AHP1

TgAHP1 is a peroxiredoxin-family antioxidant enzyme that protects Toxoplasma gondii against oxidative stress by acting as a cysteine-dependent peroxidase, using the active-site Cys166 to reduce hydroperoxides through a thiol-based redox mechanism (PMID:34603593). It displays substrate preference for alkyl hydroperoxides over H2O2, and its loss sensitizes parasites specifically to tert-butyl hydroperoxide, leading to ROS accumulation, apoptosis, and impaired invasion, proliferation, and virulence in mice (PMID:34603593). The enzyme localizes in a punctate pattern anterior to the nucleus consistent with peroxisomal targeting (PMID:34603593), and under oxidative stress it is a target of the urmylation pathway, interacting with the ubiquitin-related modifier TgURM1 in a stress-dependent manner (PMID:35323018, PMID:36142209). Beyond these findings, the structural basis of catalysis and the functional consequence of urmylation on TgAHP1 activity have not been characterized in the available corpus.

1. 2021 Medium

   Establishing that TgAHP1 is an active peroxidase answered whether the protein has intrinsic antioxidant enzymatic function and pinpointed the catalytic residue driving it.

   Evidence Active-site mutagenesis of Cys166 with in vitro and cellular peroxidase activity assays in T. gondii

   PMID:34603593

   Open questions at the time

   • No structural model or resolving-cysteine/regeneration partner identified
   • Abstract-level detail from a single lab
   • Catalytic cycle and electron donor not defined
2. 2021 Medium

   Subcellular imaging addressed where the enzyme acts, placing TgAHP1 in a punctate compartment consistent with peroxisomal localization.

   Evidence Fluorescence microscopy of fixed parasites showing punctate staining anterior to the nucleus

   PMID:34603593

   Open questions at the time

   • Peroxisomal identity inferred from pattern, not confirmed by co-localization with a peroxisomal marker
   • No functional consequence directly tied to localization
3. 2021 Medium

   Genetic knockout defined the physiological substrate preference and the biological importance of TgAHP1, showing alkyl hydroperoxide detoxification is required for parasite fitness and virulence.

   Evidence ΔAhp1 knockout with ROS measurement, invasion/proliferation assays, and mouse virulence model

   PMID:34603593

   Open questions at the time

   • Molecular mechanism linking ROS accumulation to apoptosis not defined
   • Single lab; substrate preference shown phenotypically rather than by enzyme kinetics
4. 2022 Medium

   Interaction proteomics connected TgAHP1 to the urmylation pathway, revealing a post-translational modification layer engaged specifically during oxidative stress.

   Evidence Proximity-based labelling (BioID) and mass spectrometry proteomics in T. gondii; TgURM1 interaction validated under oxidative stress

   PMID:35323018 PMID:36142209

   Open questions at the time

   • Functional consequence of urmylation on TgAHP1 activity, stability, or localization not established
   • Urmylated residue not mapped

• How urmylation modulates TgAHP1 peroxidase activity and how the enzyme is regenerated in its catalytic cycle remain unresolved.
• No structural characterization of the active site
• Reducing/regeneration partner unknown
• Direct link between urmylation and enzymatic outcome untested