{"gene":"RH88_085320","gene_symbol":"","run_date":"2026-09-07T10:25:42","timeline":{"discoveries":[{"year":2000,"finding":"Cloning and sequencing of the T. gondii cytochrome b gene revealed that atovaquone-resistant mutants carry mutations M129L and I254L within the Qo domain (ubiquinol-binding pocket) of cytochrome b, indicating that atovaquone inhibits electron transport at the cytochrome bc1 complex (complex III) in T. gondii.","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 identifying specific active-site mutations, structural modeling, first complete characterization of the gene, foundational work replicated across multiple subsequent studies","pmids":["10802314"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1999,"finding":"A cytochrome b mutation was identified in a decoquinate-resistant mutant of T. gondii, indicating that cytochrome b mutations can confer resistance to quinolone-class compounds acting on the 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, single method (sequencing of resistant mutant), no abstract text available for full method evaluation","pmids":["10519259"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1998,"finding":"Atovaquone at 0.03 µM totally inhibited respiration and disrupted mitochondrial membrane potential in T. gondii tachyzoites, and was shown biochemically to inhibit the respiratory chain between cytochrome b and c1, consistent with inhibition of the cytochrome bc1 complex.","method":"In situ respiration assay with digitonin-permeabilized tachyzoites, safranin O membrane potential measurement, antimycin A and cyanide sensitivity assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct biochemical (enzymatic) assay in permeabilized parasites with pharmacological dissection, multiple inhibitors used as controls, single lab","pmids":["9813002"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2003,"finding":"Using yeast bc1 complex as a surrogate, atovaquone was shown to bind competitively at the ubiquinol oxidation (Qo) pocket of cytochrome b where it interacts with the Rieske iron-sulfur protein; introduction of the L275F mutation in yeast cytochrome b (equivalent to the naturally occurring phenylalanine in bovine) decreased atovaquone sensitivity, providing a molecular explanation for species-differential inhibition relevant to T. gondii drug targeting.","method":"Enzymatic inhibition kinetics, EPR spectroscopy, site-directed mutagenesis of cytochrome b, energy-minimized structural modeling","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstituted bc1 complex inhibition assay, mutagenesis, EPR, and structural modeling in a single rigorous study; mechanistically informs T. gondii cytochrome b Qo binding","pmids":["12791689"],"is_preprint":false,"attribution":[{"species":"Saccharomyces cerevisiae","strain":""}]},{"year":2017,"finding":"Selection of T. gondii under ELQ-316 pressure identified a Thr222Pro substitution in cytochrome b that confers 49-fold resistance to ELQ-316 and 19-fold resistance to antimycin, a well-characterized Qi site inhibitor, providing genetic evidence that ELQ-316 inhibits the Qi site of the T. gondii cytochrome bc1 complex.","method":"Drug-pressure selection of resistant clones, whole-genome/cytochrome b sequencing, cross-resistance assay with antimycin","journal":"Antimicrobial agents and chemotherapy","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic epistasis via resistant mutant sequencing plus cross-resistance with a known Qi-site inhibitor, two orthogonal lines of evidence, single lab","pmids":["27919897"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2016,"finding":"Mutant yeast and co-crystallographic studies demonstrated that novel 4(1H)-quinolone scaffolds bind to the cytochrome bc1 complex Qi site, and bradyzoite cysts of T. gondii highly express cytochrome b, which is targeted by these compounds.","method":"Mutant yeast bc1 complex biochemical assay, co-crystallography of bc1 complex with quinolone compounds, in vitro and in vivo drug efficacy assays","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 1 / Moderate — co-crystal structure plus mutant yeast biochemistry, two orthogonal methods, single lab","pmids":["27412848"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":"EGS"}]},{"year":2019,"finding":"Artemisinin-resistant T. gondii lines generated by in vitro evolution showed dramatic amplification of the mitochondrial genome including cytochrome b and cytochrome c oxidase I genes; treatment of wild-type parasites with high concentrations of artemisinin decreased mitochondrial membrane potential, and this phenotype was stably altered in resistant parasites, implicating the mitochondrion (and cytochrome b-containing complexes) as a target of artemisinin action.","method":"In vitro evolution under drug pressure, whole-genome sequencing, CRISPR/Cas9 genome editing, mitochondrial membrane potential measurement","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — whole-genome sequencing plus direct mitochondrial membrane potential measurement and genome editing, but cytochrome b amplification is correlative rather than directly mutagenized","pmids":["31806760"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2017,"finding":"Knockout of TgMSH1 (mitochondrial MutS homolog) in T. gondii resulted in accumulation of missense mutations in cytochrome b and reduced mitochondrial DNA content; none of the cytochrome b mutations affected known atovaquone-sensitivity determinants, yet parasites survived atovaquone by shutting down oxidative phosphorylation entirely rather than through target-site resistance, revealing that atovaquone sensitivity depends on functional electron transport through cytochrome b.","method":"TgMSH1 knockout, Sanger sequencing of cytochrome b, electron transport (respiration) assay, atovaquone resistance phenotyping","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic KO with sequencing and functional electron transport assay, single lab, two orthogonal methods","pmids":["29141004"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2025,"finding":"Cryo-EM structures of the T. gondii III2-IV respiratory supercomplex and isolated complex III2 were solved; atovaquone binding to the Qo site of cytochrome b was visualized, revealing species-specific interactions underpinning differential inhibition. ELQ-300 was shown to bind the Qi site of cytochrome b in a flipped orientation compared to related compounds in the mammalian enzyme. Knockout of the supercomplex interface subunit disrupted III2-IV assembly but did not reduce electron transfer efficiency, though it impaired parasite fitness.","method":"Cryo-electron microscopy structure determination, genetic knockout, drug-binding co-structures with atovaquone and ELQ-300, structure-guided design of new ELQ compounds","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structures with functional validation by KO and drug co-structures, multiple orthogonal methods in a single rigorous study","pmids":["40389671"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2018,"finding":"Structure-activity relationship studies with 4(1H)-quinolones against wild-type and drug-resistant T. gondii strains showed that Qo site-active compounds are most potent, and comparison of parasite cytochrome b sequences identified amino acids naturally present in T. gondii that correspond to resistance-conferring positions in P. falciparum, indicating that sequence differences in cytochrome b between the two parasites influence intrinsic drug susceptibility.","method":"Drug susceptibility assays against wild-type and resistant parasite strains, cytochrome b sequence comparison between species","journal":"ACS infectious diseases","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pharmacological profiling against defined resistant strains plus sequence analysis, single lab, two complementary approaches","pmids":["30117728"],"is_preprint":false,"attribution":[{"species":"Toxoplasma gondii","strain":""},{"species":"Plasmodium falciparum","strain":""}]}],"current_model":"Toxoplasma gondii cytochrome b is a mitochondrially encoded subunit of the cytochrome bc1 complex (complex III) that mediates electron transfer from ubiquinol at the Qo oxidation site (where atovaquone and Qo-active endochin-like quinolones bind) and at the Qi reduction site (where ELQ-316-class quinolones and antimycin bind); cryo-EM structures have revealed the precise binding modes and parasite-specific interactions at both sites, mutations within either domain confer drug resistance, and functional electron transport through cytochrome b is essential for maintaining mitochondrial membrane potential and parasite viability."},"narrative":{"mechanistic_narrative":"Toxoplasma gondii cytochrome b is a subunit of the cytochrome bc1 complex (complex III) that catalyzes electron transfer within the parasite mitochondrial respiratory chain, a function essential for maintaining mitochondrial membrane potential and parasite viability [PMID:9813002, PMID:29141004]. Biochemical dissection of permeabilized tachyzoites localized atovaquone inhibition to the respiratory chain between cytochrome b and c1, where the drug collapses membrane potential [PMID:9813002], and drug-resistant mutants carry substitutions (M129L, I254L) mapping to the Qo ubiquinol-oxidation pocket [PMID:10802314]. Cytochrome b thus contains two functionally and pharmacologically distinct catalytic sites: the Qo site, where atovaquone and Qo-active 4(1H)-quinolones bind and interact with the Rieske iron-sulfur protein [PMID:12791689, PMID:30117728], and the Qi site, where antimycin and ELQ-class quinolones act, defined genetically by the Thr222Pro substitution conferring cross-resistance to ELQ-316 and antimycin [PMID:27919897, PMID:27412848]. Cryo-EM structures of the T. gondii III2-IV supercomplex and complex III2 visualized atovaquone bound at the Qo site and ELQ-300 bound at the Qi site in a flipped orientation relative to the mammalian enzyme, defining the parasite-specific interactions that underpin differential drug inhibition [PMID:40389671]. Sensitivity to atovaquone depends on functional electron transport rather than solely on target-site occupancy, since parasites can survive the drug by shutting down oxidative phosphorylation entirely [PMID:29141004].","teleology":[{"year":1998,"claim":"Establishing where atovaquone acts on the parasite resolved whether the mitochondrial respiratory chain is a viable drug target in T. gondii.","evidence":"In situ respiration and safranin O membrane-potential assays in digitonin-permeabilized tachyzoites with inhibitor controls","pmids":["9813002"],"confidence":"High","gaps":["Did not identify the molecular residues bound by atovaquone","Did not distinguish Qo from Qi site involvement"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":1999,"claim":"A cytochrome b mutation in a decoquinate-resistant mutant extended the target-site logic beyond atovaquone to quinolone-class bc1 inhibitors.","evidence":"Drug-resistant mutant generation and cytochrome b sequencing","pmids":["10519259"],"confidence":"Medium","gaps":["Single lab, single method with no full method detail","Specific residue and its site (Qo/Qi) not clearly resolved"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2000,"claim":"Sequencing atovaquone-resistant mutants pinpointed the Qo ubiquinol-binding pocket as the atovaquone-binding site and gave the first full gene characterization.","evidence":"Resistant-mutant generation, gene cloning/sequencing (M129L, I254L), and Qo-domain structural modeling","pmids":["10802314"],"confidence":"High","gaps":["Structural model, not an experimental structure","Did not address the Qi site"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2003,"claim":"A reconstituted surrogate enzyme showed atovaquone binds competitively at Qo and interacts with the Rieske protein, explaining species-differential inhibition relevant to parasite targeting.","evidence":"Yeast bc1 inhibition kinetics, EPR, site-directed L275F mutagenesis, and structural modeling","pmids":["12791689"],"confidence":"High","gaps":["Performed in yeast bc1, not the T. gondii enzyme","Does not address Qi-site inhibitors"],"attribution":[{"species":"Saccharomyces cerevisiae","strain":""}]},{"year":2016,"claim":"Defining a second druggable site established that 4(1H)-quinolones can inhibit the Qi site, with bradyzoite cysts highly expressing the target.","evidence":"Mutant yeast bc1 biochemistry, co-crystallography of bc1 with quinolones, and drug efficacy assays","pmids":["27412848"],"confidence":"High","gaps":["Structural work used surrogate bc1 rather than the T. gondii complex","Cyst-stage expression not linked to a mechanistic function change"],"attribution":[{"species":"Toxoplasma gondii","strain":"EGS"}]},{"year":2017,"claim":"Genetic selection under ELQ-316 gave in vivo evidence for Qi-site inhibition in the parasite itself via a resistance mutation shared with antimycin.","evidence":"Drug-pressure selection, cytochrome b sequencing (Thr222Pro), and antimycin cross-resistance assay","pmids":["27919897"],"confidence":"High","gaps":["No structural confirmation of ELQ-316 at the Qi site at this stage","Fitness cost of the mutation not quantified"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2017,"claim":"Disrupting mitochondrial genome maintenance revealed that atovaquone sensitivity depends on functional electron transport, not merely on target-site sequence.","evidence":"TgMSH1 knockout with cytochrome b sequencing, respiration assay, and atovaquone phenotyping","pmids":["29141004"],"confidence":"Medium","gaps":["Accumulated cytochrome b missense mutations not individually validated","Mechanism of oxidative-phosphorylation shutdown not defined"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2018,"claim":"Comparative pharmacology and sequence analysis showed intrinsic cytochrome b sequence differences shape drug susceptibility across apicomplexan species.","evidence":"Drug susceptibility assays against wild-type and resistant strains and cross-species cytochrome b sequence comparison","pmids":["30117728"],"confidence":"Medium","gaps":["Sequence-susceptibility links are correlative","Causality of individual residues not tested by mutagenesis here"],"attribution":[{"species":"Toxoplasma gondii","strain":""},{"species":"Plasmodium falciparum","strain":""}]},{"year":2019,"claim":"Artemisinin-resistant evolution implicated cytochrome b-containing mitochondrial complexes in artemisinin action through genome amplification and membrane-potential changes.","evidence":"In vitro evolution, whole-genome sequencing, CRISPR/Cas9 editing, and membrane-potential measurement","pmids":["31806760"],"confidence":"Medium","gaps":["Cytochrome b amplification is correlative, not directly mutagenized","Direct mechanism of artemisinin on cytochrome b not established"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":2025,"claim":"Cryo-EM structures resolved the atomic binding modes at both catalytic sites and the supercomplex context, replacing surrogate-enzyme models with the native parasite enzyme.","evidence":"Cryo-EM of III2-IV supercomplex and complex III2, atovaquone and ELQ-300 co-structures, and supercomplex-interface knockout","pmids":["40389671"],"confidence":"High","gaps":["Turnover kinetics of the parasite enzyme not resolved from structures","Functional role of supercomplex assembly beyond fitness not fully defined"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]},{"year":null,"claim":"How the two catalytic sites are coordinated during turnover and how supercomplex assembly modulates electron flux under physiological and drug-stress conditions remain open.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No kinetic model of coupled Qo/Qi turnover in the T. gondii enzyme","Physiological role of III2-IV supercomplex assembly incompletely defined"],"attribution":[{"species":"Toxoplasma gondii","strain":""}]}],"mechanism_profile":{"molecular_activity":[],"localization":[],"pathway":[],"complexes":["Cytochrome bc1 complex (complex III / III2)","III2-IV respiratory supercomplex"],"partners":["RIESKE IRON-SULFUR PROTEIN","CYTOCHROME C1","CYTOCHROME C OXIDASE (COMPLEX IV)"],"other_free_text":["Contains two distinct catalytic sites: the Qo ubiquinol-oxidation site (atovaquone, Qo-active quinolones) and the Qi ubiquinone-reduction site (antimycin, ELQ-class quinolones).","Qo-site resistance mutations: M129L, I254L; Qi-site resistance mutation: Thr222Pro (also confers antimycin cross-resistance).","Mitochondrially encoded; genome amplification observed under artemisinin pressure.","Atovaquone sensitivity requires functional electron transport, not just target-site sequence."]},"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 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Institute","url":"https://pubmed.ncbi.nlm.nih.gov/42058244","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":14653,"output_tokens":2860,"usd":0.04343,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":8773,"output_tokens":3989,"usd":0.071795,"stage2_stop_reason":"end_turn"},"total_usd":0.115225,"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\": \"Cloning and sequencing of the T. gondii cytochrome b gene revealed that atovaquone-resistant mutants carry mutations M129L and I254L within the Qo domain (ubiquinol-binding pocket) of cytochrome b, indicating that atovaquone inhibits electron transport at the cytochrome bc1 complex (complex III) in T. gondii.\",\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 identifying specific active-site mutations, structural modeling, first complete characterization of the gene, foundational work replicated across multiple subsequent studies\",\n      \"pmids\": [\"10802314\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"A cytochrome b mutation was identified in a decoquinate-resistant mutant of T. gondii, indicating that cytochrome b mutations can confer resistance to quinolone-class compounds acting on the 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, single method (sequencing of resistant mutant), no abstract text available for full method evaluation\",\n      \"pmids\": [\"10519259\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Atovaquone at 0.03 µM totally inhibited respiration and disrupted mitochondrial membrane potential in T. gondii tachyzoites, and was shown biochemically to inhibit the respiratory chain between cytochrome b and c1, consistent with inhibition of the cytochrome bc1 complex.\",\n      \"method\": \"In situ respiration assay with digitonin-permeabilized tachyzoites, safranin O membrane potential measurement, antimycin A and cyanide sensitivity assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct biochemical (enzymatic) assay in permeabilized parasites with pharmacological dissection, multiple inhibitors used as controls, single lab\",\n      \"pmids\": [\"9813002\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Using yeast bc1 complex as a surrogate, atovaquone was shown to bind competitively at the ubiquinol oxidation (Qo) pocket of cytochrome b where it interacts with the Rieske iron-sulfur protein; introduction of the L275F mutation in yeast cytochrome b (equivalent to the naturally occurring phenylalanine in bovine) decreased atovaquone sensitivity, providing a molecular explanation for species-differential inhibition relevant to T. gondii drug targeting.\",\n      \"method\": \"Enzymatic inhibition kinetics, EPR spectroscopy, site-directed mutagenesis of cytochrome b, energy-minimized structural modeling\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstituted bc1 complex inhibition assay, mutagenesis, EPR, and structural modeling in a single rigorous study; mechanistically informs T. gondii cytochrome b Qo binding\",\n      \"pmids\": [\"12791689\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Saccharomyces cerevisiae\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Selection of T. gondii under ELQ-316 pressure identified a Thr222Pro substitution in cytochrome b that confers 49-fold resistance to ELQ-316 and 19-fold resistance to antimycin, a well-characterized Qi site inhibitor, providing genetic evidence that ELQ-316 inhibits the Qi site of the T. gondii cytochrome bc1 complex.\",\n      \"method\": \"Drug-pressure selection of resistant clones, whole-genome/cytochrome b sequencing, cross-resistance assay with antimycin\",\n      \"journal\": \"Antimicrobial agents and chemotherapy\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis via resistant mutant sequencing plus cross-resistance with a known Qi-site inhibitor, two orthogonal lines of evidence, single lab\",\n      \"pmids\": [\"27919897\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Mutant yeast and co-crystallographic studies demonstrated that novel 4(1H)-quinolone scaffolds bind to the cytochrome bc1 complex Qi site, and bradyzoite cysts of T. gondii highly express cytochrome b, which is targeted by these compounds.\",\n      \"method\": \"Mutant yeast bc1 complex biochemical assay, co-crystallography of bc1 complex with quinolone compounds, in vitro and in vivo drug efficacy assays\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — co-crystal structure plus mutant yeast biochemistry, two orthogonal methods, single lab\",\n      \"pmids\": [\"27412848\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"EGS\"}]\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Artemisinin-resistant T. gondii lines generated by in vitro evolution showed dramatic amplification of the mitochondrial genome including cytochrome b and cytochrome c oxidase I genes; treatment of wild-type parasites with high concentrations of artemisinin decreased mitochondrial membrane potential, and this phenotype was stably altered in resistant parasites, implicating the mitochondrion (and cytochrome b-containing complexes) as a target of artemisinin action.\",\n      \"method\": \"In vitro evolution under drug pressure, whole-genome sequencing, CRISPR/Cas9 genome editing, mitochondrial membrane potential measurement\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — whole-genome sequencing plus direct mitochondrial membrane potential measurement and genome editing, but cytochrome b amplification is correlative rather than directly mutagenized\",\n      \"pmids\": [\"31806760\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Knockout of TgMSH1 (mitochondrial MutS homolog) in T. gondii resulted in accumulation of missense mutations in cytochrome b and reduced mitochondrial DNA content; none of the cytochrome b mutations affected known atovaquone-sensitivity determinants, yet parasites survived atovaquone by shutting down oxidative phosphorylation entirely rather than through target-site resistance, revealing that atovaquone sensitivity depends on functional electron transport through cytochrome b.\",\n      \"method\": \"TgMSH1 knockout, Sanger sequencing of cytochrome b, electron transport (respiration) assay, atovaquone resistance phenotyping\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO with sequencing and functional electron transport assay, single lab, two orthogonal methods\",\n      \"pmids\": [\"29141004\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cryo-EM structures of the T. gondii III2-IV respiratory supercomplex and isolated complex III2 were solved; atovaquone binding to the Qo site of cytochrome b was visualized, revealing species-specific interactions underpinning differential inhibition. ELQ-300 was shown to bind the Qi site of cytochrome b in a flipped orientation compared to related compounds in the mammalian enzyme. Knockout of the supercomplex interface subunit disrupted III2-IV assembly but did not reduce electron transfer efficiency, though it impaired parasite fitness.\",\n      \"method\": \"Cryo-electron microscopy structure determination, genetic knockout, drug-binding co-structures with atovaquone and ELQ-300, structure-guided design of new ELQ compounds\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structures with functional validation by KO and drug co-structures, multiple orthogonal methods in a single rigorous study\",\n      \"pmids\": [\"40389671\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Structure-activity relationship studies with 4(1H)-quinolones against wild-type and drug-resistant T. gondii strains showed that Qo site-active compounds are most potent, and comparison of parasite cytochrome b sequences identified amino acids naturally present in T. gondii that correspond to resistance-conferring positions in P. falciparum, indicating that sequence differences in cytochrome b between the two parasites influence intrinsic drug susceptibility.\",\n      \"method\": \"Drug susceptibility assays against wild-type and resistant parasite strains, cytochrome b sequence comparison between species\",\n      \"journal\": \"ACS infectious diseases\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pharmacological profiling against defined resistant strains plus sequence analysis, single lab, two complementary approaches\",\n      \"pmids\": [\"30117728\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}, {\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    }\n  ],\n  \"current_model\": \"Toxoplasma gondii cytochrome b is a mitochondrially encoded subunit of the cytochrome bc1 complex (complex III) that mediates electron transfer from ubiquinol at the Qo oxidation site (where atovaquone and Qo-active endochin-like quinolones bind) and at the Qi reduction site (where ELQ-316-class quinolones and antimycin bind); cryo-EM structures have revealed the precise binding modes and parasite-specific interactions at both sites, mutations within either domain confer drug resistance, and functional electron transport through cytochrome b is essential for maintaining mitochondrial membrane potential and parasite viability.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"Toxoplasma gondii cytochrome b is a subunit of the cytochrome bc1 complex (complex III) that catalyzes electron transfer within the parasite mitochondrial respiratory chain, a function essential for maintaining mitochondrial membrane potential and parasite viability [#2, #7]. Biochemical dissection of permeabilized tachyzoites localized atovaquone inhibition to the respiratory chain between cytochrome b and c1, where the drug collapses membrane potential [#2], and drug-resistant mutants carry substitutions (M129L, I254L) mapping to the Qo ubiquinol-oxidation pocket [#0]. Cytochrome b thus contains two functionally and pharmacologically distinct catalytic sites: the Qo site, where atovaquone and Qo-active 4(1H)-quinolones bind and interact with the Rieske iron-sulfur protein [#3, #9], and the Qi site, where antimycin and ELQ-class quinolones act, defined genetically by the Thr222Pro substitution conferring cross-resistance to ELQ-316 and antimycin [#4, #5]. Cryo-EM structures of the T. gondii III2-IV supercomplex and complex III2 visualized atovaquone bound at the Qo site and ELQ-300 bound at the Qi site in a flipped orientation relative to the mammalian enzyme, defining the parasite-specific interactions that underpin differential drug inhibition [#8]. Sensitivity to atovaquone depends on functional electron transport rather than solely on target-site occupancy, since parasites can survive the drug by shutting down oxidative phosphorylation entirely [#7].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Establishing where atovaquone acts on the parasite resolved whether the mitochondrial respiratory chain is a viable drug target in T. gondii.\",\n      \"evidence\": \"In situ respiration and safranin O membrane-potential assays in digitonin-permeabilized tachyzoites with inhibitor controls\",\n      \"pmids\": [\"9813002\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify the molecular residues bound by atovaquone\", \"Did not distinguish Qo from Qi site involvement\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"A cytochrome b mutation in a decoquinate-resistant mutant extended the target-site logic beyond atovaquone to quinolone-class bc1 inhibitors.\",\n      \"evidence\": \"Drug-resistant mutant generation and cytochrome b sequencing\",\n      \"pmids\": [\"10519259\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab, single method with no full method detail\", \"Specific residue and its site (Qo/Qi) not clearly resolved\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Sequencing atovaquone-resistant mutants pinpointed the Qo ubiquinol-binding pocket as the atovaquone-binding site and gave the first full gene characterization.\",\n      \"evidence\": \"Resistant-mutant generation, gene cloning/sequencing (M129L, I254L), and Qo-domain structural modeling\",\n      \"pmids\": [\"10802314\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural model, not an experimental structure\", \"Did not address the Qi site\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"A reconstituted surrogate enzyme showed atovaquone binds competitively at Qo and interacts with the Rieske protein, explaining species-differential inhibition relevant to parasite targeting.\",\n      \"evidence\": \"Yeast bc1 inhibition kinetics, EPR, site-directed L275F mutagenesis, and structural modeling\",\n      \"pmids\": [\"12791689\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Performed in yeast bc1, not the T. gondii enzyme\", \"Does not address Qi-site inhibitors\"],\n      \"attribution\": [{\"species\": \"Saccharomyces cerevisiae\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Defining a second druggable site established that 4(1H)-quinolones can inhibit the Qi site, with bradyzoite cysts highly expressing the target.\",\n      \"evidence\": \"Mutant yeast bc1 biochemistry, co-crystallography of bc1 with quinolones, and drug efficacy assays\",\n      \"pmids\": [\"27412848\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural work used surrogate bc1 rather than the T. gondii complex\", \"Cyst-stage expression not linked to a mechanistic function change\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"EGS\"}]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Genetic selection under ELQ-316 gave in vivo evidence for Qi-site inhibition in the parasite itself via a resistance mutation shared with antimycin.\",\n      \"evidence\": \"Drug-pressure selection, cytochrome b sequencing (Thr222Pro), and antimycin cross-resistance assay\",\n      \"pmids\": [\"27919897\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural confirmation of ELQ-316 at the Qi site at this stage\", \"Fitness cost of the mutation not quantified\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Disrupting mitochondrial genome maintenance revealed that atovaquone sensitivity depends on functional electron transport, not merely on target-site sequence.\",\n      \"evidence\": \"TgMSH1 knockout with cytochrome b sequencing, respiration assay, and atovaquone phenotyping\",\n      \"pmids\": [\"29141004\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Accumulated cytochrome b missense mutations not individually validated\", \"Mechanism of oxidative-phosphorylation shutdown not defined\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Comparative pharmacology and sequence analysis showed intrinsic cytochrome b sequence differences shape drug susceptibility across apicomplexan species.\",\n      \"evidence\": \"Drug susceptibility assays against wild-type and resistant strains and cross-species cytochrome b sequence comparison\",\n      \"pmids\": [\"30117728\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Sequence-susceptibility links are correlative\", \"Causality of individual residues not tested by mutagenesis here\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}, {\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Artemisinin-resistant evolution implicated cytochrome b-containing mitochondrial complexes in artemisinin action through genome amplification and membrane-potential changes.\",\n      \"evidence\": \"In vitro evolution, whole-genome sequencing, CRISPR/Cas9 editing, and membrane-potential measurement\",\n      \"pmids\": [\"31806760\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cytochrome b amplification is correlative, not directly mutagenized\", \"Direct mechanism of artemisinin on cytochrome b not established\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Cryo-EM structures resolved the atomic binding modes at both catalytic sites and the supercomplex context, replacing surrogate-enzyme models with the native parasite enzyme.\",\n      \"evidence\": \"Cryo-EM of III2-IV supercomplex and complex III2, atovaquone and ELQ-300 co-structures, and supercomplex-interface knockout\",\n      \"pmids\": [\"40389671\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Turnover kinetics of the parasite enzyme not resolved from structures\", \"Functional role of supercomplex assembly beyond fitness not fully defined\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the two catalytic sites are coordinated during turnover and how supercomplex assembly modulates electron flux under physiological and drug-stress conditions remain open.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No kinetic model of coupled Qo/Qi turnover in the T. gondii enzyme\", \"Physiological role of III2-IV supercomplex assembly incompletely defined\"],\n      \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008121\", \"supporting_discovery_ids\": [2, 3, 8], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]},\n      {\"term_id\": \"GO:0006122\", \"supporting_discovery_ids\": [2, 7], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005750\", \"supporting_discovery_ids\": [8], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]},\n      {\"term_id\": \"GO:0005743\", \"supporting_discovery_ids\": [2, 8], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0022904\", \"supporting_discovery_ids\": [2, 7], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]},\n      {\"term_id\": \"GO:0006119\", \"supporting_discovery_ids\": [7], \"attribution\": [{\"species\": \"Toxoplasma gondii\", \"strain\": \"\"}]}\n    ],\n    \"complexes\": [\n      \"Cytochrome bc1 complex (complex III / III2)\",\n      \"III2-IV respiratory supercomplex\"\n    ],\n    \"partners\": [\n      \"Rieske iron-sulfur protein\",\n      \"Cytochrome c1\",\n      \"Cytochrome c oxidase (complex IV)\"\n    ],\n    \"other_free_text\": [\n      \"Contains two distinct catalytic sites: the Qo ubiquinol-oxidation site (atovaquone, Qo-active quinolones) and the Qi ubiquinone-reduction site (antimycin, ELQ-class quinolones).\",\n      \"Qo-site resistance mutations: M129L, I254L; Qi-site resistance mutation: Thr222Pro (also confers antimycin cross-resistance).\",\n      \"Mitochondrially encoded; genome amplification observed under artemisinin pressure.\",\n      \"Atovaquone sensitivity requires functional electron transport, not just target-site sequence.\"\n    ]\n  },\n  \"strain_conflicts\": []\n}","audit_flag":null,"strains":[{"strain":"RH88","gene_id":"TGRH88_085320","name":null,"product":"cytochrome b","uniprot_acc":null,"chromosome":"JAAUHK010000041","start_pos":18476}]}