GAPDH2

GAPDH2 is an apicoplast-localized enzyme of Toxoplasma gondii that supplies reducing power to organellar metabolism and is required for tachyzoite proliferation (PMID:36449552). Endogenous tagging places the protein within the apicoplast (PMID:36449552), and conditional depletion arrests parasite growth in the lytic cycle (PMID:36449552). Functionally, GAPDH2 provides NADPH to apicoplast pathways: its loss reduces isoprenoid precursor synthesis through the MEP pathway and diminishes de novo fatty acid synthesis in the organelle (PMID:36449552). Beyond this metabolic role, no further mechanistic detail has been characterized in the available corpus.

1. 2022 High

   Establishing where GAPDH2 acts was the first step; endogenous tagging localized it to the apicoplast, distinguishing it from cytosolic glycolytic enzymes and framing a role in organellar metabolism.

   Evidence Endogenous tagging and fluorescence localization in T. gondii

   PMID:36449552

   Open questions at the time

   • Sub-organellar localization within the apicoplast not resolved
   • Import/targeting mechanism not defined
2. 2022 High

   Whether GAPDH2 is dispensable was tested by conditional knockdown, which arrested tachyzoite growth and established the enzyme as essential to the lytic cycle.

   Evidence Conditional depletion with growth-arrest phenotype readout in T. gondii

   PMID:36449552

   Open questions at the time

   • Whether growth arrest reflects a single metabolic deficit or pleiotropic effects not dissected
   • Requirement in other life-cycle stages (bradyzoite, in vivo) not addressed
3. 2022 High

   The metabolic consequence of depletion was measured to define GAPDH2's biochemical output, showing it supplies NADPH that feeds the MEP isoprenoid pathway and de novo fatty acid synthesis in the apicoplast.

   Evidence Conditional depletion combined with quantification of MEP pathway intermediates and fatty acid synthesis assays in T. gondii

   PMID:36449552

   Open questions at the time

   • Direct enzymatic characterization (NADPH-dependent kinetics) not reported
   • Contribution to the ferredoxin redox system not directly measured
   • Whether NADPH supply is the sole essential function unresolved
4. 2023 Low

   Whether GAPDH2 activity is post-translationally regulated was probed by identifying it as a candidate interactor/substrate of the apicoplast deacetylase TgKDAC4, raising the possibility of acetylation-based control.

   Evidence Immunoprecipitation followed by mass spectrometry identifying TgGAPDH2 with acetylation sites in T. gondii

   PMID:37375060

   Open questions at the time

   • Single Co-IP/MS identification with no functional validation of the interaction or acetylation
   • Effect of acetylation on GAPDH2 enzymatic activity not tested
   • No reciprocal validation of the TgKDAC4 interaction

• How GAPDH2 is enzymatically regulated and whether its acetylation state modulates apicoplast NADPH supply remain open.
• No functional link established between TgKDAC4-mediated deacetylation and GAPDH2 activity
• No structural or kinetic model of the enzyme
• Role across life-cycle stages and in vivo not characterized