ISPG

ISPG (PF3D7_1022800) is the Plasmodium 4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase, a [4Fe-4S] cluster-dependent enzyme that catalyzes the penultimate step of the non-mevalonate (MEP) isoprenoid biosynthesis pathway, converting 2-C-methyl-D-erythritol 2,4-cyclodiphosphate to 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate, as established for the E. coli ortholog through in vitro reconstitution, metal analysis, and cysteine mutagenesis (PMID:16268586). In Plasmodium the enzyme localizes to the apicoplast, where FeS-cluster–dependent activity has been reconstituted for the P. vivax ortholog (PMID:28007455), and it receives its [4Fe-4S] cofactor from the apicoplast SUF biogenesis pathway, accepting clusters transferred from the scaffold protein SufA in vitro (PMID:30721667). Conditional genetic deletion in P. falciparum showed that ISPG is essential for blood-stage survival yet dispensable for apicoplast organelle maintenance, and that it obtains its cofactor redundantly from either SufA or NfuApi, since only their combined loss is lethal (PMID:35164549). The Plasmodium falciparum enzyme carries a large parasite-specific insert of ~170–320 residues relative to eubacterial orthologs (PMID:16268586), whose functional significance has not been characterized in the available corpus.

1. 2005 Medium

   Establishing the catalytic identity of the IspG family answered what reaction and cofactor requirements define this enzyme, providing the mechanistic template later applied to the Plasmodium protein.

   Evidence Anaerobic purification, radiochemical and 13C NMR assays, cysteine mutagenesis and metal analysis on the E. coli ortholog, with sequence comparison to P. falciparum

   PMID:16268586

   Open questions at the time

   • Catalytic activity and cofactor requirements demonstrated on the E. coli ortholog, not on the Plasmodium enzyme
   • Function of the ~170–320 residue Plasmodium-specific insert not addressed
   • Physiological electron donor in Plasmodium not tested
2. 2016 Low

   Localizing the Plasmodium ortholog established where the enzyme acts and confirmed its dependence on [4Fe-4S] clusters, connecting it to apicoplast metabolism.

   Evidence Immune-localization on P. vivax-infected clinical blood smears plus in vitro FeS reconstitution and in silico docking

   PMID:28007455

   Open questions at the time

   • Apicoplast localization by immunostaining only, no orthogonal tag or fractionation
   • FeS requirement supported by docking and reconstitution but not by mutagenesis or functional enzyme assay
3. 2019 Low

   Demonstrating cluster transfer from SufA to apo-ISPG answered how the enzyme acquires its cofactor within the apicoplast, placing it downstream of the SUF biogenesis machinery.

   Evidence In vitro Fe-S cluster transfer assay using P. vivax clinical isolate proteins

   PMID:30721667

   Open questions at the time

   • Single in vitro transfer assay without structural or mutagenesis corroboration
   • Does not establish in vivo dependence or exclusivity of SufA as donor
4. 2022 Medium

   Conditional deletion resolved whether ISPG is essential and how its cofactor delivery is organized, showing it is required for blood-stage survival independent of apicoplast maintenance and served redundantly by two carrier proteins.

   Evidence Conditional genetic deletion in the PfMev apicoplast bypass line with individual and double SufA/NfuApi deletions in P. falciparum

   PMID:35164549

   Open questions at the time

   • In vivo enzymatic output and metabolite flux not directly measured
   • Structural basis of redundant cofactor loading by SufA versus NfuApi not defined
   • Role of the Fd/FNR redox system in supplying reducing power not tested here

• The physiological electron donor for Plasmodium ISPG and the function of its parasite-specific insert remain unresolved.
• No direct demonstration of the in vivo reductant (Fd/FNR) driving Plasmodium ISPG catalysis
• Functional and structural role of the ~170–320 residue insert uncharacterized
• No structure of the Plasmodium enzyme or its cofactor-loaded state