PYKI

PYK1 is the cytosolic pyruvate kinase of Toxoplasma gondii and the central node at which multiple carbon sources converge to supply pyruvate for parasite metabolism (PMID:31186321). Structurally, it is a tetramer whose catalytic cycle couples B-domain open/closed transitions to changes in the buried surface area between adjacent monomers; uniquely among pyruvate kinases, its fully closed conformation is inactive without bound substrate, and it is allosterically activated by glucose-6-phosphate through an ordered apicomplexan-specific insertion in the allosteric domain that accommodates the mono-phosphorylated sugar (PMID:20856875). Conditional depletion of PYK1 globally reshapes carbon metabolism, causing amylopectin accumulation, reduced cellular ATP, and severe growth impairment, establishing PYK1 as essential for pyruvate homeostasis; growth is partially rescued by lactate or alanine, with lactate rescue requiring lactate dehydrogenase to regenerate pyruvate (PMID:31186321). PYK1 acts cooperatively with the apicoplast-localized paralog PYK2: loss of PYK2 alone is tolerated, but combined loss of PYK1 and PYK2 accentuates apicoplast loss and arrests growth, indicating that cytosolic and apicoplast pyruvate pools are functionally linked (PMID:31186321).

1. 2010 High

   Establishing how a pyruvate kinase in an apicomplexan parasite is regulated required resolving its catalytic conformations and its unusual allosteric activator; the structure revealed a domain-coupled, glucose-6-phosphate-responsive enzyme distinct from canonical PKs.

   Evidence X-ray crystallography of TgPYK1 in open and closed B-domain states with enzymatic activity and allosteric activator assays

   PMID:20856875

   Open questions at the time

   • Structure does not address the in vivo metabolic role or essentiality of PYK1
   • The physiological trigger driving glucose-6-phosphate activation in the intracellular parasite was not defined
2. 2019 High

   Whether PYK1 is metabolically essential and how it integrates carbon flux was unknown; conditional depletion showed it is the primary pyruvate source and required for pyruvate homeostasis, ATP levels, and growth.

   Evidence Conditional knockdown with metabolomics, ATP measurement, amylopectin staining, and lactate/alanine rescue coupled to lactate dehydrogenase inhibition

   PMID:31186321

   Open questions at the time

   • Does not resolve how flux is partitioned among glucose, lactate, and alanine inputs in vivo
   • The regulatory link between glucose-6-phosphate activation and the observed metabolic phenotype was not directly tested
3. 2019 High

   The functional relationship between cytosolic PYK1 and the apicoplast paralog PYK2 was unclear; genetic combination showed the two cooperate to maintain apicoplast integrity.

   Evidence Genetic deletion of PYK2 with and without conditional PYK1 depletion, assayed for apicoplast integrity and growth

   PMID:31186321

   Open questions at the time

   • The mechanism by which pyruvate pools are shared or shuttled between cytosol and apicoplast is not defined
   • Why PYK2 loss alone is dispensable but becomes critical upon PYK1 depletion is unexplained

• How glucose-6-phosphate allosteric activation is physiologically deployed to tune PYK1 flux during the intracellular lytic cycle, and the molecular basis of cytosol-apicoplast pyruvate cooperation, remain open.
• No study directly links the structural allosteric mechanism to in vivo metabolic regulation
• No transporter or shuttle mediating cytosol-to-apicoplast pyruvate supply has been identified