TGRH88_085320

Annotated: 2026-09-07

23 papers in source corpus 10 papers cited in narrative 10 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

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).

Mechanistic history

Synthesis pass · year-by-year structured walk · 10 steps

1998 High
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

PMID:9813002
Open questions at the time
- Did not identify the molecular residues bound by atovaquone
- Did not distinguish Qo from Qi site involvement
1999 Medium
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

PMID:10519259
Open questions at the time
- Single lab, single method with no full method detail
- Specific residue and its site (Qo/Qi) not clearly resolved
2000 High
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

PMID:10802314
Open questions at the time
- Structural model, not an experimental structure
- Did not address the Qi site
2003 High
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

PMID:12791689
Open questions at the time
- Performed in yeast bc1, not the T. gondii enzyme
- Does not address Qi-site inhibitors
2016 High
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

PMID:27412848
Open questions at the time
- Structural work used surrogate bc1 rather than the T. gondii complex
- Cyst-stage expression not linked to a mechanistic function change
2017 High
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

PMID:27919897
Open questions at the time
- No structural confirmation of ELQ-316 at the Qi site at this stage
- Fitness cost of the mutation not quantified
2017 Medium
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

PMID:29141004
Open questions at the time
- Accumulated cytochrome b missense mutations not individually validated
- Mechanism of oxidative-phosphorylation shutdown not defined
2018 Medium
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

PMID:30117728
Open questions at the time
- Sequence-susceptibility links are correlative
- Causality of individual residues not tested by mutagenesis here
2019 Medium
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

PMID:31806760
Open questions at the time
- Cytochrome b amplification is correlative, not directly mutagenized
- Direct mechanism of artemisinin on cytochrome b not established
2025 High
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

PMID:40389671
Open questions at the time
- Turnover kinetics of the parasite enzyme not resolved from structures
- Functional role of supercomplex assembly beyond fitness not fully defined

Open questions

Synthesis pass · forward-looking unresolved questions

How the two catalytic sites are coordinated during turnover and how supercomplex assembly modulates electron flux under physiological and drug-stress conditions remain open.
No kinetic model of coupled Qo/Qi turnover in the T. gondii enzyme
Physiological role of III2-IV supercomplex assembly incompletely defined

Mechanism profile

Synthesis pass · controlled-vocabulary classification

Partners

CYTOCHROME C OXIDASE (COMPLEX IV)CYTOCHROME C1 RIESKE IRON-SULFUR PROTEIN

Complex memberships

Cytochrome bc1 complex (complex III / III2)III2-IV respiratory supercomplex

Evidence

Reading pass · 10 per-paper findings extracted from the source corpus

Year	Finding	Method	Journal	Conf	PMIDs
2000	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.	Drug-resistant mutant generation, gene cloning and sequencing, structural modeling of Qo domain	Molecular and biochemical parasitology	High	10802314
1999	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.	Drug-resistant mutant generation and gene sequencing	The Journal of eukaryotic microbiology	Medium	10519259
1998	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.	In situ respiration assay with digitonin-permeabilized tachyzoites, safranin O membrane potential measurement, antimycin A and cyanide sensitivity assays	The Journal of biological chemistry	High	9813002
2003	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.	Enzymatic inhibition kinetics, EPR spectroscopy, site-directed mutagenesis of cytochrome b, energy-minimized structural modeling	The Journal of biological chemistry	High	12791689
2017	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.	Drug-pressure selection of resistant clones, whole-genome/cytochrome b sequencing, cross-resistance assay with antimycin	Antimicrobial agents and chemotherapy	High	27919897
2016	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.	Mutant yeast bc1 complex biochemical assay, co-crystallography of bc1 complex with quinolone compounds, in vitro and in vivo drug efficacy assays	Scientific reports	High	27412848
2019	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.	In vitro evolution under drug pressure, whole-genome sequencing, CRISPR/Cas9 genome editing, mitochondrial membrane potential measurement	Proceedings of the National Academy of Sciences of the United States of America	Medium	31806760
2017	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.	TgMSH1 knockout, Sanger sequencing of cytochrome b, electron transport (respiration) assay, atovaquone resistance phenotyping	PloS one	Medium	29141004
2025	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.	Cryo-electron microscopy structure determination, genetic knockout, drug-binding co-structures with atovaquone and ELQ-300, structure-guided design of new ELQ compounds	Nature structural & molecular biology	High	40389671
2018	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.	Drug susceptibility assays against wild-type and resistant parasite strains, cytochrome b sequence comparison between species	ACS infectious diseases	Medium	30117728

Source papers

Stage 0 corpus · 23 papers · ranked by NIH iCite citations

Year	Title	Journal	Citations	PMID
2000	Characterization of cytochrome b from Toxoplasma gondii and Q(o) domain mutations as a mechanism of atovaquone-resistance.	Molecular and biochemical parasitology	124	10802314
2003	Molecular basis for atovaquone binding to the cytochrome bc1 complex.	The Journal of biological chemistry	113	12791689
2008	In vitro susceptibility of various genotypic strains of Toxoplasma gondii to pyrimethamine, sulfadiazine, and atovaquone.	Antimicrobial agents and chemotherapy	106	18212105
1998	Respiration and oxidative phosphorylation in the apicomplexan parasite Toxoplasma gondii.	The Journal of biological chemistry	91	9813002
2007	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.	Antimicrobial agents and chemotherapy	66	17242151
2014	Molecular characterisation of Sarcocystis lutrae n. sp. and Toxoplasma gondii from the musculature of two Eurasian otters (Lutra lutra) in Norway.	Parasitology research	60	25512210
2007	Modeling the molecular basis of atovaquone resistance in parasites and pathogenic fungi.	Trends in parasitology	52	17826334
2017	Genetic Evidence for Cytochrome b Qi Site Inhibition by 4(1H)-Quinolone-3-Diarylethers and Antimycin in Toxoplasma gondii.	Antimicrobial agents and chemotherapy	49	27919897
2018	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.	ACS infectious diseases	40	30117728
2016	New paradigms for understanding and step changes in treating active and chronic, persistent apicomplexan infections.	Scientific reports	37	27412848
2019	Evolution of resistance in vitro reveals mechanisms of artemisinin activity in Toxoplasma gondii.	Proceedings of the National Academy of Sciences of the United States of America	35	31806760
2011	Design of anti-parasitic and anti-fungal hydroxy-naphthoquinones that are less susceptible to drug resistance.	Molecular and biochemical parasitology	31	21251932
2013	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).	Parasitology research	30	23358734
2020	Potent Tetrahydroquinolone Eliminates Apicomplexan Parasites.	Frontiers in cellular and infection microbiology	26	32626661
2020	Orally Bioavailable Endochin-Like Quinolone Carbonate Ester Prodrug Reduces Toxoplasma gondii Brain Cysts.	Antimicrobial agents and chemotherapy	26	32540978
2020	Activities of Endochin-Like Quinolones Against in vitro Cultured Besnoitia besnoiti Tachyzoites.	Frontiers in veterinary science	16	32161765
1999	Cytochrome b mutation identified in a decoquinate-resistant mutant of Toxoplasma gondii.	The Journal of eukaryotic microbiology	12	10519259
2025	Structure, assembly and inhibition of the Toxoplasma gondii respiratory chain supercomplex.	Nature structural & molecular biology	10	40389671
2024	Molecular prevalence, phylogeny and hematological impact of Toxoplasma gondii and Plasmodium spp. in common quails from Punjab, Pakistan.	PloS one	10	38820542
2017	Lack of mitochondrial MutS homolog 1 in Toxoplasma gondii disrupts maintenance and fidelity of mitochondrial DNA and reveals metabolic plasticity.	PloS one	9	29141004
2001	Are cytochrome b gene mutations the only cause of atovaquone resistance in Pneumocystis?	Drug resistance updates : reviews and commentaries in antimicrobial and anticancer chemotherapy	7	11991686
2016	Molecular phylogeny of Toxoplasmatinae: comparison between inferences based on mitochondrial and apicoplast genetic sequences.	Revista brasileira de parasitologia veterinaria = Brazilian journal of veterinary parasitology : Orgao Oficial do Colegio Brasileiro de Parasitologia Veterinaria	6	27007245
2025	Phylogenetic study of the protozoan Toxoplasma gondii using cytochrome b and GRA20 genes.	Archives of Razi Institute	0	42058244