KASIII

KASIII (PF3D7_0211400) is a beta-ketoacyl-ACP synthase III that catalyzes the initiating decarboxylative condensation step of type II fatty acid biosynthesis in the P. falciparum apicoplast, an organelle targeting inferred from its N-terminal presequence and homology to plastid enzymes (PMID:9770490). In reconstituted reactions, KASIII condenses malonyl-pfACP with short-chain acyl-CoA primers, using acetyl-CoA and butyryl-CoA efficiently but isobutyryl-CoA poorly, and its acyl-acceptor substrate malonyl-pfACP is supplied by pfMCAT acting on the mature (transit-peptide-cleaved) form of pfACP (PMID:12549938). An active-site cysteine is essential for catalysis, since Cys→Ala or Cys→Ser substitution abolishes activity; the enzyme additionally performs a slow acyl-CoA:ACP transacylase reaction (~700-fold slower than condensation) (PMID:12270624, PMID:12549938). Pharmacologically, KASIII is not inhibited by thiolactomycin but is inhibited by 1,2-dithiole-3-one compounds that also block parasite growth, whereas thiolactomycin analogues inhibit the enzyme without a clear correlation to growth inhibition, indicating KASIII is not the primary parasite target of those analogues (PMID:12549938, PMID:16161997).

1. 1998 Medium

   Establishing where KASIII acts addressed whether P. falciparum harbors a plastid-like fatty acid pathway, placing the enzyme in the apicoplast as a candidate drug target.

   Evidence Sequence analysis and apicoplast localization of the related ACP protein, with thiolactomycin growth inhibition as pathway evidence

   PMID:9770490

   Open questions at the time

   • Apicoplast targeting of KASIII itself was inferred from presequence, not directly imaged
   • No direct enzymatic characterization of KASIII in this study
2. 2002 High

   Recombinant reconstitution and active-site mutagenesis answered whether KASIII is a genuine catalytic enzyme and identified the residue required for catalysis.

   Evidence Recombinant expression of a truncated active enzyme, in vitro activity assays, and Cys→Ala/Cys→Ser active-site mutagenesis

   PMID:12270624

   Open questions at the time

   • Full substrate specificity and kinetics not resolved here
   • Transacylase versus condensation activities not quantitatively separated
3. 2003 High

   Detailed kinetics and reconstitution defined the reaction KASIII catalyzes, its acyl-CoA primer preferences, and the source of its acyl-acceptor substrate, establishing its role as the pathway-initiating condensing enzyme.

   Evidence In vitro kinetic characterization across acyl-CoA substrates, a reconstituted pfMCAT-to-KASIII two-step assay, and inhibitor IC50 determinations with parasite growth assays

   PMID:12549938

   Open questions at the time

   • In vitro kinetics do not establish physiological flux or in vivo substrate identity
   • Little discrimination between pfACP and E. coli ACP leaves acceptor specificity determinants unclear
4. 2005 Medium

   Testing thiolactomycin analogues clarified whether KASIII is the parasite target of these inhibitors, concluding it is not the primary target despite in vitro inhibition.

   Evidence Recombinant KASIII inhibition assays, parasite growth inhibition assays, and structure-activity analysis

   PMID:16161997

   Open questions at the time

   • The actual parasite target of the TLM analogues is not identified
   • Lack of enzyme-growth correlation is not mechanistically explained

• Whether KASIII is genetically essential for blood-stage parasite survival and what its validated small-molecule vulnerability is remain open.
• No genetic knockout/knockdown phenotype for KASIII in the corpus
• No structural model of KASIII reported
• Direct apicoplast localization of KASIII protein not demonstrated