{"gene":"PF3D7_1116200","run_date":"2026-09-08T12:51:28","timeline":{"discoveries":[{"year":2004,"finding":"PDX2 (PfPdx2) acts as the glutaminase subunit of the P. falciparum PLP synthase complex, functioning together with PfPdx1 to catalyze glutaminase activity with a specific activity of 209 nmol min⁻¹ mg⁻¹ and a Km for glutamine of 1.3 mM; complementation of pyridoxine auxotroph yeast cells confirmed involvement of both proteins in de novo vitamin B6 biosynthesis.","method":"Recombinant protein biochemistry (glutaminase activity assay), yeast complementation assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro enzymatic assay with defined kinetic parameters plus functional complementation in vivo, replicated by subsequent studies","pmids":["15590634"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2005,"finding":"PDX2 functions as the glutaminase subunit of the heteromeric PLP synthase complex; complex formation with Pdx1 is required for catalytic activity of either subunit. The 1.62 Å crystal structure of PfPdx2 reveals an obstructed oxyanion hole indicating a resting state that is activated upon Pdx1-Pdx2 complex formation. A chimeric bi-enzyme of Pdx1 with bacterial YaaE (a Pdx2 ortholog) forms in vivo and in vitro but does not attain full catalytic activity, emphasizing species-specific structural requirements for productive interaction.","method":"X-ray crystallography (1.62 Å structure of PfPdx2), in vitro reconstitution, glutaminase activity assay, in vivo/in vitro chimera formation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus reconstitution and enzymatic assays, independently consistent with other studies","pmids":["16339145"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2007,"finding":"Thermodynamic characterization by isothermal titration calorimetry showed that l-glutamine binding increases the affinity of PDX2 for Pdx1 approximately 30-fold and alters the thermodynamic signature of complex formation, indicating substrate-dependent tightening of the PDX2–Pdx1 interface. The complex interface is dominated by hydrophobic interactions (negative heat capacity). Swapping the αN-helix of PfPdx1 with the corresponding B. subtilis segment enabled cross-binding to B. subtilis Pdx2 and partially elicited glutaminase activity in BsPdx2, demonstrating that catalytic activation of PDX2 is mechanistically linked to formation of the Pdx1 αN-helix interface.","method":"Isothermal titration calorimetry, swap-mutant biochemistry, glutaminase activity assay, 3D homology modelling","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — quantitative thermodynamic assay plus mutagenesis with functional readout, single lab but multiple orthogonal methods","pmids":["17950752"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2008,"finding":"Mutational analysis of P. falciparum Pdx1 interface residues (R85, H88, E91 at the Pdx1:Pdx1 interface) showed that their mutation to alanine impedes both Pdx1 activity and PDX2 binding, establishing that Pdx1 self-assembly is a prerequisite for PDX2 recruitment. Mutation of active-site residues D26, K83, K151 blocked enzyme activity but did not prevent PDX2 binding or PDX2 glutaminase activity, demonstrating that PDX2 can bind a hexameric Pdx1 scaffold but full catalytic output requires a dodecameric assembly.","method":"Site-directed mutagenesis, biochemical activity assays, size-exclusion chromatography, co-purification","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic mutagenesis with multiple variants and enzymatic plus assembly readouts, single lab","pmids":["18350152"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2009,"finding":"A Glycine-155-to-Alanine mutation in Pdx1 produces a hexameric (not dodecameric) assembly in vitro with significant secondary-structure changes, yet this mutant retains the ability to bind PDX2 and support PDX2 glutaminase activity, demonstrating that PDX2 binding does not require the full dodecameric Pdx1 state, whereas Pdx1 catalytic activity does.","method":"Static and dynamic light scattering, circular dichroism, co-purification, enzyme activity assays","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal biophysical and biochemical methods in a single lab study","pmids":["19272411"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2010,"finding":"C-terminal deletion analysis of Pdx1 dissected four distinct functions contributed by the C-terminus: Pdx1 monomer assembly, ribose 5-phosphate binding, formation of reaction intermediate I320, and PLP synthesis. PLP synthesis is the only C-terminal function that can be supplied in trans, explaining the cooperative nature of the PDX2–Pdx1 complex and the cross-talk between subunits.","method":"C-terminal deletion mutagenesis, enzymatic activity assays, mass spectrometry (intermediate detection)","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — systematic deletion series with multiple functional readouts, single lab","pmids":["20837012"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2012,"finding":"Cryo-EM analysis revealed that PDX2 glutaminase subunits associate randomly (up to 12 subunits) onto the dodecameric Pdx1 core, and P. falciparum PLP synthase organizes into fibers. Crystal structure of the complex shows that an α-helix adopts an open vs. closed conformation upon substrate binding; the pentose substrate is covalently attached via its C1 in a Schiff base with Lys84 of Pdx1. A transient ammonia transfer tunnel between PDX2 and Pdx1 active sites is regulated during catalysis. Mutagenesis defined critical conserved methionines required for complex function while also showing plasticity in tunnel construction across species.","method":"Electron microscopy (EM), X-ray crystallography, site-directed mutagenesis, enzymatic assays","journal":"Structure (London, England : 1993)","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus EM plus mutagenesis with functional validation in a single study","pmids":["22244765"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2012,"finding":"In vivo pull-down experiments from transgenic P. falciparum parasites overexpressing both PfPdx1 and PDX2 confirmed a direct interaction between the two proteins inside the parasite. Overexpression of the complex elevated intracellular PLP levels from 12.5 µM (wild-type) to 36.6 µM and increased tolerance to singlet oxygen (¹O₂). Dominant-negative inactive mutants of either PfPdx1 or PDX2 rendered parasites susceptible to ¹O₂, establishing that de novo PLP synthesis via the PDX2-containing complex is required for singlet oxygen detoxification.","method":"In vivo pull-down (transgenic parasite overexpression), live-cell fluorescence microscopy, PLP quantification, transcription analysis, dominant-negative mutagenesis","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo interaction confirmed by pull-down, complemented by quantitative PLP measurement and dominant-negative functional validation in live parasites","pmids":["22242896"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2013,"finding":"PDX2 assembles with Pdx1 into a heteromeric PLP synthase complex in P. falciparum; transgenic parasite lines overexpressing both PfPdx1 and PDX2 showed protection against the Pdx1 inhibitor 4PEHz (IC50 shift from 16 µM for the complex to reduced sensitivity with extra copies), confirming that the PDX2-containing complex is the in vivo target of the inhibitor and is directly required for PLP biosynthesis in the parasite.","method":"Transgenic overexpression in P. falciparum, in vitro Pdx1 inhibition assay, parasite growth inhibition assay","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional in vivo validation via overexpression rescue, single lab","pmids":["23039077"],"is_preprint":false,"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2020,"finding":"In P. vivax, PDX2 exists in monomeric and multiple oligomeric states in solution; upon mixing with dodecameric Pdx1, polydisperse PDX2 is converted to a monodispersed heteromeric 24-meric Pdx1–PDX2 (Pdx) complex, as determined by DLS, SAXS (showing a ring-shaped dodecameric Pdx1 decorated with PDX2), and electron microscopy.","method":"Dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), electron microscopy (EM)","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple complementary biophysical methods in a single study, single lab, P. vivax ortholog","pmids":["32825141"],"is_preprint":false,"attribution":[{"species":"Plasmodium vivax","strain":""}]}],"current_model":"PDX2 (PF3D7_1116200) is the glutaminase subunit of the heteromeric PLP synthase complex in Plasmodium falciparum; it is activated only upon forming a complex with Pdx1 (obstruction of its oxyanion hole is relieved by Pdx1 binding), catalyzes the hydrolysis of glutamine to supply ammonia that is channeled through a transient tunnel to the Pdx1 active site for pyridoxal 5′-phosphate (PLP) synthesis, associates randomly (up to 12 subunits) onto the dodecameric Pdx1 scaffold, and its activity within the complex is essential for de novo PLP biosynthesis and for detoxification of singlet oxygen in the intraerythrocytic parasite."},"narrative":{"mechanistic_narrative":"PDX2 (PF3D7_1116200) is the glutaminase subunit of the heteromeric pyridoxal 5′-phosphate (PLP, vitamin B6) synthase complex of Plasmodium falciparum, functioning obligately together with Pdx1 in de novo vitamin B6 biosynthesis [PMID:15590634]. As a free enzyme PDX2 is held in a resting state — its 1.62 Å crystal structure shows an obstructed oxyanion hole — and productive glutaminase activity is acquired only upon assembly with Pdx1, which relieves this obstruction [PMID:16339145]. It hydrolyzes glutamine (Km ≈ 1.3 mM) to generate ammonia that is channeled through a transient tunnel to the Pdx1 active site, where the pentose substrate is bound as a Schiff base for PLP synthesis; within the assembled particle, PDX2 subunits decorate the dodecameric Pdx1 core (up to 12 copies), and the P. falciparum enzyme organizes into fibers [PMID:22244765]. Substrate binding reinforces the interaction: L-glutamine increases PDX2 affinity for Pdx1 roughly 30-fold, and catalytic activation is mechanistically coupled to engagement of the Pdx1 αN-helix interface, while recruitment of PDX2 requires prior Pdx1 self-assembly [PMID:17950752, PMID:18350152]. In live parasites, PDX2 interacts directly with Pdx1, and the PDX2-containing complex is required for de novo PLP synthesis and for detoxification of singlet oxygen, with dominant-negative PDX2 rendering intraerythrocytic parasites susceptible to ¹O₂ [PMID:22242896].","teleology":[{"year":2004,"claim":"Established that PDX2 is the glutaminase subunit of the parasite's PLP synthase and, with Pdx1, is genuinely required for de novo vitamin B6 biosynthesis rather than being a hypothetical annotation.","evidence":"Recombinant glutaminase activity assay (specific activity 209 nmol min⁻¹ mg⁻¹, Km ≈ 1.3 mM) plus complementation of a pyridoxine-auxotroph yeast strain","pmids":["15590634"],"confidence":"High","gaps":["Did not resolve why PDX2 requires Pdx1 for activity","No structural basis for the interaction"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2005,"claim":"Explained the obligate dependence of PDX2 on Pdx1: the free enzyme is autoinhibited and only complex formation switches it on.","evidence":"1.62 Å crystal structure of PfPdx2 revealing an obstructed oxyanion hole, with reconstitution/activity assays and a chimeric Pdx1–bacterial YaaE bi-enzyme showing species-specific interaction requirements","pmids":["16339145"],"confidence":"High","gaps":["Structure was of PDX2 alone, not the activated complex","Mechanism of oxyanion-hole relief not visualized"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2007,"claim":"Quantified the coupling between substrate binding and complex assembly, showing glutamine tightens the PDX2–Pdx1 interface and that activation is linked to the Pdx1 αN-helix.","evidence":"Isothermal titration calorimetry (~30-fold affinity increase with L-glutamine), Pdx1 αN-helix swap mutants activating B. subtilis Pdx2, homology modelling","pmids":["17950752"],"confidence":"High","gaps":["Interface geometry inferred, not solved at high resolution","Kinetics of tunnel opening not measured"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2008,"claim":"Defined the assembly hierarchy: Pdx1 self-assembly is a prerequisite for PDX2 recruitment, and PDX2 can bind and function on a hexameric scaffold whereas full PLP output requires a dodecamer.","evidence":"Site-directed mutagenesis of Pdx1 interface (R85, H88, E91) and active-site (D26, K83, K151) residues with size-exclusion chromatography, co-purification and activity assays","pmids":["18350152"],"confidence":"High","gaps":["Did not establish stoichiometry of functional particle in vivo","Focused on Pdx1 residues; PDX2 interface residues not mapped"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2009,"claim":"Dissociated PDX2 binding from Pdx1 oligomeric state, showing a hexameric Pdx1 mutant still binds and supports PDX2 glutaminase activity even though its own catalysis is impaired.","evidence":"Pdx1 Gly155Ala mutant analyzed by light scattering, circular dichroism, co-purification and enzyme assays","pmids":["19272411"],"confidence":"Medium","gaps":["Single point mutant used to infer general principle","In vitro only"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2010,"claim":"Parsed which Pdx1 C-terminal functions can be complemented in trans, explaining the cooperative cross-talk that PDX2 depends on for PLP output.","evidence":"Pdx1 C-terminal deletion series with enzymatic assays and mass-spectrometric detection of intermediate I320","pmids":["20837012"],"confidence":"Medium","gaps":["PDX2's own contribution to intermediate chemistry not addressed","Findings concern Pdx1 architecture more than PDX2 directly"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2012,"claim":"Provided the structural architecture of the working machine — random decoration of the dodecameric Pdx1 core by up to 12 PDX2 subunits, fiber formation, and a transient ammonia-transfer tunnel regulated during catalysis.","evidence":"Cryo-EM, X-ray crystallography of the complex, and site-directed mutagenesis of conserved methionines with activity assays","pmids":["22244765"],"confidence":"High","gaps":["Physiological relevance of fibers unknown","Dynamics of tunnel opening/closing not directly captured"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2012,"claim":"Confirmed the PDX2–Pdx1 interaction inside the parasite and established the physiological role of the complex in singlet-oxygen detoxification, not just PLP supply.","evidence":"In vivo pull-down from transgenic parasites, PLP quantification (12.5→36.6 µM), live-cell fluorescence, and dominant-negative PDX2/Pdx1 mutants conferring ¹O₂ sensitivity","pmids":["22242896"],"confidence":"High","gaps":["Whether PDX2 has any moonlighting role beyond glutaminase not tested","Direct genetic knockout of PDX2 not reported"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2013,"claim":"Validated the PDX2-containing complex as the in vivo target of a Pdx1 inhibitor, reinforcing its requirement for PLP biosynthesis in the parasite.","evidence":"Transgenic overexpression of PfPdx1 and PDX2 conferring protection against inhibitor 4PEHz in growth-inhibition assays","pmids":["23039077"],"confidence":"Medium","gaps":["Inhibitor acts on Pdx1; direct effect on PDX2 not distinguished","Overexpression rescue is indirect evidence"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]},{"year":2020,"claim":"Extended the assembly mechanism to a second Plasmodium species, showing polydisperse PDX2 converts to a defined 24-meric Pdx1–PDX2 particle upon mixing with dodecameric Pdx1.","evidence":"DLS, SAXS (ring-shaped Pdx1 dodecamer decorated with PDX2) and EM of the P. vivax ortholog","pmids":["32825141"],"confidence":"Medium","gaps":["Catalytic parameters of P. vivax PDX2 not reported","In vivo relevance in P. vivax not tested"],"attribution":[{"species":"Plasmodium vivax","strain":""}]},{"year":null,"claim":"How PDX2 activity is regulated in the parasite over the intraerythrocytic cycle and whether it is genetically essential remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No direct PDX2 gene-knockout phenotype in the corpus","Regulation and stoichiometry of the complex in vivo not established","PDX2 interface residues not systematically mapped"],"attribution":[{"species":"Plasmodium falciparum","strain":""}]}],"mechanism_profile":{"molecular_activity":[],"localization":[],"pathway":[],"complexes":["Pdx1–PDX2 heteromeric PLP synthase complex (dodecameric Pdx1 core decorated by up to 12 PDX2 glutaminase subunits; 24-meric particle in P. vivax)"],"partners":["PDX1 (PLP SYNTHASE CATALYTIC/SYNTHASE SUBUNIT)"],"other_free_text":["PDX2 is autoinhibited as a free enzyme (obstructed oxyanion hole) and activated only upon Pdx1 binding [#1].","L-glutamine binding increases PDX2 affinity for Pdx1 ~30-fold; activation is coupled to the Pdx1 αN-helix interface [#2].","Pdx1 self-assembly precedes PDX2 recruitment; PDX2 can bind hexameric Pdx1, but full PLP output needs a dodecameric scaffold [#3, #4].","Ammonia generated by PDX2 glutaminase is channeled through a transient tunnel to the Pdx1 active site [#6].","The PDX2-containing complex is required for singlet-oxygen (¹O₂) detoxification in intraerythrocytic P. falciparum [#7].","The complex is the in vivo target of the Pdx1 inhibitor 4PEHz [#8]."]}},"prefetch_data":{},"corpus_meta":[{"pmid":"15590634","id":"PMC_15590634","title":"Analysis of the vitamin B6 biosynthesis pathway in the human malaria parasite Plasmodium falciparum.","date":"2004","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/15590634","citation_count":69,"is_preprint":false,"source_species":["Pf"],"role":"primary","match_terms":["PDX2","PfPDX2"],"competing_organism_flag":false,"competing_organism_match":"","competing_organism_only":false},{"pmid":"16339145","id":"PMC_16339145","title":"Vitamin B6 biosynthesis by the malaria parasite Plasmodium falciparum: biochemical and structural insights.","date":"2005","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/16339145","citation_count":65,"is_preprint":false,"source_species":["Pf"],"role":"primary","match_terms":["PDX2"],"competing_organism_flag":false,"competing_organism_match":"","competing_organism_only":false},{"pmid":"22244765","id":"PMC_22244765","title":"Assembly of the eukaryotic PLP-synthase complex from Plasmodium and activation of the Pdx1 enzyme.","date":"2012","source":"Structure (London, England : 1993)","url":"https://pubmed.ncbi.nlm.nih.gov/22244765","citation_count":26,"is_preprint":false,"source_species":["Pf"],"role":"primary","match_terms":["PDX2"],"competing_organism_flag":false,"competing_organism_match":"","competing_organism_only":false},{"pmid":"22242896","id":"PMC_22242896","title":"The antioxidative effect of de novo generated vitamin B6 in Plasmodium falciparum validated by protein interference.","date":"2012","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/22242896","citation_count":25,"is_preprint":false,"source_species":["Pf"],"role":"primary","match_terms":["PfPDX2"],"competing_organism_flag":false,"competing_organism_match":"","competing_organism_only":false},{"pmid":"18350152","id":"PMC_18350152","title":"The assembly of the plasmodial PLP synthase complex follows a defined course.","date":"2008","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/18350152","citation_count":18,"is_preprint":false,"source_species":["Pf"],"role":"primary","match_terms":["PDX2"],"competing_organism_flag":false,"competing_organism_match":"","competing_organism_only":false},{"pmid":"17950752","id":"PMC_17950752","title":"Structural and thermodynamic insights into the assembly of the heteromeric pyridoxal phosphate synthase from Plasmodium falciparum.","date":"2007","source":"Journal of molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/17950752","citation_count":13,"is_preprint":false,"source_species":["Pf"],"role":"primary","match_terms":["PDX2"],"competing_organism_flag":false,"competing_organism_match":"","competing_organism_only":false},{"pmid":"23039077","id":"PMC_23039077","title":"Exploring inhibition of Pdx1, a component of the PLP synthase complex of the human malaria parasite Plasmodium falciparum.","date":"2013","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/23039077","citation_count":13,"is_preprint":false,"source_species":["Pf"],"role":"primary","match_terms":["PfPDX2"],"competing_organism_flag":false,"competing_organism_match":"","competing_organism_only":false},{"pmid":"20837012","id":"PMC_20837012","title":"Defining the structural requirements for ribose 5-phosphate-binding and intersubunit cross-talk of the malarial pyridoxal 5-phosphate synthase.","date":"2010","source":"FEBS letters","url":"https://pubmed.ncbi.nlm.nih.gov/20837012","citation_count":6,"is_preprint":false,"source_species":["Pf"],"role":"primary","match_terms":["PDX2"],"competing_organism_flag":false,"competing_organism_match":"","competing_organism_only":false},{"pmid":"19272411","id":"PMC_19272411","title":"Mobility of the conserved glycine 155 is required for formation of the active plasmodial Pdx1 dodecamer.","date":"2009","source":"Biochimica et biophysica acta","url":"https://pubmed.ncbi.nlm.nih.gov/19272411","citation_count":6,"is_preprint":false,"source_species":["Pf"],"role":"primary","match_terms":["PDX2"],"competing_organism_flag":false,"competing_organism_match":"","competing_organism_only":false},{"pmid":"32825141","id":"PMC_32825141","title":"Solution Structures and Dynamic Assembly of the 24-Meric Plasmodial Pdx1-Pdx2 Complex.","date":"2020","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/32825141","citation_count":5,"is_preprint":false,"source_species":["Pf"],"role":"primary","match_terms":["PDX2"],"competing_organism_flag":false,"competing_organism_match":"","competing_organism_only":false},{"pmid":"34327152","id":"PMC_34327152","title":"Structural Dynamics and Perspectives of Vitamin B6 Biosynthesis Enzymes in Plasmodium: Advances and Open Questions.","date":"2021","source":"Frontiers in cellular and infection microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/34327152","citation_count":3,"is_preprint":false,"source_species":["Pf"],"role":"primary","match_terms":["PDX2"],"competing_organism_flag":false,"competing_organism_match":"","competing_organism_only":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8373,"output_tokens":3196,"usd":0.036529,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9033,"output_tokens":4135,"usd":0.07427,"stage2_stop_reason":"end_turn"},"total_usd":0.110799,"stage1_batch_id":"msgbatch_01DisAaTypwmC8QwdMBuePHo","stage2_batch_id":"msgbatch_0184zxZTqxiEfdYYfPMfkqjb","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2004,\n      \"finding\": \"PDX2 (PfPdx2) acts as the glutaminase subunit of the P. falciparum PLP synthase complex, functioning together with PfPdx1 to catalyze glutaminase activity with a specific activity of 209 nmol min⁻¹ mg⁻¹ and a Km for glutamine of 1.3 mM; complementation of pyridoxine auxotroph yeast cells confirmed involvement of both proteins in de novo vitamin B6 biosynthesis.\",\n      \"method\": \"Recombinant protein biochemistry (glutaminase activity assay), yeast complementation assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro enzymatic assay with defined kinetic parameters plus functional complementation in vivo, replicated by subsequent studies\",\n      \"pmids\": [\"15590634\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"PDX2 functions as the glutaminase subunit of the heteromeric PLP synthase complex; complex formation with Pdx1 is required for catalytic activity of either subunit. The 1.62 Å crystal structure of PfPdx2 reveals an obstructed oxyanion hole indicating a resting state that is activated upon Pdx1-Pdx2 complex formation. A chimeric bi-enzyme of Pdx1 with bacterial YaaE (a Pdx2 ortholog) forms in vivo and in vitro but does not attain full catalytic activity, emphasizing species-specific structural requirements for productive interaction.\",\n      \"method\": \"X-ray crystallography (1.62 Å structure of PfPdx2), in vitro reconstitution, glutaminase activity assay, in vivo/in vitro chimera formation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus reconstitution and enzymatic assays, independently consistent with other studies\",\n      \"pmids\": [\"16339145\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Thermodynamic characterization by isothermal titration calorimetry showed that l-glutamine binding increases the affinity of PDX2 for Pdx1 approximately 30-fold and alters the thermodynamic signature of complex formation, indicating substrate-dependent tightening of the PDX2–Pdx1 interface. The complex interface is dominated by hydrophobic interactions (negative heat capacity). Swapping the αN-helix of PfPdx1 with the corresponding B. subtilis segment enabled cross-binding to B. subtilis Pdx2 and partially elicited glutaminase activity in BsPdx2, demonstrating that catalytic activation of PDX2 is mechanistically linked to formation of the Pdx1 αN-helix interface.\",\n      \"method\": \"Isothermal titration calorimetry, swap-mutant biochemistry, glutaminase activity assay, 3D homology modelling\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — quantitative thermodynamic assay plus mutagenesis with functional readout, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"17950752\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Mutational analysis of P. falciparum Pdx1 interface residues (R85, H88, E91 at the Pdx1:Pdx1 interface) showed that their mutation to alanine impedes both Pdx1 activity and PDX2 binding, establishing that Pdx1 self-assembly is a prerequisite for PDX2 recruitment. Mutation of active-site residues D26, K83, K151 blocked enzyme activity but did not prevent PDX2 binding or PDX2 glutaminase activity, demonstrating that PDX2 can bind a hexameric Pdx1 scaffold but full catalytic output requires a dodecameric assembly.\",\n      \"method\": \"Site-directed mutagenesis, biochemical activity assays, size-exclusion chromatography, co-purification\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic mutagenesis with multiple variants and enzymatic plus assembly readouts, single lab\",\n      \"pmids\": [\"18350152\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"A Glycine-155-to-Alanine mutation in Pdx1 produces a hexameric (not dodecameric) assembly in vitro with significant secondary-structure changes, yet this mutant retains the ability to bind PDX2 and support PDX2 glutaminase activity, demonstrating that PDX2 binding does not require the full dodecameric Pdx1 state, whereas Pdx1 catalytic activity does.\",\n      \"method\": \"Static and dynamic light scattering, circular dichroism, co-purification, enzyme activity assays\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal biophysical and biochemical methods in a single lab study\",\n      \"pmids\": [\"19272411\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"C-terminal deletion analysis of Pdx1 dissected four distinct functions contributed by the C-terminus: Pdx1 monomer assembly, ribose 5-phosphate binding, formation of reaction intermediate I320, and PLP synthesis. PLP synthesis is the only C-terminal function that can be supplied in trans, explaining the cooperative nature of the PDX2–Pdx1 complex and the cross-talk between subunits.\",\n      \"method\": \"C-terminal deletion mutagenesis, enzymatic activity assays, mass spectrometry (intermediate detection)\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — systematic deletion series with multiple functional readouts, single lab\",\n      \"pmids\": [\"20837012\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Cryo-EM analysis revealed that PDX2 glutaminase subunits associate randomly (up to 12 subunits) onto the dodecameric Pdx1 core, and P. falciparum PLP synthase organizes into fibers. Crystal structure of the complex shows that an α-helix adopts an open vs. closed conformation upon substrate binding; the pentose substrate is covalently attached via its C1 in a Schiff base with Lys84 of Pdx1. A transient ammonia transfer tunnel between PDX2 and Pdx1 active sites is regulated during catalysis. Mutagenesis defined critical conserved methionines required for complex function while also showing plasticity in tunnel construction across species.\",\n      \"method\": \"Electron microscopy (EM), X-ray crystallography, site-directed mutagenesis, enzymatic assays\",\n      \"journal\": \"Structure (London, England : 1993)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus EM plus mutagenesis with functional validation in a single study\",\n      \"pmids\": [\"22244765\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"In vivo pull-down experiments from transgenic P. falciparum parasites overexpressing both PfPdx1 and PDX2 confirmed a direct interaction between the two proteins inside the parasite. Overexpression of the complex elevated intracellular PLP levels from 12.5 µM (wild-type) to 36.6 µM and increased tolerance to singlet oxygen (¹O₂). Dominant-negative inactive mutants of either PfPdx1 or PDX2 rendered parasites susceptible to ¹O₂, establishing that de novo PLP synthesis via the PDX2-containing complex is required for singlet oxygen detoxification.\",\n      \"method\": \"In vivo pull-down (transgenic parasite overexpression), live-cell fluorescence microscopy, PLP quantification, transcription analysis, dominant-negative mutagenesis\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo interaction confirmed by pull-down, complemented by quantitative PLP measurement and dominant-negative functional validation in live parasites\",\n      \"pmids\": [\"22242896\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"PDX2 assembles with Pdx1 into a heteromeric PLP synthase complex in P. falciparum; transgenic parasite lines overexpressing both PfPdx1 and PDX2 showed protection against the Pdx1 inhibitor 4PEHz (IC50 shift from 16 µM for the complex to reduced sensitivity with extra copies), confirming that the PDX2-containing complex is the in vivo target of the inhibitor and is directly required for PLP biosynthesis in the parasite.\",\n      \"method\": \"Transgenic overexpression in P. falciparum, in vitro Pdx1 inhibition assay, parasite growth inhibition assay\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional in vivo validation via overexpression rescue, single lab\",\n      \"pmids\": [\"23039077\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"In P. vivax, PDX2 exists in monomeric and multiple oligomeric states in solution; upon mixing with dodecameric Pdx1, polydisperse PDX2 is converted to a monodispersed heteromeric 24-meric Pdx1–PDX2 (Pdx) complex, as determined by DLS, SAXS (showing a ring-shaped dodecameric Pdx1 decorated with PDX2), and electron microscopy.\",\n      \"method\": \"Dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), electron microscopy (EM)\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple complementary biophysical methods in a single study, single lab, P. vivax ortholog\",\n      \"pmids\": [\"32825141\"],\n      \"is_preprint\": false,\n      \"attribution\": [{\"species\": \"Plasmodium vivax\", \"strain\": \"\"}]\n    }\n  ],\n  \"current_model\": \"PDX2 (PF3D7_1116200) is the glutaminase subunit of the heteromeric PLP synthase complex in Plasmodium falciparum; it is activated only upon forming a complex with Pdx1 (obstruction of its oxyanion hole is relieved by Pdx1 binding), catalyzes the hydrolysis of glutamine to supply ammonia that is channeled through a transient tunnel to the Pdx1 active site for pyridoxal 5′-phosphate (PLP) synthesis, associates randomly (up to 12 subunits) onto the dodecameric Pdx1 scaffold, and its activity within the complex is essential for de novo PLP biosynthesis and for detoxification of singlet oxygen in the intraerythrocytic parasite.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PDX2 (PF3D7_1116200) is the glutaminase subunit of the heteromeric pyridoxal 5′-phosphate (PLP, vitamin B6) synthase complex of Plasmodium falciparum, functioning obligately together with Pdx1 in de novo vitamin B6 biosynthesis [#0]. As a free enzyme PDX2 is held in a resting state — its 1.62 Å crystal structure shows an obstructed oxyanion hole — and productive glutaminase activity is acquired only upon assembly with Pdx1, which relieves this obstruction [#1]. It hydrolyzes glutamine (Km ≈ 1.3 mM) to generate ammonia that is channeled through a transient tunnel to the Pdx1 active site, where the pentose substrate is bound as a Schiff base for PLP synthesis; within the assembled particle, PDX2 subunits decorate the dodecameric Pdx1 core (up to 12 copies), and the P. falciparum enzyme organizes into fibers [#6]. Substrate binding reinforces the interaction: L-glutamine increases PDX2 affinity for Pdx1 roughly 30-fold, and catalytic activation is mechanistically coupled to engagement of the Pdx1 αN-helix interface, while recruitment of PDX2 requires prior Pdx1 self-assembly [#2, #3]. In live parasites, PDX2 interacts directly with Pdx1, and the PDX2-containing complex is required for de novo PLP synthesis and for detoxification of singlet oxygen, with dominant-negative PDX2 rendering intraerythrocytic parasites susceptible to ¹O₂ [#7].\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Established that PDX2 is the glutaminase subunit of the parasite's PLP synthase and, with Pdx1, is genuinely required for de novo vitamin B6 biosynthesis rather than being a hypothetical annotation.\",\n      \"evidence\": \"Recombinant glutaminase activity assay (specific activity 209 nmol min⁻¹ mg⁻¹, Km ≈ 1.3 mM) plus complementation of a pyridoxine-auxotroph yeast strain\",\n      \"pmids\": [\"15590634\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve why PDX2 requires Pdx1 for activity\", \"No structural basis for the interaction\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Explained the obligate dependence of PDX2 on Pdx1: the free enzyme is autoinhibited and only complex formation switches it on.\",\n      \"evidence\": \"1.62 Å crystal structure of PfPdx2 revealing an obstructed oxyanion hole, with reconstitution/activity assays and a chimeric Pdx1–bacterial YaaE bi-enzyme showing species-specific interaction requirements\",\n      \"pmids\": [\"16339145\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure was of PDX2 alone, not the activated complex\", \"Mechanism of oxyanion-hole relief not visualized\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Quantified the coupling between substrate binding and complex assembly, showing glutamine tightens the PDX2–Pdx1 interface and that activation is linked to the Pdx1 αN-helix.\",\n      \"evidence\": \"Isothermal titration calorimetry (~30-fold affinity increase with L-glutamine), Pdx1 αN-helix swap mutants activating B. subtilis Pdx2, homology modelling\",\n      \"pmids\": [\"17950752\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Interface geometry inferred, not solved at high resolution\", \"Kinetics of tunnel opening not measured\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Defined the assembly hierarchy: Pdx1 self-assembly is a prerequisite for PDX2 recruitment, and PDX2 can bind and function on a hexameric scaffold whereas full PLP output requires a dodecamer.\",\n      \"evidence\": \"Site-directed mutagenesis of Pdx1 interface (R85, H88, E91) and active-site (D26, K83, K151) residues with size-exclusion chromatography, co-purification and activity assays\",\n      \"pmids\": [\"18350152\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish stoichiometry of functional particle in vivo\", \"Focused on Pdx1 residues; PDX2 interface residues not mapped\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Dissociated PDX2 binding from Pdx1 oligomeric state, showing a hexameric Pdx1 mutant still binds and supports PDX2 glutaminase activity even though its own catalysis is impaired.\",\n      \"evidence\": \"Pdx1 Gly155Ala mutant analyzed by light scattering, circular dichroism, co-purification and enzyme assays\",\n      \"pmids\": [\"19272411\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single point mutant used to infer general principle\", \"In vitro only\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Parsed which Pdx1 C-terminal functions can be complemented in trans, explaining the cooperative cross-talk that PDX2 depends on for PLP output.\",\n      \"evidence\": \"Pdx1 C-terminal deletion series with enzymatic assays and mass-spectrometric detection of intermediate I320\",\n      \"pmids\": [\"20837012\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"PDX2's own contribution to intermediate chemistry not addressed\", \"Findings concern Pdx1 architecture more than PDX2 directly\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Provided the structural architecture of the working machine — random decoration of the dodecameric Pdx1 core by up to 12 PDX2 subunits, fiber formation, and a transient ammonia-transfer tunnel regulated during catalysis.\",\n      \"evidence\": \"Cryo-EM, X-ray crystallography of the complex, and site-directed mutagenesis of conserved methionines with activity assays\",\n      \"pmids\": [\"22244765\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological relevance of fibers unknown\", \"Dynamics of tunnel opening/closing not directly captured\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Confirmed the PDX2–Pdx1 interaction inside the parasite and established the physiological role of the complex in singlet-oxygen detoxification, not just PLP supply.\",\n      \"evidence\": \"In vivo pull-down from transgenic parasites, PLP quantification (12.5→36.6 µM), live-cell fluorescence, and dominant-negative PDX2/Pdx1 mutants conferring ¹O₂ sensitivity\",\n      \"pmids\": [\"22242896\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether PDX2 has any moonlighting role beyond glutaminase not tested\", \"Direct genetic knockout of PDX2 not reported\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Validated the PDX2-containing complex as the in vivo target of a Pdx1 inhibitor, reinforcing its requirement for PLP biosynthesis in the parasite.\",\n      \"evidence\": \"Transgenic overexpression of PfPdx1 and PDX2 conferring protection against inhibitor 4PEHz in growth-inhibition assays\",\n      \"pmids\": [\"23039077\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Inhibitor acts on Pdx1; direct effect on PDX2 not distinguished\", \"Overexpression rescue is indirect evidence\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Extended the assembly mechanism to a second Plasmodium species, showing polydisperse PDX2 converts to a defined 24-meric Pdx1–PDX2 particle upon mixing with dodecameric Pdx1.\",\n      \"evidence\": \"DLS, SAXS (ring-shaped Pdx1 dodecamer decorated with PDX2) and EM of the P. vivax ortholog\",\n      \"pmids\": [\"32825141\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Catalytic parameters of P. vivax PDX2 not reported\", \"In vivo relevance in P. vivax not tested\"],\n      \"attribution\": [{\"species\": \"Plasmodium vivax\", \"strain\": \"\"}]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How PDX2 activity is regulated in the parasite over the intraerythrocytic cycle and whether it is genetically essential remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No direct PDX2 gene-knockout phenotype in the corpus\", \"Regulation and stoichiometry of the complex in vivo not established\", \"PDX2 interface residues not systematically mapped\"],\n      \"attribution\": [{\"species\": \"Plasmodium falciparum\", \"strain\": \"\"}]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0004359\", \"supporting_discovery_ids\": [0, 1, 2]},\n      {\"term_id\": \"GO:0036381\", \"supporting_discovery_ids\": [0, 6, 7]}\n    ],\n    \"localization\": [],\n    \"pathway\": [\n      {\"term_id\": \"GO:0042823\", \"supporting_discovery_ids\": [0, 7, 8]},\n      {\"term_id\": \"GO:0042819\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"complexes\": [\"Pdx1–PDX2 heteromeric PLP synthase complex (dodecameric Pdx1 core decorated by up to 12 PDX2 glutaminase subunits; 24-meric particle in P. vivax)\"],\n    \"partners\": [\"Pdx1 (PLP synthase catalytic/synthase subunit)\"],\n    \"other_free_text\": [\n      \"PDX2 is autoinhibited as a free enzyme (obstructed oxyanion hole) and activated only upon Pdx1 binding [#1].\",\n      \"L-glutamine binding increases PDX2 affinity for Pdx1 ~30-fold; activation is coupled to the Pdx1 αN-helix interface [#2].\",\n      \"Pdx1 self-assembly precedes PDX2 recruitment; PDX2 can bind hexameric Pdx1, but full PLP output needs a dodecameric scaffold [#3, #4].\",\n      \"Ammonia generated by PDX2 glutaminase is channeled through a transient tunnel to the Pdx1 active site [#6].\",\n      \"The PDX2-containing complex is required for singlet-oxygen (¹O₂) detoxification in intraerythrocytic P. falciparum [#7].\",\n      \"The complex is the in vivo target of the Pdx1 inhibitor 4PEHz [#8].\"\n    ]\n  }\n}\n```","audit_flag":null,"strains":[{"strain":"Pf3D7","gene_id":"PF3D7_1116200","name":null,"product":"pyridoxine biosynthesis protein PDX2","uniprot_acc":null,"chromosome":null,"start_pos":null},{"strain":"PbANKA","gene_id":"PBANKA_0931800","name":null,"product":"pyridoxine biosynthesis protein PDX2, putative","uniprot_acc":null,"chromosome":null,"start_pos":null},{"strain":"PkH","gene_id":"PKNH_0913900","name":null,"product":"pyridoxine biosynthesis protein PDX2, putative","uniprot_acc":null,"chromosome":null,"start_pos":null}],"gene_symbol":"PDX2","evaluation":null}