The initiating steps of a type II fatty acid synthase in Plasmodium falciparum are catalyzed by pfACP, pfMCAT, and pfKASIII

The mitochondrion of Plasmodium falciparum is required for cellular acetyl-CoA metabolism and protein acetylation

Coenzyme A (CoA) biosynthesis is an excellent target for antimalarial intervention. While most studies have focused on the use of CoA to produce acetyl-CoA in the apicoplast and the cytosol of malaria parasites, mitochondrial acetyl-CoA production is less well understood. In the current study, we performed metabolite-labeling experiments to measure endogenous metabolites in Plasmodium falciparum lines with genetic deletions affecting mitochondrial dehydrogenase activity. Our results show that the mitochondrion is required for cellular acetyl-CoA biosynthesis and identify a synthetic lethal relationship between the two main ketoacid dehydrogenase enzymes. The activity of these enzymes is dependent on the lipoate attachment enzyme LipL2, which is essential for parasite survival solely based on its role in supporting acetyl-CoA metabolism. We also find that acetyl-CoA produced in the mitochondrion is essential for the acetylation of histones and other proteins outside of the mitochondrion. Taken together, our results demonstrate that the mitochondrion is required for cellular acetyl-CoA metabolism and protein acetylation essential for parasite survival.

The nutrient games - Plasmodium metabolism during hepatic development

Malaria is a febrile illness caused by species of the protozoan parasite Plasmodium and is characterized by recursive infections of erythrocytes, leading to clinical symptoms and pathology. In mammals, Plasmodium parasites undergo a compulsory intrahepatic development stage before infecting erythrocytes. Liver-stage parasites have a metabolic configuration to facilitate the replication of several thousand daughter parasites. Their metabolism is of interest to identify cellular pathways essential for liver infection, to kill the parasite before onset of the disease. In this review, we summarize the current knowledge on nutrient acquisition and biosynthesis by liver-stage parasites mostly generated in murine malaria models, gaps in knowledge, and challenges to create a holistic view of the development and deficiencies in this field.

Metabolic responses in blood-stage malaria parasites associated with increased and decreased sensitivity to PfATP4 inhibitors

BACKGROUND

Spiroindolone and pyrazoleamide antimalarial compounds target Plasmodium falciparum P-type ATPase (PfATP4) and induce disruption of intracellular Na⁺ homeostasis. Recently, a PfATP4 mutation was discovered that confers resistance to a pyrazoleamide while increasing sensitivity to a spiroindolone. Transcriptomic and metabolic adaptations that underlie this seemingly contradictory response of P. falciparum to sublethal concentrations of each compound were examined to understand the different cellular accommodation to PfATP4 disruptions.

METHODS

A genetically engineered P. falciparum Dd2 strain (Dd2^A211V) carrying an Ala211Val (A211V) mutation in PfATP4 was used to identify metabolic adaptations associated with the mutation that results in decreased sensitivity to PA21A092 (a pyrazoleamide) and increased sensitivity to KAE609 (a spiroindolone). First, sublethal doses of PA21A092 and KAE609 causing substantial reduction (30-70%) in Dd2^A211V parasite replication were identified. Then, at this sublethal dose of PA21A092 (or KAE609), metabolomic and transcriptomic data were collected during the first intraerythrocytic developmental cycle. Finally, the time-resolved data were integrated with a whole-genome metabolic network model of P. falciparum to characterize antimalarial-induced physiological adaptations.

RESULTS

Sublethal treatment with PA21A092 caused significant (p < 0.001) alterations in the abundances of 91 Plasmodium gene transcripts, whereas only 21 transcripts were significantly altered due to sublethal treatment with KAE609. In the metabolomic data, a substantial alteration (≥ fourfold) in the abundances of carbohydrate metabolites in the presence of either compound was found. The estimated rates of macromolecule syntheses between the two antimalarial-treated conditions were also comparable, except for the rate of lipid synthesis. A closer examination of parasite metabolism in the presence of either compound indicated statistically significant differences in enzymatic activities associated with synthesis of phosphatidylcholine, phosphatidylserine, and phosphatidylinositol.

CONCLUSION

The results of this study suggest that malaria parasites activate protein kinases via phospholipid-dependent signalling in response to the ionic perturbation induced by the Na⁺ homeostasis disruptor PA21A092. Therefore, targeted disruption of phospholipid signalling in PA21A092-resistant parasites could be a means to block the emergence of resistance to PA21A092.

New insights into apicoplast metabolism in blood-stage malaria parasites

The apicoplast of Plasmodium falciparum is the only source of essential isoprenoid precursors and Coenzyme A (CoA) in the parasite. Isoprenoid precursor synthesis relies on the iron-sulfur cluster (FeS) cofactors produced within the apicoplast, rendering FeS synthesis an essential function of this organelle. Recent reports provide important insights into the roles of FeS cofactors and the use of isoprenoid precursors and CoA both inside and outside the apicoplast. Here, we review the recent insights into the roles of these metabolites in blood-stage malaria parasites and discuss new questions that have been raised in light of these discoveries.

Transcriptome Analysis and GC-MS Profiling of Key Fatty Acid Biosynthesis Genes in Akebia trifoliata (Thunb.) Koidz Seeds

Simple Summary

Plant oil is an important renewable energy substance, and A. trifoliata seeds are of value in this regard. A. trifoliata fruits have many seeds with high oil content, but research progress on A. trifoliata seed oil is slow. Fatty acid biosynthesis is the most important factor affecting plant oil content. Therefore, analysis of the key genes for fatty acid biosynthesis is beneficial for breeding A. trifoliata varieties with high oil content. Here, we report changes in seed oil and key oil biosynthesis genes in the growth period of A. trifoliata based on transcriptome analysis. We found that the development of A. trifoliata seeds and fruits was not synchronized, and when the fruit was ripe, the seed oil content was not the highest. With the development of A. trifoliata seeds, linoleic and oleic acid content was found to decrease and increase, respectively. Subsequently, several key genes for oil biosynthesis in A. trifoliata were identified. These results further our understanding of the mechanism underlying oil biosynthesis in A. trifoliata seeds.

Abstract

Akebia trifoliata (Thunb.) Koidz is an important Chinese medicinal and economic crop. Its seeds, which are rich in fatty acids, are usually discarded. As of now, A. trifoliata lipid biosynthesis pathways and genes have not been clearly described. In this work, we found that seed and fruit development of A. trifoliata were not synchronized, and that when the fruit was ripe, seed oil content was not at its highest. As seeds developed, linoleic and oleic acid content was found to decrease and increase, respectively. RNA sequencing yielded 108.45 GB of clean reads from 15 cDNA libraries, containing 8756 differentially expressed genes. We identified 65 unigenes associated with lipid biosynthesis, including fatty acid and triacylglycerol biosynthesis. The 65 unigenes were mapped to the A. trifoliata lipid synthesis pathway. There were 20 AtrFAD family members in A. trifoliata, which could be divided into four sub-groups with the highest number of AtrSADs. Our study revealed the dynamic changes in A. trifoliata seed oil content and composition during its growth period and provides large-scale and comprehensive transcriptome data of A. trifoliata seeds. These findings provide a basis for the improvement of A. trifoliata seed oil yield and quality.

Dephospho-CoA kinase, a nuclear-encoded apicoplast protein, remains active and essential after Plasmodium falciparum apicoplast disruption

Malaria parasites contain an essential organelle called the apicoplast that houses metabolic pathways for fatty acid, heme, isoprenoid, and iron–sulfur cluster synthesis. Surprisingly, malaria parasites can survive without the apicoplast as long as the isoprenoid precursor isopentenyl pyrophosphate (IPP) is supplemented in the growth medium, making it appear that isoprenoid synthesis is the only essential function of the organelle in blood‐stage parasites. In the work described here, we localized an enzyme responsible for coenzyme A synthesis, DPCK, to the apicoplast, but we were unable to delete DPCK, even in the presence of IPP. However, once the endogenous DPCK was complemented with the E. coli DPCK (EcDPCK), we were successful in deleting it. We were then able to show that DPCK activity is required for parasite survival through knockdown of the complemented EcDPCK. Additionally, we showed that DPCK enzyme activity remains functional and essential within the vesicles present after apicoplast disruption. These results demonstrate that while the apicoplast of blood‐stage P. falciparum parasites can be disrupted, the resulting vesicles remain biochemically active and are capable of fulfilling essential functions.

Roles of Ferredoxin-Dependent Proteins in the Apicoplast of Plasmodium falciparum Parasites

Ferredoxin (Fd) and ferredoxin-NADP+ reductase (FNR) form a redox system that is hypothesized to play a central role in the maintenance and function of the apicoplast organelle of malaria parasites. The Fd/FNR system provides reducing power to various iron-sulfur cluster (FeS)-dependent proteins in the apicoplast and is believed to help to maintain redox balance in the organelle. While the Fd/FNR system has been pursued as a target for antimalarial drug discovery, Fd, FNR, and the FeS proteins presumably reliant on their reducing power play an unknown role in parasite survival and apicoplast maintenance. To address these questions, we generated genetic deletions of these proteins in a parasite line containing an apicoplast bypass system. Through these deletions, we discovered that Fd, FNR, and certain FeS proteins are essential for parasite survival but found that none are required for apicoplast maintenance. Additionally, we addressed the question of how Fd and its downstream FeS proteins obtain FeS cofactors by deleting the FeS transfer proteins SufA and NfuApi. While individual deletions of these proteins revealed their dispensability, double deletion resulted in synthetic lethality, demonstrating a redundant role in providing FeS clusters to Fd and other essential FeS proteins. Our data support a model in which the reducing power from the Fd/FNR system to certain downstream FeS proteins is essential for the survival of blood-stage malaria parasites but not for organelle maintenance, while other FeS proteins are dispensable for this stage of parasite development. IMPORTANCE Ferredoxin (Fd) and ferredoxin-NADP+ reductase (FNR) form one of the few known redox systems in the apicoplast of malaria parasites and provide reducing power to iron-sulfur (FeS) cluster proteins within the organelle. While the Fd/FNR system has been explored as a drug target, the essentiality and roles of this system and the identity of its downstream FeS proteins have not been determined. To answer these questions, we generated deletions of these proteins in an apicoplast metabolic bypass line (PfMev) and determined the minimal set of proteins required for parasite survival. Moving upstream of this pathway, we also generated individual and dual deletions of the two FeS transfer proteins that deliver FeS clusters to Fd and downstream FeS proteins. We found that both transfer proteins are dispensable, but double deletion displayed a synthetic lethal phenotype, demonstrating their functional redundancy. These findings provide important insights into apicoplast biochemistry and drug development.

Antioxidant Metabolism, Photosystem II, and Fatty Acid Composition of Two Tall Fescue Genotypes With Different Heat Tolerance Under High Temperature Stress

Tall fescue (Festuca arundinacea Schreb.) is a typical and widely used cool-season turf grass. High temperature is a key factor that limits its utility. The objectives of this study were to investigate the behaviors of fatty acid composition and its gene expression patterns in heat-resistant genotype "TF71" and heat-sensitive genotype "TF133" exposed to heat stress (40/35°C, 14/10 h), and to broaden our comprehension about the relationship between heat tolerance and fatty acid function. The result showed that heat stress increased the malondialdehyde (MDA) content and relative electrolyte leakage (EL), but decreased the level of chlorophyll and the activity of superoxide dismutase (SOD) and peroxidase (POD) when compared to the controls, to a greater extent in "TF133." This result proved that "TF71" had superior high-temperature resistance. Furthermore, comparing the changes in the composition of fatty acid and the expression of the genes involved in its synthesis between the two different genotypes under heat stress, we found that heat stress increased the degree of unsaturation, UFA/SFA, and double bond index (DBI) in "TF71." Moreover, quantitative RT-PCR revealed that heat stress altered the expression of the genes involved in fatty acid synthesis, including ACAC, FabD, FabF, FabH, FabI, and FatA. According to these findings, we can speculate that increasing the unsaturation degree of fatty acid or controlling the equilibrium ratio of UFA/SFA might be closely associated with the improving of the heat resistance in tall fescue.

Host biotin is required for liver stage development in malaria parasites

Malaria parasites require certain host nutrients for growth and survival. In this project, we examined the role of the human vitamin biotin in all stages of the malaria life cycle. We cultured blood- and liver-stage malaria parasites in the absence of biotin and found that, whereas blood-stage replication was unaffected, liver-stage parasites deprived of biotin were no longer capable of establishing a blood-stage infection. Interestingly, biotin depletion resulted in more severe developmental defects than the genetic disruption of parasite biotin metabolism. This finding suggests that host biotin metabolism also contributes to parasite development. Because neither the parasite nor the human host can synthesize biotin, parasite infectivity may be affected by the nutritional status of the host.

Acetyl-CoA carboxylase (ACC) is a biotin-dependent enzyme that is the target of several classes of herbicides. Malaria parasites contain a plant-like ACC, and this is the only protein predicted to be biotinylated in the parasite. We found that ACC is expressed in the apicoplast organelle in liver- and blood-stage malaria parasites; however, it is activated through biotinylation only in the liver stages. Consistent with this observation, deletion of the biotin ligase responsible for ACC biotinylation does not impede blood-stage growth, but results in late liver-stage developmental defects. Biotin depletion increases the severity of the developmental defects, demonstrating that parasite and host biotin metabolism are required for normal liver-stage progression. This finding may link the development of liver-stage malaria parasites to the nutritional status of the host, as neither the parasite nor the human host can synthesize biotin.

Relict plastidic metabolic process as a potential therapeutic target

The alignment of the evolutionary history of parasites with that of plants provides a different panorama in the drug development process. The housing of different metabolic processes, essential for parasite survival, adds to the indispensability of the apicoplast. The different pathways responsible for fueling the apicoplast and parasite offer a myriad of proteins responsible for the apicoplast function. The studies emphasizing the target-based approaches might help in the discovery of antimalarials. The different putative drug targets and their roles are highlighted. In addition, the origin of the apicoplast and metabolic processes are reviewed and the different drugs acting upon the enzymes of the apicoplast are discussed.

A high yield optimized method for the production of acylated ACPs enabling the analysis of enzymes involved in P. falciparum fatty acid biosynthesis

The natural substrates of the enzymes involved in type-II fatty acid biosynthesis (FAS-II) are acylated acyl carrier proteins (acyl-ACPs). The state of the art method to produce acyl-ACPs involves the transfer of a phosphopantetheine moiety from CoA to apo-ACP by E. coli holo-ACP synthase (EcACPS), yielding holo-ACP which subsequently becomes thioesterified with free fatty acids by the E. coli acyl-ACP synthase (EcAAS). Alternatively, acyl-ACPs can be synthesized by direct transfer of acylated phosphopantetheine moieties from acyl-CoA to apo-ACP by means of EcACPS. The need for native substrates to characterize the FAS-II enzymes of P. falciparum prompted us to investigate the potential and limit of the two methods to efficiently acylate P. falciparum ACP (PfACP) with respect to chain length and β-modification and in preparative amounts. The EcAAS activity is found to be independent from the oxidation state at the β-position and accepts fatty acids as substrates with chain lengths starting from C8 to C20, whereas EcACPS accepts very efficiently acyl-CoAs with chain lengths up to C16, and with decreasing activity also longer chains (C18 to C20). Methods were developed to synthesize and purify preparative amounts of high quality natural substrates that are fully functional for the enzymes of the P. falciparum FAS-II system.

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The apo-form of P. falciparum ACP (PfACP) has been purified to homogeneity.

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PfACP can be acylated very efficiently and in preparative amounts using the improved EcACPS and EcAAS methods.

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Small and long chain fatty acids can be transferred.

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The acylation reaction is independent of the oxidation state at the β-position of the acyl-chains.

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Acyl-PfACPs are fully functional substrates of the corresponding P. falciparum FAS-II enzymes.

Structural and dynamical aspects of Streptococcus gordonii FabH through molecular docking and MD simulations

β-Ketoacyl-ACP-synthase III (FabH or KAS III) has become an attractive target for the development of new antibacterial agents which can overcome the multidrug resistance. Unraveling the fatty acid biosynthesis (FAB) metabolic pathway and understanding structural coordinates of FabH will provide valuable insights to target Streptococcus gordonii for curing oral infection. In this study, we designed inhibitors against therapeutic target FabH, in order to block the FAB pathway. As compared to other targets, FabH has more interactions with other proteins, located on the leading strand with higher codon adaptation index value and associated with lipid metabolism category of COG. Current study aims to gain in silico insights into the structural and dynamical aspect of S. gordonii FabH via molecular docking and molecular dynamics (MD) simulations. The FabH protein is catalytically active in dimerization while it can lock in monomeric state. Current study highlights two residues Pro88 and Leu315 that are close to each other by dimerization. The active site of FabH is composed of the catalytic triad formed by residues Cys112, His249, and Asn279 in which Cys112 is involved in acetyl transfer, while His249 and Asn279 play an active role in decarboxylation. Docking analysis revealed that among the studied compounds, methyl-CoA disulfide has highest GOLD score (82.75), binding affinity (-11 kcal/mol) and exhibited consistently better interactions. During MD simulations, the FabH structure remained stable with the average RMSD value of 1.7 Å and 1.6 Å for undocked protein and docked complex, respectively. Further, crucial hydrogen bonding of the conserved catalytic triad for exhibiting high affinity between the FabH protein and ligand is observed by RDF analysis. The MD simulation results clearly demonstrated that binding of the inhibitor with S. gordonii FabH enhanced the structure and stabilized the dimeric FabH protein. Therefore, the inhibitor has the potential to become a lead compound.

Fatty acid biosynthesis revisited: structure elucidation and metabolic engineering

Fatty acids are primary metabolites synthesized by complex, elegant, and essential biosynthetic machinery. Fatty acid synthases resemble an iterative assembly line, with an acyl carrier protein conveying the growing fatty acid to necessary enzymatic domains for modification. Each catalytic domain is a unique enzyme spanning a wide range of folds and structures. Although they harbor the same enzymatic activities, two different types of fatty acid synthase architectures are observed in nature. During recent years, strained petroleum supplies have driven interest in engineering organisms to either produce more fatty acids or specific high value products. Such efforts require a fundamental understanding of the enzymatic activities and regulation of fatty acid synthases. Despite more than one hundred years of research, we continue to learn new lessons about fatty acid synthases' many intricate structural and regulatory elements. In this review, we summarize each enzymatic domain and discuss efforts to engineer fatty acid synthases, providing some clues to important challenges and opportunities in the field.

Celastrol inhibits Plasmodium falciparum enoyl-acyl carrier protein reductase

Enoyl-acyl carrier protein reductase (ENR), a critical enzyme in type II fatty acid biosynthesis, is a promising target for drug discovery against hepatocyte-stage Plasmodium falciparum. In order to identify PfENR-specific inhibitors, we docked 70 FDA-approved, bioactive, and/or natural product small molecules known to inhibit the growth of whole-cell blood-stage P. falciparum into several PfENR crystallographic structures. Subsequent in vitro activity assays identified a noncompetitive low-micromolar PfENR inhibitor, celastrol, from this set of compounds.

Chemoenzymatic exchange of phosphopantetheine on protein and peptide

Evaluation of new acyl carrier protein hydrolase (AcpH, EC 3.1.4.14) homologs from proteobacteria and cyanobacteria reveals significant variation in substrate selectivity and kinetic parameters for phosphopantetheine hydrolysis from carrier proteins. Evaluation with carrier proteins from both primary and secondary metabolic pathways reveals an overall preference for acyl carrier protein (ACP) substrates from type II fatty acid synthases, as well as variable activity for polyketide synthase ACPs and peptidyl carrier proteins (PCP) from non-ribosomal peptide synthases. We also demonstrate the kinetic parameters of these homologs for AcpP and the 11-mer peptide substrate YbbR. These findings enable the fully reversible labeling of all three classes of natural product synthase carrier proteins as well as full and minimal fusion protein constructs.

The suf iron-sulfur cluster synthesis pathway is required for apicoplast maintenance in malaria parasites

The apicoplast organelle of the malaria parasite Plasmodium falciparum contains metabolic pathways critical for liver-stage and blood-stage development. During the blood stages, parasites lacking an apicoplast can grow in the presence of isopentenyl pyrophosphate (IPP), demonstrating that isoprenoids are the only metabolites produced in the apicoplast which are needed outside of the organelle. Two of the isoprenoid biosynthesis enzymes are predicted to rely on iron-sulfur (FeS) cluster cofactors, however, little is known about FeS cluster synthesis in the parasite or the roles that FeS cluster proteins play in parasite biology. We investigated two putative FeS cluster synthesis pathways (Isc and Suf) focusing on the initial step of sulfur acquisition. In other eukaryotes, these proteins can be located in multiple subcellular compartments, raising the possibility of cross-talk between the pathways or redundant functions. In P. falciparum, SufS and its partner SufE were found exclusively the apicoplast and SufS was shown to have cysteine desulfurase activity in a complementation assay. IscS and its effector Isd11 were solely mitochondrial, suggesting that the Isc pathway cannot contribute to apicoplast FeS cluster synthesis. The Suf pathway was disrupted with a dominant negative mutant resulting in parasites that were only viable when supplemented with IPP. These parasites lacked the apicoplast organelle and its organellar genome--a phenotype not observed when isoprenoid biosynthesis was specifically inhibited with fosmidomycin. Taken together, these results demonstrate that the Suf pathway is essential for parasite survival and has a fundamental role in maintaining the apicoplast organelle in addition to any role in isoprenoid biosynthesis.

Equal opportunity for low-degree network nodes: a PageRank-based method for protein target identification in metabolic graphs

Biological network data, such as metabolic-, signaling- or physical interaction graphs of proteins are increasingly available in public repositories for important species. Tools for the quantitative analysis of these networks are being developed today. Protein network-based drug target identification methods usually return protein hubs with large degrees in the networks as potentially important targets. Some known, important protein targets, however, are not hubs at all, and perturbing protein hubs in these networks may have several unwanted physiological effects, due to their interaction with numerous partners. Here, we show a novel method applicable in networks with directed edges (such as metabolic networks) that compensates for the low degree (non-hub) vertices in the network, and identifies important nodes, regardless of their hub properties. Our method computes the PageRank for the nodes of the network, and divides the PageRank by the in-degree (i.e., the number of incoming edges) of the node. This quotient is the same in all nodes in an undirected graph (even for large- and low-degree nodes, that is, for hubs and non-hubs as well), but may differ significantly from node to node in directed graphs. We suggest to assign importance to non-hub nodes with large PageRank/in-degree quotient. Consequently, our method gives high scores to nodes with large PageRank, relative to their degrees: therefore non-hub important nodes can easily be identified in large networks. We demonstrate that these relatively high PageRank scores have biological relevance: the method correctly finds numerous already validated drug targets in distinct organisms (Mycobacterium tuberculosis, Plasmodium falciparum and MRSA Staphylococcus aureus), and consequently, it may suggest new possible protein targets as well. Additionally, our scoring method was not chosen arbitrarily: its value for all nodes of all undirected graphs is constant; therefore its high value captures importance in the directed edge structure of the graph.

Novel antimalarial drug targets: hope for new antimalarial drugs

Malaria is a major global threat, that results in more than 2 million deaths each year. The treatment of malaria is becoming extremely difficult due to the emergence of drug-resistant parasites, the absence of an effective vaccine, and the spread of insecticide-resistant vectors. Thus, malarial therapy needs new chemotherapeutic approaches leading to the search for new drug targets. Here, we discuss different approaches to identifying novel antimalarial drug targets. We have also given due attention to the existing validated targets with a view to develop novel, rationally designed lead molecules. Some of the important parasite proteins are claimed to be the targets; however, further in vitro or in vivo structure-function studies of such proteins are crucial to validate these proteins as suitable targets. The interactome analysis among apicoplast, mitochondrion and genomic DNA will also be useful in identifying vital pathways or proteins regulating critical pathways for parasite growth and survival, and could be attractive targets. Molecules responsible for parasite invasion to host erythrocytes and ion channels of infected erythrocytes, essential for intra-erythrocyte survival and stage progression of parasites are also becoming attractive targets. This review will discuss and highlight the current understanding regarding the potential antimalarial drug targets, which could be utilized to develop novel antimalarials.

Babesia bovis: a bipartite signal directs the glutamyl-tRNA synthetase to the apicoplast

Babesia bovis contains a prokaryotic derived organelle known as the apicoplast. Many participants of the metabolic pathways within the apicoplast are encoded in the nuclear genome and post-translationally imported with the help of a bipartite signal. Recently, an all encompassing algorithm was derived to predict apicoplast targeted proteins for many non-Plasmodium apicomplexans in which it reported the presence of 260 apicoplast targeted proteins in Babesia. One of these proteins is glutamyl tRNA synthetase (GltX). This study investigates if the putative bipartite signal of GltX alone is sufficient to direct proteins into the apicoplast. Using a transient transfection system consisting of a green fluorescent protein as the reporter, we tested the signal and transit portions of the bipartite signal in apicoplastic transport. We first identified the transcript of gltX to be expressed during the asexual blood stages and subsequently confirmed that the complete bipartite signal is responsible for directing the reporter protein into a compartment distinct from the nucleus and the mitochondrion. As GltX bipartite signal successfully guided the reporter protein into the apicoplast, our finding implies that it also directs native GltX into the same organelle.

Functional characterizations of malonyl-CoA:acyl carrier protein transacylase (MCAT) in Eimeria tenella

Eimeria tenella, an apicomplexan parasite in chickens, possesses an apicoplast and its associated metabolic pathways including the Type II fatty acid synthesis (FAS II). Malonyl-CoA:acyl-carry protein transacylase (MCAT) encoded by the fabD gene is one of the essential enzymes in the FAS II system. In the present study, the entire E. tenella MCAT gene (EtfabD) was cloned and sequenced. Immunolabeling located this protein in the apicoplast organelle in coccidial sporozoites. Functional replacement of the fabD gene with amber mutation of E. coli temperature-sensitive LA2-89 strain by E. tenella EtMCAT demonstrated that EcFabD and EtMCAT perform the same biochemical function. The recombinant EtMCAT protein was expressed and its general biochemical features were also determined. An alkaloid natural product corytuberine (CAS: 517-56-6) could specifically inhibit the EtMCAT activity (IC(50)=16.47μM), but the inhibition of parasite growth in vitro by corytuberine was very weak (the predicted MIC(50)=0.65mM).

Apicoplast and endoplasmic reticulum cooperate in fatty acid biosynthesis in apicomplexan parasite Toxoplasma gondii

Apicomplexan parasites are responsible for high impact human diseases such as malaria, toxoplasmosis, and cryptosporidiosis. These obligate intracellular pathogens are dependent on both de novo lipid biosynthesis as well as the uptake of host lipids for biogenesis of parasite membranes. Genome annotations and biochemical studies indicate that apicomplexan parasites can synthesize fatty acids via a number of different biosynthetic pathways that are differentially compartmentalized. However, the relative contribution of each of these biosynthetic pathways to total fatty acid composition of intracellular parasite stages remains poorly defined. Here, we use a combination of genetic, biochemical, and metabolomic approaches to delineate the contribution of fatty acid biosynthetic pathways in Toxoplasma gondii. Metabolic labeling studies with [(13)C]glucose showed that intracellular tachyzoites synthesized a range of long and very long chain fatty acids (C14:0-26:1). Genetic disruption of the apicoplast-localized type II fatty-acid synthase resulted in greatly reduced synthesis of saturated fatty acids up to 18 carbons long. Ablation of type II fatty-acid synthase activity resulted in reduced intracellular growth that was partially restored by addition of long chain fatty acids. In contrast, synthesis of very long chain fatty acids was primarily dependent on a fatty acid elongation system comprising three elongases, two reductases, and a dehydratase that were localized to the endoplasmic reticulum. The function of these enzymes was confirmed by heterologous expression in yeast. This elongase pathway appears to have a unique role in generating very long unsaturated fatty acids (C26:1) that cannot be salvaged from the host.

Antimalarial drug targets in Plasmodium falciparum predicted by stage-specific metabolic network analysis

BACKGROUND

Despite enormous efforts to combat malaria the disease still afflicts up to half a billion people each year of which more than one million die. Currently no approved vaccine is available and resistances to antimalarials are widely spread. Hence, new antimalarial drugs are urgently needed.

RESULTS

Here, we present a computational analysis of the metabolism of Plasmodium falciparum, the deadliest malaria pathogen. We assembled a compartmentalized metabolic model and predicted life cycle stage specific metabolism with the help of a flux balance approach that integrates gene expression data. Predicted metabolite exchanges between parasite and host were found to be in good accordance with experimental findings when the parasite's metabolic network was embedded into that of its host (erythrocyte). Knock-out simulations identified 307 indispensable metabolic reactions within the parasite. 35 out of 57 experimentally demonstrated essential enzymes were recovered and another 16 enzymes, if additionally the assumption was made that nutrient uptake from the host cell is limited and all reactions catalyzed by the inhibited enzyme are blocked. This predicted set of putative drug targets, shown to be enriched with true targets by a factor of at least 2.75, was further analyzed with respect to homology to human enzymes, functional similarity to therapeutic targets in other organisms and their predicted potency for prophylaxis and disease treatment.

CONCLUSIONS

The results suggest that the set of essential enzymes predicted by our flux balance approach represents a promising starting point for further drug development.

Lactococcus lactis fabH, encoding beta-ketoacyl-acyl carrier protein synthase, can be functionally replaced by the Plasmodium falciparum congener

Plasmodium falciparum, in addition to scavenging essential fatty acids from its intra- and intercellular environments, possesses a functional complement of type II fatty acid synthase (FAS) enzymes targeted to the apicoplast organelle. Recent evidence suggests that products of the plasmodial FAS II system may be critical for the parasite's liver-to-blood cycle transition, and it has been speculated that endogenously generated fatty acids may be precursors for essential cofactors, such as lipoate, in the apicoplast. beta-Ketoacyl-acyl carrier protein (ACP) synthase III (pfKASIII or FabH) is one of the key enzymes in the initiating steps of the FAS II pathway, possessing two functions in P. falciparum: the decarboxylative thio-Claisen condensation of malonyl-ACP and various acyl coenzymes A (acyl-CoAs; KAS activity) and the acetyl-CoA:ACP transacylase reaction (ACAT). Here, we report the generation and characterization of a hybrid Lactococcus lactis strain that translates pfKASIII instead of L. lactis fabH to initiate fatty acid biosynthesis. The L. lactis expression vector pMG36e was modified for the efficient overexpression of the plasmodial gene in L. lactis. Transcriptional analysis indicated high-efficiency overexpression, and biochemical KAS and ACAT assays confirm these activities in cell extracts. Phenotypically, the L. lactis strain expressing pfKASIII has a growth rate and fatty acid profiles that are comparable to those of the strain complemented with its endogenous gene, suggesting that pfKASIII can use L. lactis ACP as substrate and perform near-normal function in L. lactis cells. This strain may have potential application as a bacterial model for pfKASIII inhibitor prescreening.

Plasmodium falciparum acyl carrier protein crystal structures in disulfide-linked and reduced states and their prevalence during blood stage growth

Acyl Carrier Protein (ACP) has a single reactive sulfhydryl necessary for function in covalently binding nascent fatty acids during biosynthesis. In Plasmodium falciparum, the causative agent of the most lethal form of malaria, fatty acid biosynthesis occurs in the apicoplast organelle during the liver stage of the parasite life cycle. During the blood stage, fatty acid biosynthesis is inactive and the redox state of the apicoplast has not been determined. We solved the crystal structure of ACP from P. falciparum in reduced and disulfide-linked forms, and observe the surprising result that the disulfide in the PfACP cross-linked dimer is sequestered from bulk solvent in a tight molecular interface. We assessed solvent accessibility of the disulfide with small molecule reducing agents and found that the disulfide is protected from BME but less so for other common reducing agents. We examined cultured P. falciparum parasites to determine which form of PfACP is prevalent during the blood stages. We readily detected monomeric PfACP in parasite lysate, but do not observe the disulfide-linked form, even under conditions of oxidative stress. To demonstrate that PfACP contains a free sulfhydryl and is not acylated or in the apo state, we treated blood stage parasites with the disulfide forming reagent diamide. We found that the effects of diamide are reversed with reducing agent. Together, these results suggest that the apicoplast is a reducing compartment, as suggested by models of P. falciparum metabolism, and that PfACP is maintained in a reduced state during blood stage growth.

The amidase domain of lipoamidase specifically inactivates lipoylated proteins in vivo

BACKGROUND

In the 1950s, Reed and coworkers discovered an enzyme activity in Streptococcus faecalis (Enterococcus faecalis) extracts that inactivated the Escherichia. coli and E. faecalis pyruvate dehydrogenase complexes through cleavage of the lipoamide bond. The enzyme that caused this lipoamidase activity remained unidentified until Jiang and Cronan discovered the gene encoding lipoamidase (Lpa) through the screening of an expression library. Subsequent cloning and characterization of the recombinant enzyme revealed that lipoamidase is an 80 kDa protein composed of an amidase domain containing a classic Ser-Ser-Lys catalytic triad and a carboxy-terminal domain of unknown function. Here, we show that the amidase domain can be used as an in vivo probe which specifically inactivates lipoylated enzymes.

METHODOLOGY/PRINCIPAL FINDINGS

We evaluated whether Lpa could function as an inducible probe of alpha-ketoacid dehydrogenase inactivation using E. coli as a model system. Lpa expression resulted in cleavage of lipoic acid from the three lipoylated proteins expressed in E. coli, but did not result in cleavage of biotin from the sole biotinylated protein, the biotin carboxyl carrier protein. When expressed in lipoylation deficient E. coli, Lpa is not toxic, indicating that Lpa does not interfere with any other critical metabolic pathways. When truncated to the amidase domain, Lpa retained lipoamidase activity without acquiring biotinidase activity, indicating that the carboxy-terminal domain is not essential for substrate recognition or function. Substitution of any of the three catalytic triad amino acids with alanine produced inactive Lpa proteins.

CONCLUSIONS/SIGNIFICANCE

The enzyme lipoamidase is active against a broad range of lipoylated proteins in vivo, but does not affect the growth of lipoylation deficient E. coli. Lpa can be truncated to 60% of its original size with only a partial loss of activity, resulting in a smaller probe that can be used to study the effects of alpha-ketoacid dehydrogenase inactivation in vivo.

Targeting the fatty acid biosynthesis enzyme, beta-ketoacyl-acyl carrier protein synthase III (PfKASIII), in the identification of novel antimalarial agents

The importance of fatty acids to the human malaria parasite, Plasmodium falciparum, and differences due to a type I fatty acid synthesis (FAS) pathway in the parasite, make it an attractive drug target. In the present study, we developed and a utilized a pharmacophore to select compounds for testing against PfKASIII, the initiating enzyme of FAS. This effort identified several PfKASIII inhibitors that grouped into various chemical classes of sulfides, sulfonamides, and sulfonyls. Approximately 60% of the submicromolar inhibitors of PfKASIII inhibited in vitro growth of the malaria parasite. These compounds inhibited both drug sensitive and resistant parasites and testing against a mammalian cell line revealed an encouraging in vitro therapeutic index for the most active compounds. Docking studies into the active site of PfKASIII suggest a potential binding mode that exploits amino acid residues at the mouth of the substrate tunnel.

Targeting the Lipid Metabolic Pathways for the Treatment of Malaria

The control and eventual eradication of human malaria is considered one of the most important global public health goals of the 21st Century. Malaria, caused by intraerythrocytic protozoan parasites of the genus Plasmodium, is by far the most lethal and among the most prevalent of the infectious diseases. Four species of Plasmodium (P. falciparum, P. malariae, P. ovale, and P. vivax) are known to be infectious to humans, and more recent cases of infection due to P. knowlesi also have been reported. These species cause approximately 300 million annual cases of clinical malaria resulting in around one million deaths mostly caused by P. falciparum. The rapid emergence of drug-resistant Plasmodium strains has severely reduced the potency of medicines commonly used to treat and block the transmission of malaria and threatens the effectiveness of combination therapy in the field. New drugs that target important parasite functions, which are not the target of current antimalarial drugs, and have the potential to act against multi-drug-resistant Plasmodium strains are urgently needed. Recent studies in P. falciparum have unraveled new metabolic pathways for the synthesis of the parasite phospholipids and fatty acids. The present review summarizes our current understanding of these pathways in Plasmodium development and pathogenesis, and provides an update on the efforts underway to characterize their importance using genetic means and to develop antimalarial therapies targeting lipid metabolic pathways.

Synthesis and biological evaluation of novel sulfonyl-naphthalene-1,4-diols as FabH inhibitors

A series of analogs of 2-tosylnaphthalene-1,4-diol were prepared and were found to be potent 10-20 nM reversible inhibitors of the Escherichia coli FabH enzyme. The inhibitors were also effective but to a lesser degree (30 nM-5 microM), against the Mycobacterium tuberculosis and Plasmodium falciparum FabH enzymes. Preliminary SAR studies demonstrated that the sulfonyl group and naphthalene-1,4 diol were required for activity against all enzymes but the toluene portion could be significantly altered and leads to either modest increases or decreases in activity against the three enzymes. The in vitro activity of the analogs against E. coli FabH parallel the in vivo activity against E. coli TolC strain and many of the compounds were also shown to have antimalarial activity against P. falciparum.

Malonyl-CoA: acyl carrier protein transacylase from Helicobacter pylori: Crystal structure and its interaction with acyl carrier protein

Malonyl-CoA: acyl carrier protein transacylase (MCAT) is a critical enzyme responsible for the transfer of the malonyl moiety to holo-acyl carrier protein (ACP) forming the malonyl-ACP intermediates in the initiation step of type II fatty acid synthesis (FAS II) in bacteria. MCAT has been considered as an attractive drug target in the discovery of antibacterial agents. In this study, the crystal structure of MCAT from Helicobacter pylori (Hp) at 2.5 angstroms resolution is reported, and the interaction of HpMCAT with HpACP is extensively investigated by using computational docking, GST-pull-down, and surface plasmon resonance (SPR) technology-based assays. The crystal structure results reveal that HpMCAT has a compact folding composed of a large subdomain with a similar core as in alpha/beta hydrolases, and a similar ferredoxin-like small subdomain as in acylphosphatases. The docking result suggests two positively charged areas near the entrance of the active site of HpMCAT as the ACP-binding region. Binding assay research shows that HpMCAT demonstrates a moderately binding ability against HpACP. The solved 3D structure of HpMCAT is expected to supply useful information for the structure-based discovery of novel inhibitors against MCAT, and the quantitative study of HpMCAT interaction with HpACP is hoped to give helpful hints in the understanding of the detailed catalytic mechanisms for HpMCAT.

Mass spectrometry-based systems approach for identification of inhibitors of Plasmodium falciparum fatty acid synthase

The emergence of strains of Plasmodium falciparum resistant to the commonly used antimalarials warrants the development of new antimalarial agents. The discovery of type II fatty acid synthase (FAS) in Plasmodium distinct from the FAS in its human host (type I FAS) opened up new avenues for the development of novel antimalarials. The process of fatty acid synthesis takes place by iterative elongation of butyryl-acyl carrier protein (butyryl-ACP) by two carbon units, with the successive action of four enzymes constituting the elongation module of FAS until the desired acyl length is obtained. The study of the fatty acid synthesis machinery of the parasite inside the red blood cell culture has always been a challenging task. Here, we report the in vitro reconstitution of the elongation module of the FAS of malaria parasite involving all four enzymes, FabB/F (beta-ketoacyl-ACP synthase), FabG (beta-ketoacyl-ACP reductase), FabZ (beta-ketoacyl-ACP dehydratase), and FabI (enoyl-ACP reductase), and its analysis by matrix-assisted laser desorption-time of flight mass spectrometry (MALDI-TOF MS). That this in vitro systems approach completely mimics the in vivo machinery is confirmed by the distribution of acyl products. Using known inhibitors of the enzymes of the elongation module, cerulenin, triclosan, NAS-21/91, and (-)-catechin gallate, we demonstrate that accumulation of intermediates resulting from the inhibition of any of the enzymes can be unambiguously followed by MALDI-TOF MS. Thus, this work not only offers a powerful tool for easier and faster throughput screening of inhibitors but also allows for the study of the biochemical properties of the FAS pathway of the malaria parasite.

In vivo functional analyses of the type II acyl carrier proteins of fatty acid biosynthesis

Acyl carrier protein (ACP) is a key component of the fatty acid synthesis pathways of both type I and type II synthesis systems. A large number of structure-function studies of various type II ACPs have been reported, but all are in vitro studies that assayed function or interaction of mutant ACPs with various enzymes of fatty acid synthesis or transfer. Hence in these studies functional properties of various mutant ACPs were assayed with only a subset of the many ACP-interacting proteins, which may not give an accurate overall view of the function of these proteins in vivo. This is especially so because Escherichia coli ACP has been reported to interact with several proteins that have no known roles in lipid metabolism. We therefore tested a large number of mutant derivatives of E. coli ACP carrying single amino acid substitutions for their abilities to restore growth to an E. coli strain carrying a temperature-sensitive mutation in acpP, the gene that encodes ACP. Many of these mutant proteins had previously been tested in vitro thus providing data for comparison with our results. We found that several mutant ACPs containing substitutions of ACP residues reported previously to be required for ACP function in vitro support normal growth of the acpP mutant strain. However, several mutant proteins reported to be severely defective in vitro failed to support growth of the acpP strain in vivo (or supported only weak growth). A collection of ACPs from diverse bacteria and from three eukaryotic organelles was also tested. All of the bacterial ACPs tested restored growth to the E. coli acpP mutant strain except those from two related bacteria, Enterococcus faecalis and Lactococcus lactis. Only one of the three eukaryotic organellar ACPs allowed growth. Strikingly the ACP is that of the apicoplast of Plasmodium falciparum (the protozoan that causes malaria). The fact that an ACP from a such diverse organism can replace AcpP function in E. coli suggests that some of the protein-protein interactions detected for AcpP may be not be essential for growth of E. coli.

Alkyl-CoA Disulfides as Inhibitors and Mechanistic Probes for FabH Enzymes

The first step of the reaction catalyzed by the homodimeric FabH from a dissociated fatty acid synthase is acyl transfer from acyl-CoA to an active site cysteine. We report that C1 to C10 alkyl-CoA disulfides irreversibly inhibit Escherichia coli FabH (ecFabH) and Mycobacterium tuberculosis FabH with relative efficiencies that reflect these enzymes' differential acyl-group specificity. Crystallographic and kinetic studies with MeSSCoA show rapid inhibition of one monomer of ecFabH through formation of a methyl disulfide conjugate with this cysteine. Reaction of the second subunit with either MeSSCoA or acetyl-CoA is much slower. In the presence of malonyl-ACP, the acylation rate of the second subunit is restored to that of the native ecFabH. These observations suggest a catalytic model in which a structurally disordered apo-ecFabH dimer orders on binding either the first substrate, acetyl-CoA, or the inhibitor MeSSCoA, and is restored to a disordered state on binding of malonyl-ACP.

The ubiquitous carrier protein--a window to metabolite biosynthesis

Nature has developed a remarkable strategy to isolate metabolites from the milieu of the cell for chemical modification through the use of carrier proteins. Common to both primary and secondary metabolic pathways, acyl-carrier proteins constitute a conserved protein architecture which mediate the biosynthesis of a variety of metabolic products. Analogies have been made between the carrier protein and solid phase resin for chemical synthesis, as both entities provide a mechanism to separate compounds of interest from complex mixtures for selective chemical modification. However, there is significantly more to the carrier protein than an attachment point. In this review, we aim to systematically characterize the role of carrier proteins in various metabolic pathways and outline their utility in biosynthesis and biotechnology; 185 references are cited.

Synthesis and biological evaluation of thiazolidine-2-one 1,1-dioxide as inhibitors of Escherichia coli β-ketoacyl-ACP-synthase III (FabH)

A series of cyclic sulfones has been synthesized and their activity against beta-ketoacyl-ACP-synthase III (FabH) has been investigated. The compounds are selectively active against Escherichia coli FabH (ecFabH), but not Mycobacterium tuberculosis FabH (mtFabH) or Plasmodium falciparum KASIII (PfKASIII). The activity against ecFabH ranges from 0.9 to >100microM and follows a consistent general SAR trend. Many of the compounds were shown to have antimalarial activity against chloroquine (CQ)-sensitive (D6) P. falciparum (IC(50)=5.3microM for the most potent inhibitor) and some were active against E. coli (MIC=6.6microg/ml for the most potent inhibitor).

Antimalarial drugs - host targets (re)visited

Every year, forty percent of the world population is at risk of contracting malaria. Hopes for the erradication of this disease during the 20th century were dashed by the ability of Plasmodium falciparum, its most deadly causative agent, to develop resistance to available drugs. Efforts to produce an effective vaccine have so far been unsuccessful, enhancing the need to develop novel antimalarial drugs. In this review, we summarize our knowledge concerning existing antimalarials, mechanisms of drug-resistance development, the use of drug combination strategies and the quest for novel anti-plasmodial compounds. We emphasize the potential role of host genes and molecules as novel targets for newly developed drugs. Recent results from our laboratory have shown Hepatocyte Growth Factor/MET signaling to be essential for the establishment of infection in hepatocytes. We discuss the potential use of this pathway in the prophylaxis of malaria infection.

Membrane transporters in the relict plastid of malaria parasites

Malaria parasites contain a nonphotosynthetic plastid homologous to chloroplasts of plants. The parasite plastid synthesizes fatty acids, heme, iron sulfur clusters and isoprenoid precursors and is indispensable, making it an attractive target for antiparasite drugs. How parasite plastid biosynthetic pathways are fuelled in the absence of photosynthetic capture of energy and carbon was not clear. Here, we describe a pair of parasite transporter proteins, PfiTPT and PfoTPT, that are homologues of plant chloroplast innermost membrane transporters responsible for moving phosphorylated C3, C5, and C6 compounds across the plant chloroplast envelope. PfiTPT is shown to be localized in the innermost membrane of the parasite plastid courtesy of a cleavable N-terminal targeting sequence. PfoTPT lacks such a targeting sequence, but is shown to localize in the outermost parasite plastid membrane with its termini projecting into the cytosol. We have identified these membrane proteins in the parasite plastid and determined membrane orientation for PfoTPT. PfiTPT and PfoTPT are proposed to act in tandem to transport phosphorylated C3 compounds from the parasite cytosol into the plastid. Thus, the transporters could shunt glycolytic derivatives of glucose scavenged from the host into the plastid providing carbon, reducing equivalents and ATP to power the organelle.

The plastid of Plasmodium spp.: a target for inhibitors

Determined efforts are being made to explore the non-photosynthetic plastid organelle of Plasmodium falciparum as a target for drug development. Certain antibiotics that block organellar protein synthesis are already in clinical use as antimalarials. However, all the indications are that these should be used only in combination with conventional antimalarials. The use of antibiotics such as doxycycline and clindamycin may reduce the development of drug resistant parasites and such means to avoid drug resistance should be explored hand-in-hand with drug development. Genomic information predicts that fatty acid type II (FAS II) and isoprenoid biosynthetic pathways are localized to the plastid. However, clinical trials with fosmidomycin (a specific inhibitor of DOXP reductase in the non-mevalonate pathway for isoprenoids) suggest it too should only be used in drug combinations. Prospects for more potent antimalarial compounds have emerged from studies of several of the enzymes involved in the FAS II pathway. Lead antibiotics such as thiolactomycin (an inhibitor of beta-ketoacyl-ACP synthase) and triclosan (a specific inhibitor of enoyl-ACP reductase) have led to structurally similar, active compounds that rapidly kill ring- and trophozoite-stage parasites. The FAS II pathway is of particular interest to the pharma-industry.

Recombinant expression and biochemical characterization of the unique elongating beta-ketoacyl-acyl carrier protein synthase involved in fatty acid biosynthesis of Plasmodium falciparum using natural and artificial substrates

The human malaria parasite Plasmodium falciparum synthesizes fatty acids by using a type II synthase that is structurally different from the type I system found in eukaryotes. Because of this difference and the vital role of fatty acids, the enzymes involved in fatty acid biosynthesis of P. falciparum represent interesting targets for the development of new antimalarial drugs. beta-Ketoacyl-acyl carrier protein (ACP) synthase (PfFabBF), being the only elongating beta-ketoacyl-ACP synthase in P. falciparum, is a potential candidate for inhibition. In this study we present the cloning, expression, purification, and characterization of PfFabBF. Soluble protein was obtained when PfFabBF was expressed as a NusA fusion protein in Escherichia coli BL21(DE3)-CodonPlus-RIL cells under conditions of osmotic stress. The fusion protein was purified by affinity and ion exchange chromatography. Various acyl-P. falciparum acyl carrier protein (PfACP) substrates were tested for their specific activities, and their kinetic parameters were determined. Activity of PfFabBF was highest with C(4:0)- to C(10:0)-acyl-PfACPs and decreased with use of longer chain acyl-PfACPs. Consistent with the fatty acid synthesis profile found in the parasite cell, no activity could be detected with C(16:0)-PfACP, indicating that the enzyme is lacking the capability of elongating acyl chains that are longer than 14 carbon atoms. PfFabBF was found to be specific for acyl-PfACPs, and it displayed much lower activities with the corresponding acyl-CoAs. Furthermore, PfFabBF was shown to be sensitive to cerulenin and thiolactomycin, known inhibitors of beta-ketoacyl-ACP synthases. These results represent an important step toward the evaluation of P. falciparum beta-ketoacyl-ACP synthase as a novel antimalaria target.

Kinetic, inhibition and structural studies on 3-oxoacyl-ACP reductase from Plasmodium falciparum, a key enzyme in fatty acid biosynthesis

Type II fatty acid biosynthesis represents an attractive target for the discovery of new antimalarial drugs. Previous studies have identified malarial ENR (enoyl acyl-carrier-protein reductase, or FabI) as the target for the antiseptic triclosan. In the present paper, we report the biochemical properties and 1.5 A (1 A=0.1 nm) crystal structure of OAR (3-oxoacyl acyl-carrier-protein reductase, or FabG), the second reductive step in fatty acid biosynthesis and its inhibition by hexachlorophene. Under optimal conditions of pH and ionic strength, Plasmodium falciparum OAR displays kinetic properties similar to those of OAR from bacteria or plants. Activity with NADH is <3% of that with NADPH. Fluorescence enhancement studies indicate that NADPH can bind to the free enzyme, consistent with kinetic and product inhibition studies suggesting a steady-state ordered mechanism. The crystal structure reveals a tetramer with a sulphate ion bound in the cofactor-binding site such that the side chains of the catalytic triad of serine, tyrosine and lysine are aligned in an active conformation, as previously observed in the Escherichia coli OAR-NADP+ complex. A cluster of positively charged residues is positioned at the entrance to the active site, consistent with the proposed recognition site for the physiological substrate (3-oxoacyl-acyl-carrier protein) in E. coli OAR. The antibacterial and anthelminthic agent hexachlorophene is a potent inhibitor of OAR (IC50 2.05 microM) displaying non-linear competitive inhibition with respect to NADPH. Hexachlorophene (EC50 6.2 microM) and analogues such as bithionol also have antimalarial activity in vitro, suggesting they might be useful leads for further development.