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 non-photosynthetic 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 characterized (3). The PfFNR/PfFd couple was shown to be catalytically active in vitro yielding reducing power to support the activity of LytB (4), 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 drugs (2). The three-dimensional structure of PfFNR has been determined by X-ray crystallography (3). 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 positively-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 α-helix localized in the NADP+-binding domain. Furthermore, the binding of NADP+ triggers the formation of a disulfide-stabilized 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. 1. Pandini V. et al. (2002) J. Biol. Chem. 277, 48463-48471 2. Seeber F. et al. (2005) Curr. Farm. Des. 11, 3159-7312 3. Milani et al. (2007) J. Mol. Biol. 367, 501-513 4. Röhrich R.C. et al. (2005) FEBS lett. 579, 6433-643