FIG 3.

In vitro parasite growth curves for mode-of-action studies. (A) The P. falciparum mitochondrial membrane potential, which is driven by the cytochrome bc₁ complex- and cytochrome c oxidase-mediated reactions, is coupled with pyrimidine biosynthesis through the PfDHODH. Following the generation of oxidized ubiquinone (Q) via the cytochrome bc₁ complex, reduction of ubiquinol (QH₂) by PfDHODH occurs during pyrimidine synthesis. The ETC and pyrimidine biosynthesis pathway are sensitive to cytochrome bc₁ complex inhibitors, such as atovaquone. Expression of the yDHODH bypasses the ETC and thus rescues pyrimidine synthesis in the presence of cytochrome bc₁ inhibitors. Inhibition of a proguanil-sensitive pathway, which contributes to mitochondrial membrane potential, restores sensitivity to cytochrome bc₁ inhibitors (adapted from reference26 with permission of the publisher). (B and C) Representative P. falciparum growth curves obtained for TCMDC-135546 (B) and TCMDC-132690 (C). The dose-response curves of wild-type P. falciparum Dd2 (blue), transgenic P. falciparum Dd2-yDHODH (red), P. falciparum Dd2 with 1 μM proguanil (brown), and P. falciparum Dd2-yDHODH with 1 μM proguanil (green) are shown. Addition of proguanil restored sensitivity to cytochrome bc₁ complex inhibitors, such as TCMDC-135546, but not to compounds with an unknown mode of action, like TCMDC-132690. The error bars indicate standard deviations.