FD

PF3D7_1318100 (FD) is a [2Fe-2S] plant-type ferredoxin of P. falciparum that functions as the central soluble electron carrier of the apicoplast, where it is delivered by an N-terminal bipartite transit peptide (PMID:11056177). Within the apicoplast, FD operates in an NADPH→FNR→FD redox relay: FNR selectively reduces FD, and FD in turn donates electrons to downstream iron-sulfur enzymes, most directly to the terminal MEP/DOXP isoprenoid pathway enzyme LytB/IspH, whose catalytic activity FD reconstitutes together with FNR and NADPH in vitro (PMID:16289098, PMID:17251200); heterologous reconstitution in E. coli confirms FD can substitute for a bacterial flavodoxin to drive the terminal IspH reaction and sustain MEP-dependent growth (PMID:40967433). FD binds FNR through a Plasmodium-specific electrostatic interface involving acidic surface patches (notably Glu29, and residues Asp65/Glu66) and C-terminal aromatic residues at positions 96/97 (PMID:17938142, PMID:32470136). Genetic deletion establishes that FD is essential for blood-stage survival but dispensable for apicoplast maintenance, and that it receives its FeS cofactor redundantly from SufA and NfuApi (PMID:35164549). The artemisinin-resistance-associated D97Y substitution tightens FD–FNR binding and thereby sequesters FNR and suppresses downstream electron flow, but FD itself is not a direct molecular target of artemisinin, and editing the related D193Y variant confers no change in artemisinin susceptibility or fitness (PMID:34415329, PMID:35204156, PMID:34279219).

1. 2000 Medium

   Establishing that the parasite encodes a bona fide apicoplast-targeted ferredoxin answered whether Plasmodium possesses a plant-type FeS electron carrier and where it acts.

   Evidence Molecular cloning, bipartite transit peptide analysis, and recombinant expression of active [2Fe-2S] protein in P. falciparum

   PMID:11056177

   Open questions at the time

   • Downstream electron acceptors not yet identified
   • In vivo localization inferred from transit peptide rather than directly imaged
2. 2005 High

   Identifying LytB as an FD electron acceptor answered what physiological process FD powers, linking the ferredoxin to the essential MEP isoprenoid pathway.

   Evidence In vitro reconstitution of NADPH/FNR/FD-dependent LytB activity and stable FD–LytB complex formation in P. falciparum

   PMID:16289098

   Open questions at the time

   • Whether LytB is the only downstream acceptor not established
   • Complex not resolved structurally
3. 2007 High

   Structural and redox characterization answered the directionality of electron flow, showing FD receives rather than donates electrons to a photosystem, consistent with an NADPH→FNR→FD cascade.

   Evidence X-ray crystallography, redox potential measurement, and reconstituted NADPH-dependent electron transfer with P. falciparum FNR and FD

   PMID:17251200

   Open questions at the time

   • Structure of the FD–FNR complex not solved
   • In-cell redox state not measured
4. 2007 High

   Mapping the FD surface answered which residues mediate FNR docking, defining the electrostatic recognition interface.

   Evidence NMR chemical shift perturbation with site-directed mutagenesis and electron transfer assays on P. falciparum FD

   PMID:17938142

   Open questions at the time

   • Reciprocal contacts on FNR not yet defined here
   • Contribution of non-acidic contacts unresolved
5. 2009 Medium

   Demonstrating FD-stimulated single-electron quinone/nitro reduction answered whether the FD–FNR system contributes to reductive activation of redox-cycling compounds.

   Evidence In vitro kinetics with purified recombinant P. falciparum FNR and FD

   PMID:19523113

   Open questions at the time

   • Physiological relevance in the parasite not established
   • Endogenous substrates of this activity unknown
6. 2018 Medium

   Identifying basic FNR surface residues answered why FD–FNR recognition is Plasmodium-specific, defining the complementary side of the interaction interface.

   Evidence Site-directed mutagenesis of P. falciparum FNR with electron transfer kinetics and thermodynamic binding measurements against FD

   PMID:29688515

   Open questions at the time

   • Structural model of the assembled complex still absent
   • Whether specificity extends to other FD partners untested
7. 2020 High

   Defining the role of FD C-terminal aromatic residues answered how binding affinity is tuned and provided the first mechanistic link to the artemisinin-resistance D97Y allele.

   Evidence Chimeric substitution and site-directed mutagenesis with kinetics, fluorescence binding, and isothermal titration calorimetry on P. falciparum FD

   PMID:32470136

   Open questions at the time

   • Consequences of tighter binding for parasite physiology not tested here
   • In vivo relevance of D97Y not yet addressed
8. 2021 High

   Characterizing D97Y biochemistry answered how the resistance-associated mutation alters function, showing tighter FNR binding sequesters FNR and suppresses downstream electron flow.

   Evidence Site-directed mutagenesis with fluorescence titration, kinetics, diaphorase, ascorbate radical scavenging, and heterologous electron transfer assays on P. falciparum FD

   PMID:34415329

   Open questions at the time

   • Whether sequestration occurs at physiological FNR/FD ratios in vivo untested
   • Link to a resistance mechanism in parasites not demonstrated
9. 2021 Medium

   Editing the FD D193Y variant into parasites answered whether this field-associated allele contributes to artemisinin resistance, and it does not.

   Evidence Point-mutation gene editing with ring-stage survival and fitness assays in P. falciparum

   PMID:34279219

   Open questions at the time

   • Negative result for a single background allele
   • Does not address D97Y in vivo
10. 2022 High

    Genetic deletion answered whether FD is essential and how its cofactor is supplied, showing FD is required for blood-stage survival independent of apicoplast maintenance and receives FeS redundantly from SufA and NfuApi.

    Evidence CRISPR/conditional deletion in the PfMev bypass line with viability, apicoplast morphology, and synthetic-lethality epistasis in P. falciparum

    PMID:35164549

    Open questions at the time

    • Precise essential downstream process not pinpointed genetically
    • Direct FeS handoff mechanism from SufA/NfuApi to FD not resolved
11. 2022 Medium

    Testing dihydroartemisinin against the FD–FNR system answered whether FD is a direct drug target, showing artemisinin acts on FNR–NADPH affinity rather than on FD-directed electron transfer.

    Evidence In vitro reconstituted NADPH/FNR/FD electron transfer and diaphorase assays with DHA plus in silico docking in P. falciparum

    PMID:35204156

    Open questions at the time

    • In-cell drug interaction not tested
    • Negative result for direct FD targeting
12. 2025 High

    Heterologous reconstitution answered whether FD is functionally sufficient to drive the terminal IspH/MEP reaction, confirming it can replace a bacterial flavodoxin and that function-abolishing FD mutations deplete MEP metabolites.

    Evidence Functional complementation in an E. coli double mutant with LC-MS metabolite analysis and conditional T. gondii Fd KO complementation, using P. falciparum FD, FNR, and IspH

    PMID:40967433

    Open questions at the time

    • Native apicoplast partner network beyond IspH not fully mapped
    • Quantitative flux control by FD in vivo not established

• It remains unresolved how FeS clusters are physically handed from SufA/NfuApi to FD and whether the tighter D97Y FD–FNR interaction has any measurable consequence for parasite physiology or drug response in vivo.
• No structure of the FD–FNR complex
• Cofactor delivery mechanism uncharacterized at the molecular level
• In vivo phenotype of D97Y not established