FS

PF3D7_1014000 (FS) encodes a GDP-L-fucose synthase that catalyzes the de novo conversion of GDP-mannose to GDP-fucose in Plasmodium falciparum, and recombinant enzyme is catalytically active in vitro, establishing that this branch of the de novo GDP-fucose biosynthetic pathway is functional in the parasite (PMID:23615908). Epitope-tagged FS localizes to the cytoplasm of intraerythrocytic-stage parasites (PMID:23615908). Genetic disruption of the upstream pathway enzyme PfGMD abolishes UEA-I lectin labelling yet does not eliminate the intracellular GDP-fucose pool, and the de novo pathway is dispensable for asexual blood-stage growth and invasion, indicating an alternative route can sustain GDP-fucose independently of the GMD–FS de novo pathway (PMID:27849032). Beyond its in vitro activity, cytoplasmic localization, and its non-essential role in the de novo pathway, no further mechanistic detail for FS — including structural determinants, catalytic-residue mutagenesis, or its physiological fucosylation targets — has been characterized in the available corpus.

1. 2013 Medium

   Whether P. falciparum possesses a functional de novo GDP-fucose biosynthetic route was unknown; demonstrating that FS is catalytically active established the parasite encodes an operative GDP-mannose-to-GDP-fucose conversion step.

   Evidence In vitro enzymatic assay of recombinant FS with LC-MS/MS quantification of sugar-nucleotide pools in intraerythrocytic stages

   PMID:23615908

   Open questions at the time

   • No mutagenesis of catalytic residues to confirm the assigned activity
   • No structural model of the enzyme
   • In vitro activity not linked to an in vivo fucosylation product
2. 2013 Medium

   The subcellular compartment in which FS acts was undefined; localizing epitope-tagged FS placed the enzyme in the cytoplasm during the intraerythrocytic cycle, defining where de novo GDP-fucose synthesis occurs.

   Evidence Fluorescence microscopy of epitope-tagged FS in transgenic 3D7 parasites

   PMID:23615908

   Open questions at the time

   • Tagging may not reflect endogenous localization
   • No functional consequence tied to cytoplasmic localization
   • Localization across other life-cycle stages not assessed
3. 2016 Medium

   Whether the GMD–FS de novo pathway is the sole and essential source of parasite GDP-fucose was untested; disrupting the upstream enzyme showed the de novo route is dispensable for blood-stage growth and that an alternative mechanism maintains the GDP-fucose pool.

   Evidence PfGMD null mutant with LC-MS/MS detection of GDP-fucose, UEA-I lectin labelling, and growth/invasion assays

   PMID:27849032

   Open questions at the time

   • The alternative GDP-fucose source (e.g. a salvage route) was not identified
   • FS itself was not directly knocked out; the finding rests on disruption of the upstream partner PfGMD
   • Role of the pathway in non-asexual stages not tested

• The physiological targets of GDP-fucose-dependent fucosylation and the identity of the alternative pathway that sustains the GDP-fucose pool when de novo synthesis is disrupted remain unresolved.
• No fucosylated glycoconjugate or protein target of FS-derived GDP-fucose identified
• Alternative (salvage) route maintaining GDP-fucose uncharacterized
• No direct genetic knockout of FS reported