Purification and characterization of recombinant Plasmodium falciparum adenylosuccinate synthetase expressed in Escherichia coli

Location Is Everything: Influence of His-Tag Fusion Site on Properties of Adenylosuccinate Synthetase from Helicobacter pylori

The requirement for fast and dependable protein purification methods is constant, either for functional studies of natural proteins or for the production of biotechnological protein products. The original procedure has to be formulated for each individual protein, and this demanding task was significantly simplified by the introduction of affinity tags. Helicobacter pylori adenylosuccinate synthetase (AdSS) is present in solution in a dynamic equilibrium of monomers and biologically active homodimers. The addition of the His6-tag on the C-terminus (C-His-AdSS) was proven to have a negligible effect on the characteristics of this enzyme. This paper shows that the same enzyme with the His6-tag fused on its N-terminus (N-His-AdSS) has a high tendency to precipitate. Circular dichroism and X-ray diffraction studies do not detect any structural change that could explain this propensity. However, the dynamic light scattering, differential scanning fluorimetry, and analytical ultracentrifugation measurements indicate that the monomer of this construct is prone to aggregation, which shifts the equilibrium towards the insoluble precipitant. In agreement, enzyme kinetics measurements showed reduced enzyme activity, but preserved affinity for the substrates, in comparison with the wild-type and C-His-AdSS. The presented results reinforce the notion that testing the influence of the tag on protein properties should not be overlooked.

ZmAdSS1 encodes adenylosuccinate synthetase and plays a critical role in maize seed development and the accumulation of nutrients

Adenylosuccinate synthetase (AdSS, EC.6.3.4.4) is a key enzyme in the de novo synthesis of purine nucleotides in organisms. Its downstream product AMP plays a critical role in the process of energy metabolism, which can affect the content of ADP and ATP. However, impacts of its loss-of-function on plant metabolism and development has been relatively poorly reported. Here, we report the identification and analysis of a maize yu18 mutant obtained by mutagenesis with ethylmethane sulfonate (EMS). The yu18 is a lethal-seed mutant. Map-based cloning and allelic testing confirmed that yu18 encodes adenylosuccinate synthetase and was named ZmAdSS1. ZmAdSS1 is constitutively expressed. In the yu18 mutant, the activity of the ZmAdSS1 enzyme was decreased, which caused AMP content reduced 33.62%. The yu18 mutation significantly suppressed endoreduplication and disrupted nutrient accumulation, resulting in lower starch and protein contents that are responsible for seed filling. Further transcriptome and metabolome analysis revealed dramatic alterations in the carbohydrate metabolic pathway and amino acid metabolic pathway in yu18 kernels. Our findings demonstrate that ZmAdSS1 participates in the synthesis of AMP and affects endosperm development and nutrient accumulation in maize seeds.

Metabolic changes accompanying the loss of fumarate hydratase and malate-quinone oxidoreductase in the asexual blood stage of Plasmodium falciparum

In the glucose-rich milieu of red blood cells, asexually replicating malarial parasites mainly rely on glycolysis for ATP production, with limited carbon flux through the mitochondrial tricarboxylic acid (TCA) cycle. By contrast, gametocytes and mosquito-stage parasites exhibit an increased dependence on the TCA cycle and oxidative phosphorylation for more economical energy generation. Prior genetic studies supported these stage-specific metabolic preferences by revealing that six of eight TCA cycle enzymes are completely dispensable during the asexual blood stages of Plasmodium falciparum, with only fumarate hydratase (FH) and malate-quinone oxidoreductase (MQO) being refractory to deletion. Several hypotheses have been put forth to explain the possible essentiality of FH and MQO, including their participation in a malate shuttle between the mitochondrial matrix and the cytosol. However, using newer genetic techniques like CRISPR and dimerizable Cre, we were able to generate deletion strains of FH and MQO in P. falciparum. We employed metabolomic analyses to characterize a double knockout mutant of FH and MQO (ΔFM) and identified changes in purine salvage and urea cycle metabolism that may help to limit fumarate accumulation. Correspondingly, we found that the ΔFM mutant was more sensitive to exogenous fumarate, which is known to cause toxicity by modifying and inactivating proteins and metabolites. Overall, our data indicate that P. falciparum is able to adequately compensate for the loss of FH and MQO, rendering them unsuitable targets for drug development.

[A family of bacteriophages uses an expanded genetic alphabet]

Bacteriophage genomes are the richest source of modified nucleobases of any life form. Of these, 2,6-diaminopurine (2-aminoadénine) that pairs with thymine by forming three hydrogen bonds is the only one violating Watson and Crick's base pairing. 2,6-diaminopurine (2-aminoadénine), initially found in the cyanophage S-2L, is more widespread than expected and has also been detected in bacteriophage infecting Gram-negative and Gram-positive bacteria. The biosynthetic pathway for aminoadenine containing DNA as well as the exclusion of adenine are now elucidated. This example of a natural deviation from the DNA canonical nucleotides represents only one of the possibilities explored by nature and provides a proof of concept for the synthetic biology of non-canonical nucleic acids.

Mechanisms supporting aminoadenine-based viral DNA genomes

Bacteriophage genomes are the richest source of modified nucleobases of any life form. Of these, 2,6 diaminopurine, which pairs with thymine by forming three hydrogen bonds violates Watson and Crick's base pairing. 2,6 diaminopurine initially found in the cyanophage S-2L is more widespread than expected and has also been detected in phage infecting Gram-negative and Gram-positive bacteria. The biosynthetic pathway for aminoadenine containing DNA as well as the exclusion of adenine are now elucidated. This example of a natural deviation from the genetic code represents only one of the possibilities explored by nature and provides a proof of concept for the synthetic biology of non-canonical nucleic acids.

Characterization of a triad of genes in cyanophage S-2L sufficient to replace adenine by 2-aminoadenine in bacterial DNA

Cyanophage S-2L is known to profoundly alter the biophysical properties of its DNA by replacing all adenines (A) with 2-aminoadenines (Z), which still pair with thymines but with a triple hydrogen bond. It was recently demonstrated that a homologue of adenylosuccinate synthetase (PurZ) and a dATP triphosphohydrolase (DatZ) are two important pieces of the metabolism of 2-aminoadenine, participating in the synthesis of ZTGC-DNA. Here, we determine that S-2L PurZ can use either dATP or ATP as a source of energy, thereby also depleting the pool of nucleotides in dATP. Furthermore, we identify a conserved gene (mazZ) located between purZ and datZ genes in S-2L and related phage genomes. We show that it encodes a (d)GTP-specific diphosphohydrolase, thereby providing the substrate of PurZ in the 2-aminoadenine synthesis pathway. High-resolution crystal structures of S-2L PurZ and MazZ with their respective substrates provide a rationale for their specificities. The Z-cluster made of these three genes - datZ, mazZ and purZ - was expressed in E. coli, resulting in a successful incorporation of 2-aminoadenine in the bacterial chromosomal and plasmidic DNA. This work opens the possibility to study synthetic organisms containing ZTGC-DNA.

Antimalarial Properties of Isoquinoline Derivative from Streptomyces hygroscopicus subsp. Hygroscopicus: An In Silico Approach

Malaria is one of the life-threatening diseases in the world. The spread of resistance to antimalarial drugs is a major challenge, and resistance to artemisinin has been reported in the Southeast Asian region. In the previous study, the active compound of Streptomyces hygroscopicus subsp. Hygroscopicus (S. hygroscopicus), eponemycin, has been shown to have antimalarial effects. To further analyze the effects of other active compounds on the Plasmodium parasite, identifying and analyzing the effectiveness of compounds contained in S. hygroscopicus through instrumentation of liquid chromatography/mass spectrometry (LC/MS) and in silico studies were very useful. This study aimed at identifying other derivative compounds from S. hygroscopicus and screening the antimalarial activity of the compound by assessing the binding affinity, pharmacokinetic profile, and bond interaction. The derivative compounds were identified using LC/MS. Protein targets for derivative compounds were found through literature studies, and the results of identification of compounds and protein targets were reconstructed into three-dimensional models. Prediction of pharmacokinetic profiles was carried out using Swiss ADME. Screening of protein targets for the derivative compound was carried out using the reverse molecular docking method. Analyzing bond interaction was done by LigPlot. One compound from S. hygroscopicus, i.e., 6,7-dinitro-2-[1, 2, 4]triazole-4-yl-benzo[de]isoquinoline-1,3-dione, was successfully identified using LC/MS. This compound was an isoquinoline derivative compound. Through literature studies with inclusion criteria, thirteen protein targets were obtained for reverse molecular docking. This isoquinoline derivative had the potential to bind to each protein target. The pharmacokinetic profile showed that this compound had the drug-likeness criteria. Conclusion. 6,7-Dinitro-2-[1, 2, 4]triazole-4-yl-benzo[de]isoquinoline-1,3-dione has antimalarial activity as shown by reverse molecular docking studies and pharmacokinetic profiles. The best inhibitory ability of compounds based on bond affinity is with adenylosuccinate synthetase.

Genetic ablation of purine salvage in Cryptosporidium parvum reveals nucleotide uptake from the host cell

The apicomplexan parasite Cryptosporidium is a leading global cause of severe diarrheal disease and an important contributor to early-childhood mortality. Waterborne outbreaks occur frequently, even in countries with advanced water treatment capabilities, and there is currently no fully effective treatment. Nucleotide pathways are attractive targets for antimicrobial development, and several laboratories are designing inhibitors of these enzymes as potential treatment for Cryptosporidium infections. Here we take advantage of newly available molecular genetics for Cryptosporidium parvum to investigate nucleotide biosynthesis by directed gene ablation. Surprisingly, we found that the parasite tolerates the loss of classical targets including dihydrofolate reductase-thymidylate synthase (DHFR-TS) and inosine monophosphate dehydrogenase (IMPDH). We show that thymidine kinase provides a route to thymidine monophosphate in the absence of DHFR-TS. In contrast, only a single pathway has been identified for C. parvum purine nucleotide salvage. Nonetheless, multiple enzymes in the purine pathway, as well as the adenosine transporter, can be ablated. The resulting mutants are viable under normal conditions but are hypersensitive to inhibition of purine nucleotide synthesis in their host cell. Cryptosporidium might use as-yet undiscovered purine transporters and salvage enzymes; however, genetic and pharmacological experiments led us to conclude that Cryptosporidium imports purine nucleotides from the host cell. The potential for ATP uptake from the host has significant impact on our understanding of parasite energy metabolism given that Cryptosporidium lacks oxidative phosphorylation and glycolytic enzymes are not constitutively expressed throughout the parasite life cycle.

Biochemical and physiological investigations on adenosine 5' monophosphate deaminase from Plasmodium spp

The interplay between ATP generating and utilizing pathways in a cell is responsible for maintaining cellular ATP/energy homeostasis that is reflected by Adenylate Energy Charge (AEC) ratio. Adenylate kinase (AK), that catalyzes inter-conversion of ADP, ATP and AMP, plays a major role in maintaining AEC and is regulated by cellular AMP levels. Hence, the enzymes AMP deaminase (AMPD) and nucleotidases, which catabolize AMP, indirectly regulate AK activity and in-turn affect AEC. Here, we present the first report on AMPD from Plasmodium, the causative agent of malaria. The recombinant enzyme expressed in Saccharomyces cerevisiae was studied using functional complementation assay and residues vital for enzyme activity have been identified. Similarities and differences between Plasmodium falciparum AMPD (PfAMPD) and its homologs from yeast, Arabidopsis and humans are also discussed. The AMPD gene was deleted in the murine malaria parasite P. berghei and was found to be dispensable during all stages of the parasite life cycle. However, when episomal expression was attempted, viable parasites were not obtained, suggesting that perturbing AMP homeostasis by over-expressing AMPD might be lethal. As AMPD is known to be allosterically modulated by ATP, GTP and phosphate, allosteric activators of PfAMPD could be developed as anti-parasitic agents.

In the quest for new targets for pathogen eradication: the adenylosuccinate synthetase from the bacterium Helicobacter pylori

Adenylosuccinate synthetase (AdSS) is an enzyme at regulatory point of purine metabolism. In pathogenic organisms which utilise only the purine salvage pathway, AdSS asserts itself as a promising drug target. One of these organisms is Helicobacter pylori, a wide-spread human pathogen involved in the development of many diseases. The rate of H. pylori antibiotic resistance is on the increase, making the quest for new drugs against this pathogen more important than ever. In this context, we describe here the properties of H. pylori AdSS. This enzyme exists in a dimeric active form independently of the presence of its ligands. Its narrow stability range and pH-neutral optimal working conditions reflect the bacterium’s high level of adaptation to its living environment. Efficient inhibition of H. pylori AdSS with hadacidin and adenylosuccinate gives hope of finding novel drugs that aim at eradicating this dangerous pathogen.

Disruption of de Novo Adenosine Triphosphate (ATP) Biosynthesis Abolishes Virulence in Cryptococcus neoformans

Opportunistic fungal pathogens such as Cryptococcus neoformans are a growing cause of morbidity and mortality among immunocompromised populations worldwide. To address the current paucity of antifungal therapeutic agents, further research into fungal-specific drug targets is required. Adenylosuccinate synthetase (AdSS) is a crucial enzyme in the adeosine triphosphate (ATP) biosynthetic pathway, catalyzing the formation of adenylosuccinate from inosine monophosphate and aspartate. We have investigated the potential of this enzyme as an antifungal drug target, finding that loss of function results in adenine auxotrophy in C. neoformans, as well as complete loss of virulence in a murine model. Cryptococcal AdSS was expressed and purified in Escherichia coli and the enzyme's crystal structure determined, the first example of a structure of this enzyme from fungi. Together with enzyme kinetic studies, this structural information enabled comparison of the fungal enzyme with the human orthologue and revealed species-specific differences potentially exploitable via rational drug design. These results validate AdSS as a promising antifungal drug target and lay a foundation for future in silico and in vitro screens for novel antifungal compounds.

Adenylosuccinate synthetase and adenylosuccinate lyase deficiencies trigger growth and infectivity deficits in Leishmania donovani

Leishmania are auxotrophic for purines, and consequently purine acquisition from the host is a requisite nutritional function for the parasite. Both adenylosuccinate synthetase (ADSS) and adenylosuccinate lyase (ASL) have been identified as vital components of purine salvage in Leishmania donovani, and therefore Δadss and Δasl null mutants were constructed to test this hypothesis. Unlike wild type L. donovani, Δadss and Δasl parasites in culture exhibited a profoundly restricted growth phenotype in which the only permissive growth conditions were a 6-aminopurine source in the presence of 2'-deoxycoformycin, an inhibitor of adenine aminohydrolase activity. Although both knock-outs showed a diminished capacity to infect murine peritoneal macrophages, only the Δasl null mutant was profoundly incapacitated in its ability to infect mice. The enormous discrepancy in parasite loads observed in livers and spleens from mice infected with either Δadss or Δasl parasites can be explained by selective accumulation of adenylosuccinate in the Δasl knock-out and consequent starvation for guanylate nucleotides. Genetic complementation of a Δasl lesion in Escherichia coli implied that the L. donovani ASL could also recognize 5-aminoimidazole-(N-succinylocarboxamide) ribotide as a substrate, and purified recombinant ASL displayed an apparent Km of ∼24 μm for adenylosuccinate. Unlike many components of the purine salvage pathway of L. donovani, both ASL and ADSS are cytosolic enzymes. Overall, these data underscore the paramount importance of ASL to purine salvage by both life cycle stages of L. donovani and authenticate ASL as a potential drug target in Leishmania.

Mutational analysis of cysteine 328 and cysteine 368 at the interface of Plasmodium falciparum adenylosuccinate synthetase

Plasmodium falciparum adenylosuccinate synthetase, a homodimeric enzyme, contains 10 cysteine residues per subunit. Among these, Cys250, Cys328 and Cys368 lie at the dimer interface and are not conserved across organisms. PfAdSS has a positively charged interface with the crystal structure showing additional electron density around Cys328 and Cys368. Biochemical characterization of site directed mutants followed by equilibrium unfolding studies permits elucidation of the role of interface cysteines and positively charged interface in dimer stability. Mutation of interface cysteines, Cys328 and Cys368 to serine, perturbed the monomer-dimer equilibrium in the protein with a small population of monomer being evident in the double mutant. Introduction of negative charge in the form of C328D mutation resulted in stabilization of protein dimer as evident by size exclusion chromatography at high ionic strength buffer and equilibrium unfolding in the presence of urea. These observations suggest that cysteines at the dimer interface of PfAdSS may indeed be charged and exist as thiolate anion.

Overexpression, purification, crystallization and preliminary crystallographic studies of a hyperthermophilic adenylosuccinate synthetase from Pyrococcus horikoshii OT3

Adenylosuccinate synthetase (AdSS) is a ubiquitous enzyme that catalyzes the first committed step in the conversion of inosine monophosphate (IMP) to adenosine monophosphate (AMP) in the purine-biosynthetic pathway. Although AdSS from the vast majority of organisms is 430-457 amino acids in length, AdSS sequences isolated from thermophilic archaea are 90-120 amino acids shorter. In this study, crystallographic studies of a short AdSS sequence from Pyrococcus horikoshii OT3 (PhAdSS) were performed in order to reveal the unusual structure of AdSS from thermophilic archaea. Crystals of PhAdSS were obtained by the microbatch-under-oil method and X-ray diffraction data were collected to 2.50 Å resolution. The crystal belonged to the trigonal space group P3(2)12, with unit-cell parameters a = b = 57.2, c = 107.9 Å. There was one molecule per asymmetric unit, giving a Matthews coefficient of 2.17 Å(3) Da(-1) and an approximate solvent content of 43%. In contrast, the results of native polyacrylamide gel electrophoresis and analytical ultracentrifugation showed that the recombinant PhAdSS formed a dimer in solution.

Identification of inhibitors for putative malaria drug targets among novel antimalarial compounds

The efficacy of most marketed antimalarial drugs has been compromised by evolution of parasite resistance, underscoring an urgent need to find new drugs with new mechanisms of action. We have taken a high-throughput approach toward identifying novel antimalarial chemical inhibitors of prioritized drug targets for Plasmodium falciparum, excluding targets which are inhibited by currently used drugs. A screen of commercially available libraries identified 5655 low molecular weight compounds that inhibit growth of P. falciparum cultures with EC(50) values below 1.25μM. These compounds were then tested in 384- or 1536-well biochemical assays for activity against nine Plasmodium enzymes: adenylosuccinate synthetase (AdSS), choline kinase (CK), deoxyuridine triphosphate nucleotidohydrolase (dUTPase), glutamate dehydrogenase (GDH), guanylate kinase (GK), N-myristoyltransferase (NMT), orotidine 5'-monophosphate decarboxylase (OMPDC), farnesyl pyrophosphate synthase (FPPS) and S-adenosylhomocysteine hydrolase (SAHH). These enzymes were selected using TDRtargets.org, and are believed to have excellent potential as drug targets based on criteria such as their likely essentiality, druggability, and amenability to high-throughput biochemical screening. Six of these targets were inhibited by one or more of the antimalarial scaffolds and may have potential use in drug development, further target validation studies and exploration of P. falciparum biochemistry and biology.

Use of thermal melt curves to assess the quality of enzyme preparations

This study sought to determine whether the quality of enzyme preparations can be determined from their melting curves, which may easily be obtained using a fluorescent probe and a standard reverse transcription-polymerase chain reaction (RT-PCR) machine. Thermal melt data on 31 recombinant enzymes from Plasmodium parasites were acquired by incrementally heating them to 90 degrees C and measuring unfolding with a fluorescent dye. Activity assays specific to each enzyme were also performed. Four of the enzymes were denatured to varying degrees with heat and sodium dodecyl sulfate (SDS) prior to the thermal melt and activity assays. In general, melting curve quality was correlated with enzyme activity; enzymes with high-quality curves were found almost uniformly to be active, whereas those with lower quality curves were more varied in their catalytic performance. Inspection of melting curves of bovine xanthine oxidase and Entamoeba histolytica cysteine protease 1 allowed active stocks to be distinguished from inactive stocks, implying that a relationship between melting curve quality and activity persists over a wide range of experimental conditions and species. Our data suggest that melting curves can help to distinguish properly folded proteins from denatured ones and, therefore, may be useful in selecting stocks for further study and in optimizing purification procedures for specific proteins.

Heterologous expression of plasmodial proteins for structural studies and functional annotation

Malaria remains the world's most devastating tropical infectious disease with as many as 40% of the world population living in risk areas. The widespread resistance of Plasmodium parasites to the cost-effective chloroquine and antifolates has forced the introduction of more costly drug combinations, such as Coartem^®. In the absence of a vaccine in the foreseeable future, one strategy to address the growing malaria problem is to identify and characterize new and durable antimalarial drug targets, the majority of which are parasite proteins. Biochemical and structure-activity analysis of these proteins is ultimately essential in the characterization of such targets but requires large amounts of functional protein. Even though heterologous protein production has now become a relatively routine endeavour for most proteins of diverse origins, the functional expression of soluble plasmodial proteins is highly problematic and slows the progress of antimalarial drug target discovery. Here the status quo of heterologous production of plasmodial proteins is presented, constraints are highlighted and alternative strategies and hosts for functional expression and annotation of plasmodial proteins are reviewed.

Kinetic characterization of adenylosuccinate synthetase from the thermophilic archaea Methanocaldococcus jannaschii

Adenylosuccinate synthetase (AdSS) catalyzes the Mg2+ dependent condensation of a molecule of IMP with aspartate to form adenylosuccinate, in a reaction driven by the hydrolysis of GTP to GDP. AdSS from the thermophilic archaea, Methanocaldococcus jannaschii (MjAdSS) is 345 amino acids long against an average length of 430-457 amino acids for most mesophilic AdSS. This short AdSS has two large deletions that map to the middle and C-terminus of the protein. This article discusses the detailed kinetic characterization of MjAdSS. Initial velocity and product inhibition studies, carried out at 70 degrees C, suggest a rapid equilibrium random AB steady-state ordered C kinetic mechanism for the MjAdSS catalyzed reaction. AdSS are known to exhibit monomer-dimer equilibrium with the dimer being implicated in catalysis. In contrast, our studies show that MjAdSS is an equilibrium mixture of dimers and tetramers with the tetramer being the catalytically active form. The tetramer dissociates into dimers with a minor increase in ionic strength of the buffer, while the dimer is extremely stable and does not dissociate even at 1.2 M NaCl. Phosphate, a product of the reaction, was found to be a potent inhibitor of MjAdSS showing biphasic inhibition of enzyme activity. The inhibition was competitive with IMP and noncompetitive with GTP. MjAdSS, like the mouse acidic isozyme, exhibits substrate inhibition, with IMP inhibiting enzyme activity at subsaturating GTP concentrations. Regulation of enzyme activity by the glycolytic intermediate, fructose 1,6 bisphosphate, was also observed with the inhibition being competitive with IMP and noncompetitive against GTP.

Targeting purine and pyrimidine metabolism in human apicomplexan parasites

Synthesis de novo, acquisition by salvage and interconversion of purines and pyrimidines represent the fundamental requirements for their eventual assembly into nucleic acids as nucleotides and the deployment of their derivatives in other biochemical pathways. A small number of drugs targeted to nucleotide metabolism, by virtue of their effect on folate biosynthesis and recycling, have been successfully used against apicomplexan parasites such as Plasmodium and Toxoplasma for many years, although resistance is now a major problem in the prevention and treatment of malaria. Many targets not involving folate metabolism have also been explored at the experimental level. However, the unravelling of the genome sequences of these eukaryotic unicellular organisms, together with increasingly sophisticated molecular analyses, opens up possibilities of introducing new drugs that could interfere with these processes. This review examines the status of established drugs of this type and the potential for further exploiting the vulnerability of apicomplexan human pathogens to inhibition of this key area of metabolism.

Unique kinetic mechanism of Plasmodium falciparum adenylosuccinate synthetase

Adenylosuccinate synthetase (AdSS) catalyses the Mg(2+) dependent formation of adenylosuccinate from IMP and aspartate, the reaction being driven by the hydrolysis of GTP to GDP. All characterized AdSS thus far exhibit a random kinetic mechanism. We present here kinetic evidence that unlike all other AdSS, Plasmodium falciparum AdSS (PfAdSS) has ordered substrate binding. Inhibition studies show that binding of GTP requires IMP binding while aspartate binds to the enzyme-IMP-GTP complex. A structural basis for this difference in mechanism is presented. Kinetically, PfAdSS is closer to the mouse acidic isozyme rather than to the mouse basic isozyme. The mouse acidic isozyme is thought to play a role in the purine nucleotide biosynthetic pathway. Regulation of PfAdSS in vivo can therefore, be expected to be similar to the mouse acidic isozyme, in agreement with the role of PfAdSS as the only pathway for the synthesis of adenine nucleotides in the parasite. However, PfAdSS differs from both the mammalian homologs in that fructose-1,6-bisphosphate, a potent inhibitor of the mammalian enzyme, is an activator of PfAdSS. The differences highlighted here are promising in terms of species-specific drug design, targeting this essential enzyme in the parasite.

Crystal structure of fully ligated adenylosuccinate synthetase from Plasmodium falciparum

In the absence of the de novo purine nucleotide biosynthetic pathway in parasitic protozoa, purine salvage is of primary importance for parasite survival. Enzymes of the salvage pathway are, therefore, good targets for anti-parasitic drugs. Adenylosuccinate synthetase (AdSS), catalysing the first committed step in the synthesis of AMP from IMP, is a potential target for anti-protozoal chemotherapy. We report here the crystal structure of adenylosuccinate synthetase from the malaria parasite, Plasmodium falciparum, complexed to 6-phosphoryl IMP, GDP, Mg2+ and the aspartate analogue, hadacidin at 2 A resolution. The overall architecture of P. falciparum AdSS (PfAdSS) is similar to the known structures from Escherichia coli, mouse and plants. Differences in substrate interactions seen in this structure provide a plausible explanation for the kinetic differences between PfAdSS and the enzyme from other species. Additional hydrogen bonding interactions of the protein with GDP may account for the ordered binding of substrates to the enzyme. The dimer interface of PfAdSS is also different, with a pronounced excess of positively charged residues. Differences highlighted here provide a basis for the design of species-specific inhibitors of the enzyme.