Trx-Px2

Trx-Px2 (PF3D7_1215000) is a mitochondrially-localized 2-Cys peroxiredoxin of Plasmodium falciparum that provides thioredoxin-dependent antioxidant defense against reactive oxygen species generated during erythrocytic-stage metabolism (PMID:15710548, PMID:16879648). The recombinant protein exhibits peroxidase/antioxidant activity in vitro and, under oxidizing conditions, converts to a faster-migrating disulfide-bonded species, consistent with redox-regulated catalysis through intramolecular/intermolecular disulfide formation (PMID:11231293). Structurally it adopts the canonical 2-Cys peroxiredoxin fold, exists predominantly as a homodecamer in solution, and forms a dimer bearing an intermolecular Cys67–Cys187 disulfide in its oxidized state (PMID:16879648). Functionally, it discriminates between reducing partners, strongly preferring the mitochondrial thioredoxin PfTrx2 (KM 11.6 µM) over cytosolic PfTrx1 (KM 130.4 µM), with residues Asn62/Tyr63 and Phe139/Ala140 contributing to this substrate selectivity (PMID:16879648). Its subcellular residence was confirmed by both antibody-based confocal microscopy and GFP-fusion live imaging in erythrocytic stages, where expression peaks in trophozoite and schizont stages (PMID:15710548, PMID:16879648). Genetic disruption of the P. berghei ortholog established that Trx-Px2 is dispensable for asexual blood-stage growth and mosquito midgut oocyst development, indicating functional redundancy in vivo despite its defined biochemical role (PMID:23146411).

1. 2001 Medium

   Whether the larger P. falciparum 2-Cys peroxiredoxin possessed enzymatic antioxidant activity was unknown; recombinant expression established it as a functional peroxidase whose migration shifts under oxidizing conditions, pointing to disulfide-based redox regulation.

   Evidence Recombinant expression in E. coli, glutamine synthetase protector assay, and SDS-PAGE under oxidizing conditions

   PMID:11231293

   Open questions at the time

   • Kinetic constants were not determined for Trx-Px2 specifically in this study
   • The physiological electron donor was not identified
   • Single lab, single in vitro assay
2. 2005 Medium

   The subcellular compartment and stage-specific expression of Trx-Px2 were undefined; localization to the mitochondrion in trophozoite and schizont stages placed it at the site of respiratory-chain ROS production.

   Evidence Confocal microscopy, Western blotting, and real-time quantitative RT-PCR in erythrocytic-stage P. falciparum

   PMID:15710548

   Open questions at the time

   • The ROS-detoxification role at the mitochondrion was inferred, not directly demonstrated
   • Reducing partner not established in this study
3. 2006 High

   The structure, oligomeric state, and reductant specificity of Trx-Px2 were unknown; a crystal structure, solution studies, kinetics, and mutagenesis established its 2-Cys peroxiredoxin fold, homodecameric assembly, Cys67–Cys187 disulfide dimer, and strong preference for mitochondrial PfTrx2, defining a mitochondrial thioredoxin-peroxiredoxin redox couple.

   Evidence X-ray crystallography, in vitro kinetics with recombinant protein, site-directed mutagenesis (Asn62/Tyr63, Phe139/Ala140), and GFP-fusion live imaging

   PMID:16879648

   Open questions at the time

   • In vivo requirement for the PfTrx2 preference was not tested
   • The peroxide substrate range in vivo was not defined
   • Regulatory role of oligomeric switching not established in cells
4. 2012 Medium

   Whether the well-characterized mitochondrial peroxiredoxin is required for parasite viability was unknown; knockout of the P. berghei ortholog showed it is dispensable for asexual blood-stage growth and mosquito oocyst development, revealing functional redundancy despite its defined biochemistry.

   Evidence Gene disruption in P. berghei with RT-PCR/Western confirmation, in vivo mouse growth assays, and mosquito infection assays

   PMID:23146411

   Open questions at the time

   • Redundant or compensating antioxidant systems were not identified
   • The minor delay in peak gametocytemia was not mechanistically explained
   • Result obtained in P. berghei, not P. falciparum

• The in vivo antioxidant substrates, the compensating pathways underlying its dispensability, and the physiological significance of its decamer–dimer redox switch remain unresolved.
• No physiological peroxide substrate identified in parasites
• Redundant antioxidant partners masking the KO phenotype are unknown
• P. falciparum genetic requirement not directly tested