Inhibition of Plasmodium falciparum fatty acid biosynthesis: evaluation of FabG, FabZ, and FabI as drug targets for flavonoids.
After the discovery of a potent natural flavonoid glucoside as a potent inhibitor of FabI, a large flavonoid library was screened against three important enzymes (i.e., FabG, FabZ, and FabI) involved in the fatty acid biosynthesis of P. falciparum. Although flavones with a simple hydroxylation pattern (compounds 4-9) showed moderate inhibitory activity toward the enzymes tested (IC50 10-100 microM), the more complex flavonoids (12-16) exhibited strong activity toward all three enzymes (IC50 0.5-8 microM). Isoflavonoids 26-28 showed moderate (IC50 7-30 microM) but selective activity against FabZ. The most active compounds were C-3 gallic acid esters of catechins (32, 33, 37, 38), which are strong inhibitors of all three enzymes (IC50 0.2-1.1 microM). Kinetic analysis using luteolin (12) and (-)-catechin gallate (37) as model compounds revealed that FabG was inhibited in a noncompetitive manner. FabZ was inhibited competitively, whereas both compounds behaved as tight-binding noncompetitive inhibitors of FabI. In addition, these polyphenols showed in vitro activity against chloroquine-sensitive (NF54) and -resistant (K1) P. falciparum strains in the low to submicromolar range.