) site and a Qi (quinone reduction) site; atovaquone competitively inhibits ubiquinol oxidation at the Qo domain (with resistance conferred by M129L/I254L mutations), endochin-like quinolones inhibit the Qi site (with resistance conferred by a T222P substitution), and cryo-EM structures have now defined the molecular basis of species-selective inhibitor binding and revealed a unique apicomplexan III2-IV supercomplex architecture critical for parasite fitness.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"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 [#2, #0]. 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 [#0, #2]; and a Qi (quinone-reduction) site, where endochin-like quinolones and antimycin act, with the T222P substitution conferring cross-resistance to both [#3, #4]. 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 [#5]. 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 [#8]. The parasite retains metabolic plasticity that permits survival when oxidative phosphorylation is shut down under atovaquone pressure, without requiring canonical Qo resistance mutations [#7].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"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.\",\n      \"evidence\": \"Digitonin-permeabilized tachyzoites with respiration and membrane-potential (safranin O) assays under inhibitor titration\",\n      \"pmids\": [\"9813002\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify the specific residues of the binding site\", \"No structural model of the inhibited complex\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Identifying the molecular target of decoquinate tested whether cytochrome b is a drug target; a resistance mutation in cytochrome b implicated the bc1 complex.\",\n      \"evidence\": \"Drug-resistant mutant generation and gene sequencing\",\n      \"pmids\": [\"10519259\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab, no in vitro reconstitution\", \"Exact binding site within cytochrome b not mapped\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"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.\",\n      \"evidence\": \"Atovaquone-resistant mutant generation, gene cloning/sequencing, structural modeling of the Qo domain\",\n      \"pmids\": [\"10802314\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural model only, no experimental structure of bound drug\", \"Qi site not characterized\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"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.\",\n      \"evidence\": \"Co-crystallography with bc1 complex, mutant yeast surrogate system, in vitro parasite growth inhibition\",\n      \"pmids\": [\"27412848\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Qi-site binding confirmed in surrogate systems rather than native T. gondii complex\", \"In vivo efficacy not established here\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"EGS\"}]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"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.\",\n      \"evidence\": \"Drug-resistant mutant selection, gene sequencing, genetic characterization of resistant clones\",\n      \"pmids\": [\"27919897\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of the T222P effect not defined\", \"Effect on native electron transfer kinetics not measured\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"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.\",\n      \"evidence\": \"TgMSH1 knockout, mitochondrial DNA sequencing, electron-transport inhibition assay, atovaquone resistance selection\",\n      \"pmids\": [\"29141004\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Metabolic pathway sustaining survival without OXPHOS not defined\", \"Single lab\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"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.\",\n      \"evidence\": \"Comparative sequence analysis, testing against wild-type and resistant T. gondii and P. falciparum, in vitro growth inhibition\",\n      \"pmids\": [\"30117728\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural confirmation of T. gondii-specific residue effects\", \"Single lab\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"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.\",\n      \"evidence\": \"Whole-genome sequencing, mitochondrial copy-number analysis, membrane-potential assay, CRISPR/Cas9 point mutation\",\n      \"pmids\": [\"31806760\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cytochrome b amplification is correlative; direct mechanistic link is indirect\", \"Causal contribution of cytochrome b versus other amplified genes unresolved\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"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.\",\n      \"evidence\": \"Cryo-EM of complex III2 and III2-IV supercomplex, inhibitor soaking, supercomplex-disassembly knockout, structure-guided chemistry\",\n      \"pmids\": [\"40389671\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise role of III2-IV in electron-transfer efficiency remained challenged by the disassembly knockout\", \"Functional consequences of clade-specific subunits not fully defined\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"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.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of OXPHOS-independent survival unresolved\", \"Role of the supercomplex in electron-transfer efficiency unsettled\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008121\", \"supporting_discovery_ids\": [2, 0, 8], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]},\n      {\"term_id\": \"GO:0045153\", \"supporting_discovery_ids\": [0, 3], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005743\", \"supporting_discovery_ids\": [0, 2], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]},\n      {\"term_id\": \"GO:0005750\", \"supporting_discovery_ids\": [8, 0], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0022904\", \"supporting_discovery_ids\": [2, 0], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]},\n      {\"term_id\": \"GO:0006119\", \"supporting_discovery_ids\": [2, 7], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"complexes\": [\n      \"Cytochrome bc1 complex (complex III2)\",\n      \"Apicomplexan III2-IV supercomplex\"\n    ],\n    \"partners\": [\n      \"Cytochrome c1 (electron transfer to c1)\",\n      \"Cytochrome c oxidase / complex IV (within III2-IV supercomplex)\"\n    ],\n    \"other_free_text\": [\n      \"Qo (ubiquinol oxidation) site targeted by atovaquone; resistance via M129L/I254L\",\n      \"Qi (quinone reduction) site targeted by endochin-like quinolones (ELQ-316/ELQ-300) and antimycin; resistance via T222P\",\n      \"Mitochondrially encoded; gene amplified in artemisinin-resistant lines\",\n      \"Parasite retains metabolic plasticity permitting survival without oxidative phosphorylation under atovaquone pressure\"\n    ]\n  },\n  \"strain_conflicts\": []\n}","audit_flag":null,"strains":[{"strain":"RH88","gene_id":"TGRH88_085370","name":null,"product":"cytochrome b","uniprot_acc":null,"chromosome":"JAAUHK010000041","start_pos":26809}]}