The emergence of drug-resistant forms of Plasmodium falciparum emphasizes the need to develop new antimalarials. In this context, the fatty acid biosynthesis (FAS) pathway of the malarial parasite has recently received a lot of attention. Due to differences in the fatty acid biosynthesis systems of Plasmodium and man, this pathway is a good target for the development of new and selective therapeutic drugs directed against malaria. In continuation of these efforts we report cloning and overexpression of P. falciparum ␤-hydroxyacyl-acyl carrier protein (ACP) dehydratase (PffabZ) gene that codes for a 17-kDa protein. The enzyme catalyzes the dehydration of ␤-hydroxyacyl-ACP to trans-2-acyl-ACP, the third step in the elongation phase of the FAS cycle. It has a K m of 199 M and k cat /K m of 80.4 M ؊1 s ؊1 for the substrate analog ␤-hydroxybutyryl-CoA but utilizes crotonoyl-CoA, the product of the reaction, more efficiently (K m ‫؍‬ 86 M, k cat / K m ‫؍‬ 220 M ؊1 s ؊1 ). More importantly, we also identify inhibitors (NAS-91 and NAS-21) for the enzyme. Both the inhibitors prevented the binding of crotonoyl-CoA to PfFabZ in a competitive fashion. Indeed these inhibitors compromised the growth of P. falciparum in cultures and inhibited the parasite fatty acid synthesis pathway both in cell-free extracts as well as in situ. We modeled the structure of PfFabZ using Escherichia coli ␤-hydroxydecanoyl thioester dehydratase (EcFabA) as a template. We also modeled the inhibitor complexes of PfFabZ to elucidate the mode of binding of these compounds to FabZ. The discovery of the inhibitors of FabZ, reported for the first time against any member of this family of enzymes, essential to the type II FAS pathway opens up new avenues for treating a number of infectious diseases including malaria.Malaria continues to exact the highest mortality and morbidity rate next only to tuberculosis. "The scourge of the tropics," malaria is endemic to around 100 countries in the world.Approximately 500 million cases of malaria are reported every year, and around 3000 children die of malaria every day (1). Emerging resistance to chloroquine and other currently prescribed drugs limits treatment of malaria today, in particular cerebral malaria, caused by Plasmodium falciparum (2, 3). The situation definitely warrants express remedial actions: extensive research on P. falciparum to identify drug targets and, ultimately, the development of a new armamentarium of antimalarials.Our recent demonstration of the occurrence of the type II fatty acid synthesis (FAS) 1 pathway in the malaria parasite and its inhibition by triclosan, an inhibitor of the rate-limiting enzyme of type II FAS, enoyl-acyl carrier protein (ACP) reductase, proved the pivotal role played by this pathway in the survival of the malarial parasite (4, 5). The essential role of fatty acids and lipids in cell growth and differentiation and the different type (type I) of fatty acid biosynthetic pathway occurring in the human host, which is distinct from type II FAS of the malaria parasit...

[1] This paper describes a five-dimensional parameterization of ion-induced nucleation (IIN) that covers the complete range of conditions relevant to the lower atmosphere. The parameters are (1) temperature T (190-300 K), (2) relative humidity RH (0.05-0.95), (3) number concentration of H 2 SO 4 (10 5 -10 8 cm À3 ), (4) first-order loss of H 2 SO 4 to particles (0.00009-0.0245 s À1 ), and (5) ion source rate (2-50 ion pairs cm À3 s À1 ). The parameterization is based on a steady state version of the kinetic aerosol model Sulphuric Acid and Water Nucleation (SAWNUC) that uses experimentally measured thermodynamics for the ion clusters. Parameterized formulas are obtained for the following variables: (1) particle nucleation rate (cm À3 s À1 ), (2) H 2 SO 4 nucleation rate (cm À3 s À1 ), (3) number of H 2 SO 4 molecules in average nucleating cluster, (4) number of H 2 O molecules in average nucleating cluster, and (5) radius (nanometers) of average nucleating cluster. The parameterization generally reproduces the modeled nucleation rate to within an order of magnitude over the whole range of conditions, except when the nucleation rate is very low (<10 À6 cm À3 s À1 ), which corresponds to a rate of less than 0.1 particle d À1 cm À3 . This parameterization speeds up IIN calculations by a factor of $10 6 , as compared to the original SAWNUC model.

Choline kinase is the first enzyme in the Kennedy pathway (CDP-choline pathway) for the biosynthesis of the most essential phospholipid, phosphatidylcholine, in Plasmodium falciparum. In addition, choline kinase also plays a pivotal role in trapping essential polar head group choline inside the malaria parasite. Recently, Plasmodium falciparum choline kinase (PfCK) has been cloned, overexpressed, and purified. However, the function of this enzyme in parasite growth and survival has not been evaluated owing to the lack of a suitable inhibitor. Purified recombinant PfCK enabled us to identify an inhibitor of PfCK, hexadecyltrimethylammonium bromide (HDTAB), which has a very close structural resemblance to hexadecylphosphocholine (miltefosin), the well-known antiproliferative and antileishmanial drug. HDTAB inhibited PfCK in a dose-dependent manner and offered very potent antimalarial activity in vitro against Plasmodium falciparum. Moreover, HDTAB exhibited profound antimalarial activity in vivo against the rodent malaria parasite Plasmodium yoelii (N-67 strain). Interestingly, parasites at the trophozoite and schizont stages were found to be particularly sensitive to HDTAB. The stage-specific antimalarial effect of HDTAB correlated well with the expression pattern of PfCK in P. falciparum, which was observed by reverse transcription-PCR and immunofluorescence microscopy. Furthermore, the antimalarial activity of HDTAB paralleled the decrease in phosphatidylcholine content, which was found to correlate with the decreased phosphocholine generation. These results suggest that inhibition of choline kinase by HDTAB leads to decreased phosphocholine, which in turn causes a decrease in phosphatidylcholine biosynthesis, resulting in death of the parasite.

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