IspH

LytB (PF3D7_0104400) catalyzes the terminal step of the apicoplast MEP isoprenoid biosynthesis pathway in Plasmodium falciparum, functioning as an Fe/S enzyme that depends on a ferredoxin/ferredoxin-NADP+ reductase (FNR) redox system as its physiological electron donor (PMID:16289098, PMID:40967433). In vitro, recombinant LytB is catalytically active only when supplied with an NADPH-dependent ferredoxin/FNR electron transfer system, and it forms a stable complex with ferredoxin, establishing this redox couple as its obligate electron source (PMID:16289098). Heterologous genetic complementation confirms this in situ: LytB together with P. falciparum plant-type ferredoxin and FNR restores growth of an E. coli flavodoxin/ispH double-mutant and reconstitutes the final MEP pathway reaction, verified by metabolite analysis (PMID:40967433). Structurally, LytB adopts a cloverleaf fold with an Fe/S-containing active site, with the [2Fe-2S] ferredoxin donor modeled at its binding site (PMID:24188825). Loss of LytB expression impairs intraerythrocytic parasite growth, indicating the enzyme is required for blood-stage proliferation (PMID:17638523).

1. 2005 High

   Establishing the physiological electron donor for LytB answered how this Fe/S enzyme is powered in the apicoplast, distinguishing it from donor systems used by other pathway enzymes.

   Evidence In vitro reconstitution of LytB catalytic activity with an NADPH/ferredoxin/FNR system plus demonstration of a stable LytB-ferredoxin complex by co-purification

   PMID:16289098

   Open questions at the time

   • Interaction demonstrated in vitro; stoichiometry and affinity of the LytB-ferredoxin complex not quantified
   • Catalytic turnover parameters and substrate (HMBPP) product ratio (IPP:DMAPP) not defined in this work
2. 2007 Medium

   Testing whether LytB expression is required for growth answered whether the enzyme is a viable target in blood-stage parasites.

   Evidence Hammerhead ribozyme cleavage of ispH/lytB mRNA in vitro coupled to a parasite growth-inhibition assay with transcript-level verification

   PMID:17638523

   Open questions at the time

   • Only partial (87%) growth inhibition at high ribozyme concentration; no clean genetic knockout
   • Off-target effects of the ribozyme not fully excluded
   • Does not distinguish requirement at specific developmental stages
3. 2013 High

   Determining the LytB structure answered how the active site is organized and where its ferredoxin donor engages, providing a template for the catalytic mechanism.

   Evidence X-ray crystallography of P. falciparum LytB with computational docking of the [2Fe-2S] ferredoxin at the binding site

   PMID:24188825

   Open questions at the time

   • Ferredoxin binding modeled by docking, not experimentally co-crystallized
   • No substrate- or product-bound structure to define catalytic geometry
4. 2025 High

   Reconstituting LytB with its ferredoxin/FNR partners in a heterologous mutant answered whether this redox system is sufficient to drive the terminal MEP step in vivo, validating the electron-donor model beyond the test tube.

   Evidence Genetic complementation of an E. coli flavodoxin/ispH double-mutant with P. falciparum LytB, ptFd and ptFNR; growth rescue and LC-MS metabolite analysis

   PMID:40967433

   Open questions at the time

   • Reconstitution performed in E. coli, not in the native apicoplast context
   • Does not address regulation or assembly of the redox system in the parasite

• How the [Fe-S] cluster of LytB is assembled and matured, and how the enzyme is regulated across the parasite life cycle, remains unresolved.
• No characterization of LytB Fe-S cluster biogenesis machinery in the corpus
• No stage-specific expression or regulatory data
• No clean genetic knockout confirming essentiality