FabZ

FabZ (PF3D7_1323000) is the beta-hydroxyacyl-ACP dehydratase of the P. falciparum apicoplast-localized type II fatty acid synthesis (FAS-II) pathway, catalyzing the third step of the elongation cycle—dehydration of beta-hydroxyacyl-ACP to trans-2-acyl-ACP (PMID:9770490, PMID:12930838). The recombinant 17-kDa enzyme was reconstituted in vitro with defined kinetic parameters and was shown to occupy its expected position in the elongation cycle when assembled with FabB/F, FabG, and FabI into the complete four-enzyme module, where its pharmacological inhibition causes accumulation of upstream acyl-ACP intermediates (PMID:12930838, PMID:17485508). Structurally, FabZ assembles as a catalytically active hexamer (a trimer of 'hot dog'-fold dimers) whose L-shaped hydrophobic active-site tunnels are formed at dimer interfaces; catalysis proceeds by an elimination mechanism involving His133 and Glu147' from the opposing subunit, with Phe169 gating the tunnel to accommodate substrates of differing acyl-chain length (PMID:15930004, PMID:21843645). The enzyme undergoes a pH-dependent switch between the active hexamer and an inactive dimer, in which cis-trans peptide flips displace the catalytic histidine and distort the active site (PMID:16643907). Genetically, FabZ is dispensable in blood and mosquito stages but essential for liver-stage development into exo-erythrocytic merozoites, defining FAS-II as critical specifically at the liver-to-blood transition (PMID:19068099). FabZ is competitively inhibited at its active site by diverse natural and synthetic compounds, and multiple inhibitor-bound crystal structures show direct contacts with the catalytic residues, making it a structurally well-defined antimalarial target (PMID:12930838, PMID:16722653, PMID:21843645).

1. 1998 Medium

   Establishing where fatty acid biosynthesis occurs answered whether the apicoplast harbors a FAS pathway, placing FabZ in an organellar compartment.

   Evidence N-terminal presequence analysis and ACP immunolocalization as a pathway proxy in P. falciparum, with GFP targeting in T. gondii

   PMID:9770490

   Open questions at the time

   • Direct immunolocalization of FabZ protein itself was not performed
   • Targeting was inferred from presequence and an ACP proxy rather than experimental FabZ import
2. 2003 High

   Recombinant expression and enzymatic assay answered what reaction FabZ catalyzes and confirmed it as the dehydratase of the FAS elongation cycle, while identifying competitive inhibitors active in culture.

   Evidence Recombinant overexpression, in vitro dehydratase kinetics, competitive inhibition by NAS-91/NAS-21, and culture and cell-free FAS growth-inhibition assays in P. falciparum

   PMID:12930838

   Open questions at the time

   • Kinetics measured on CoA surrogate substrates rather than the physiological acyl-ACP
   • Catalytic residues not yet defined by structure or mutagenesis
3. 2005 High

   Solving the crystal structure answered how FabZ is organized and how it catalyzes dehydration, defining the active hexamer, the hot-dog fold, and the catalytic machinery.

   Evidence SAD-phased 2.1 A crystal structure with 3-decynoyl-N-acetylcysteamine covalent complex and active-site analysis in P. falciparum

   PMID:14684903 PMID:15930004

   Open questions at the time

   • Catalytic roles of His133 and Glu147' inferred structurally, not tested by mutagenesis
   • No structure of a physiological acyl-ACP substrate complex
4. 2006 High

   Discovery of an inactive dimeric form answered how FabZ activity can be regulated, revealing a pH-dependent oligomeric switch coupled to peptide flips at the active site.

   Evidence 2.4 A crystal structure of the dimer plus dynamic light scattering and size-exclusion chromatography of oligomeric state in P. falciparum

   PMID:16643907

   Open questions at the time

   • Physiological relevance and trigger of the pH-dependent switch in vivo unknown
   • Whether oligomeric switching regulates FAS flux in the parasite untested
5. 2007 High

   Reconstituting the four-enzyme elongation module answered whether FabZ operates in a functional pathway context and pinpointed its step by inhibitor-induced intermediate accumulation.

   Evidence In vitro reconstitution with FabB/F, FabG, FabI and MALDI-TOF MS of acyl-ACP intermediates under FabZ inhibition in P. falciparum

   PMID:17485508

   Open questions at the time

   • Physiological chain-length preference across the cycle not fully mapped
   • Regulation of module assembly in vivo not addressed
6. 2008 High

   Genetic knockout answered when FabZ is required during the life cycle, showing FAS-II and FabZ are essential only at the liver-to-blood transition.

   Evidence Targeted gene deletion in P. yoelii with life-cycle stage phenotyping

   PMID:19068099

   Open questions at the time

   • Shown in P. yoelii; direct P. falciparum FabZ knockout not reported here
   • Molecular basis of the liver-stage-specific requirement not defined
7. 2011 High

   Inhibitor-bound structures and computational modeling answered how diverse inhibitors engage the active site, providing a structural basis for competitive inhibition and substrate accommodation.

   Evidence Crystal structures of four active-site inhibitor complexes plus steered molecular dynamics predictions validated experimentally, and flavonoid/2-HDA kinetic inhibition studies in P. falciparum

   PMID:16722653 PMID:20462212 PMID:20855214 PMID:21843645

   Open questions at the time

   • No inhibitor validated as a stage-specific antimalarial in vivo
   • Selectivity over host dehydratases not established in these structures
8. 2016 Low

   Development of acylated PfACP substrates answered whether physiological acyl-ACP species serve FabZ, enabling assays with native rather than surrogate substrates.

   Evidence Enzymatic acylation of PfACP and functional validation with FAS-II enzymes in P. falciparum

   PMID:28955970

   Open questions at the time

   • Methodological study without deep mechanistic characterization of FabZ
   • No new FabZ kinetics on native acyl-ACP reported

• Whether the pH-dependent hexamer-dimer switch operates as a physiological regulatory mechanism of FAS-II flux in the parasite, and whether FabZ can be selectively drugged to block liver-stage development in vivo, remain unresolved.
• No in vivo test of oligomeric regulation
• No inhibitor demonstrated to block liver-stage transition in an infection model
• Catalytic residue assignments not confirmed by mutagenesis