A plant-type enzyme as a putative target for novel antimalarial drugs : properties of the Plasmodium falciparum ferredoxin-NADP+ reductase

Apicomplexan parasites harbor a specific organelle, named apicoplast, which is related to plant nonphotosynthetic plastids and displays a plant-like metabolism. The apicoplast has been shown to contain typical vegetal proteins, such as ferredoxin–NADP+ reductase (FNR) and ferredoxin (Fd)1-2. Both proteins from Plasmodium falciparum (PfFNR and PfFd) have been produced in recombinant form and characterized3. The PfFNR/PfFd couple was shown to be catalytically active in vitro yielding reducing power to support the activity of LytB4, the last enzyme of the biosynthetic pathway for isoprenoid precursors, a known site of action of antiplasmodial compounds. On this basis, PfFNR has been proposed as a possible target for new antimalarial drugs2. The three-dimensional structure of PfFNR has been determined by X-ray crystallography3. Compared to other plastidic-type FNRs, PfFNR displays a significantly lower catalytic efficiency and lower selectivity against NADH. These functional features are probably the consequence of the lack of protein positive charges stabilizing the 2’- phosphate of the bound substrate. NADP(H) binding to PfFNR occurs through an induced-fit mechanism never observed in other FNRs. The conformational changes induced by binding to the enzyme of 2’-P-AMP, a NADP+ analogue, includes the partial unwinding of an alpha-helix localized in the NADP+-binding domain. Furthermore, the binding of NADP+ triggers the formation of a disulfidestabilized homodimer resulting in the inactivation of PfFNR. This process, observed in vitro, could represent a physiologic mechanism regulating the enzyme activity. Structure-based design of PfFNR inhibitors is in progress and has already yielded some active compounds, with inhibitory constants in the range of micromolar or lower.