TGRH88_085440

Toxoplasma gondii cytochrome b is a mitochondrially encoded core subunit of the cytochrome bc1 complex (complex III), where it catalyzes electron transport between ubiquinol and cytochrome c1 as part of the parasite's respiratory chain (PMID:9813002, PMID:40389671). It carries two functionally distinct catalytic sites: a ubiquinol-oxidation (Qo) site targeted by atovaquone—resistance to which arises from M129L, I254L, and L275F substitutions clustered in the Qo domain (PMID:10802314, PMID:12791689)—and a quinone-reduction (Qi) site where endochin-like quinolones act, established genetically by a T222P substitution that confers cross-resistance to the classic Qi inhibitor antimycin (PMID:27919897, PMID:27412848). Blocking the Qo site with atovaquone fully collapses respiration and mitochondrial membrane potential (PMID:9813002), and simultaneous blockade of cytochrome b together with an upstream NADH dehydrogenase is synergistic, underscoring cytochrome b's central position in ubiquinone/ubiquinol cycling (PMID:17242151). Cryo-EM structures of the T. gondii III2-IV supercomplex resolved atovaquone and ELQ-300 bound within cytochrome b, revealing parasite-specific drug-binding geometries, while supercomplex disruption confirmed that this assembly is critical for parasite fitness (PMID:40389671). Cytochrome b is also implicated as part of the mitochondrial target of artemisinin, whose activity correlates with amplification of the cytochrome b-containing mitochondrial genome and loss of membrane potential (PMID:31806760).

1. 1998 High

   It was unknown where in the T. gondii respiratory chain atovaquone acts; biochemical dissection localized the block to cytochrome b, establishing the bc1 complex as the drug target and cytochrome b as essential for respiration.

   Evidence In situ respiration and membrane-potential assays in digitonin-permeabilized tachyzoites with TMPD/ascorbate rescue

   PMID:9813002

   Open questions at the time

   • Did not resolve which cytochrome b site (Qo vs Qi) is bound
   • No structural or residue-level information
2. 1999 Medium

   The molecular target of decoquinate in T. gondii was unknown; a resistance mutation in cytochrome b implicated the bc1 complex, extending cytochrome b as a shared antiparasitic drug target.

   Evidence Selection of a decoquinate-resistant mutant and cytochrome b gene sequencing

   PMID:10519259

   Open questions at the time

   • Abstract-level detail only; exact mutation and site not defined
   • No enzymatic confirmation of altered drug binding
3. 2000 High

   The T. gondii cytochrome b sequence and the residues controlling atovaquone action were undefined; cloning of the gene from atovaquone-resistant mutants pinpointed M129L and I254L in the Qo domain, establishing atovaquone as a Qo-site inhibitor.

   Evidence Drug-resistant mutant generation, gene cloning/sequencing, and Qo-domain structural modeling

   PMID:10802314

   Open questions at the time

   • Model predicted an unusual membrane-terminating topology not experimentally validated at the time
   • Qi site not addressed
4. 2003 High

   The physical basis of atovaquone binding and species-selectivity was unresolved; enzyme kinetics, EPR, and mutagenesis in a yeast surrogate showed atovaquone occupies the Qo pocket and engages the Rieske iron-sulfur protein, with L275F reducing sensitivity while preserving activity.

   Evidence bc1 inhibition kinetics, EPR of the Rieske center, site-directed mutagenesis in yeast, and structural modeling

   PMID:12791689

   Open questions at the time

   • Performed in yeast, not native T. gondii enzyme
   • No high-resolution structure of the T. gondii binding site at this stage
5. 2007 Medium

   It was unclear how Qo-site inhibition combines with other respiratory blockades; a synergy assay with an NADH dehydrogenase inhibitor demonstrated that dual disruption of ubiquinone/ubiquinol cycling is highly potent, placing cytochrome b at a strategic pharmacological node.

   Evidence Combination drug assay with HDQ and atovaquone and fractional inhibitory concentration calculation in infected cultures

   PMID:17242151

   Open questions at the time

   • Single lab, single combination readout
   • No direct measurement of enzyme-level interaction
6. 2016 Medium

   Whether cytochrome b could be targeted at the Qi site against dormant stages was unknown; co-crystallography and parasite assays showed 4-(1H)-quinolones bind the Qi site and kill both tachyzoites and bradyzoites, which highly express cytochrome b.

   Evidence Co-crystallography with bc1, mutant yeast inhibition assays, in vitro proliferation assays, and a murine infection model

   PMID:27412848

   Open questions at the time

   • Structural work used surrogate bc1, not T. gondii enzyme
   • Qi binding not yet confirmed genetically in T. gondii
7. 2017 High

   Direct genetic evidence for a Qi-site inhibitor of T. gondii cytochrome b was missing; a T222P substitution conferring cross-resistance to ELQ-316 and the known Qi inhibitor antimycin established ELQ action at the Qi site.

   Evidence ELQ-316 resistance selection, cytochrome b sequencing, and antimycin cross-resistance testing

   PMID:27919897

   Open questions at the time

   • Structural placement of T222P relative to the Qi pocket not resolved here
   • Effect on native enzyme kinetics not quantified
8. 2017 Medium

   The stability and mutability of the mitochondrial genome encoding cytochrome b was unclear; loss of TgMSH1 caused cytochrome b missense accumulation and revealed metabolic plasticity allowing survival without oxidative phosphorylation, while atovaquone still inhibited electron transport since mutations spared known sensitivity residues.

   Evidence TgMSH1 knockout, whole mitochondrial genome sequencing, electron transport assays, and glucose-dependence growth assays

   PMID:29141004

   Open questions at the time

   • Functional consequence of individual cytochrome b missense mutations not dissected
   • Mechanism of MSH1-dependent mitochondrial DNA maintenance not defined
9. 2019 Medium

   Whether the cytochrome b-containing mitochondrion is an artemisinin target was unknown; artemisinin-resistant lines amplified the mitochondrial genome (including cytochrome b) and stably altered drug-induced membrane-potential loss, implicating mitochondrial respiratory complexes in artemisinin action.

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

   PMID:31806760

   Open questions at the time

   • Cytochrome b amplification is correlative rather than a defined mechanism
   • Direct artemisinin binding to cytochrome b not shown
10. 2025 High

    The native architecture of the T. gondii respiratory assembly and the structural basis of parasite-selective drug binding were unresolved; cryo-EM of the III2-IV supercomplex placed cytochrome b as a core subunit, visualized atovaquone and ELQ-300 in their sites (ELQ-300 in a flipped orientation versus mammalian enzyme), and knockout showed the supercomplex is critical for fitness.

    Evidence Cryo-EM structures of III2-IV and III2, drug co-structures, supercomplex-subunit knockout, and fitness assays

    PMID:40389671

    Open questions at the time

    • Functional role of the supercomplex arrangement beyond fitness not fully explained
    • Dynamics of Qo/Qi catalysis not captured by static structures

• How individual cytochrome b mutations quantitatively alter native enzyme catalysis versus drug binding, and the precise mechanism by which mitochondrial genome amplification confers artemisinin resistance, remain open.
• No kinetic dissection of native T. gondii cytochrome b with defined mutations
• Artemisinin-cytochrome b relationship remains correlative