Plasmodium falciparum ferredoxin-NADP+ reductase : induced-fit transition in the NADP-binding domain and ligand-triggered dimerization and inactivation

New antimalarial drugs are urgently needed to tackle the spread of Plasmodium falciparum strains resistant to current pharmacological treatments. The recent completion of the genome sequence of this parasite (1) has considerably boosted the search of new drug targets. Plasmodia, as most protists belonging to the phylum Apicomplexa, possess a nonphotosyntetic plastid, dubbed the apicoplast (2), which is essential for parasite survival and has no counterpart in the human host (3). We have shown that the apicoplast of both T. gondii and P. falciparum contains a short plant-type electron transport chain, comprising ferredoxin NADP+ reductase (FNR) and ferredoxin (Fd) (4,5). Furthermore, we have demonstrated that such enzyme system is able to provide electrons to the enzyme LytB, which catalyzes the last step of the mevalonate-independent isoprenoid biosynthesis in the apicoplast (6). We have cloned and overproduced in E. coli both P. falciparum FNR (PfFNR) and Fd (PfFd). The two proteins have been purified, characterized and shown to be catalytically competent as an NADPH-dependent electron transfer system, though displaying specific functional features that distinguish them from their plant homologs. Recently, we have obtained the crystal structures of PfFNR both in free form and in complex with the substrate analog 2P-AMP, at 2.4 and 2.7 Å resolution, respectively. Surprisingly, PfFNR displayed structural properties unique to the P. falciparum enzyme. 2P-AMP binding was shown to trigger an unprecedented induced-fit transition in the NADP-binding domain. Prolonged incubation with NADP+ or 2P-AMP led to the formation of a disulfide-stabilized dimeric form of the enzyme, which was catalytically inactive. Insights in its three-dimensional structure indicate that PfFNR is particularly suited for the rational design of inhibitors, which could represent valuable leads in the development of antimalarial compounds. 1. Gardner, M. J., Hall, N., Fung, E., White, O., Berriman, M., Hyman, R. W., Carlton, J. M., et al. (2002) Nature 419, 498–511. 2. Foth, B. J. & McFadden, G. I. (2003) Int. Rev. Cytol. 224, 57–110. 3. Fichera, M. E. & Roos, D. S. (1997) Nature 390, 407–409. 4. Pandini, V., Caprini, G., Thomsen, N., Aliverti, A., Seeber, F. & Zanetti, G. (2002) J. Biol. Chem. 277, 48463–48471. 5. Seeber, F., Aliverti, A. & Zanetti, G. (2005) Curr. Pharm. Des. 11, 3159–3172. 6. Röhrich, R. C., Englert, N., Troschke, K., Reichenberg, A., Hintz, M., Seeber, F., Balconi, E., Aliverti, A., Zanetti, G., Kohler, U., Pfeiffer, M., Beck, E., Jomaa, H. & Wiesner, J. (2005) FEBS Lett. 579, 6433–6438.