Plasmodium serine hydroxymethyltransferase as a potential anti-malarial target: inhibition studies using improved methods for enzyme production and assay

Structural and functional analysis of two SHMT8 variants associated with soybean cyst nematode resistance

Two amino acid variants in soybean serine hydroxymethyltransferase 8 (SHMT8) are associated with resistance to the soybean cyst nematode (SCN), a devastating agricultural pathogen with worldwide economic impacts on soybean production. SHMT8 is a cytoplasmic enzyme that catalyzes the pyridoxal 5-phosphate-dependent conversion of serine and tetrahydrofolate (THF) to glycine and 5,10-methylenetetrahydrofolate. A previous study of the P130R/N358Y double variant of SHMT8, identified in the SCN-resistant soybean cultivar (cv.) Forrest, showed profound impairment of folate binding affinity and reduced THF-dependent enzyme activity, relative to the highly active SHMT8 in cv. Essex, which is susceptible to SCN. Given the importance of SCN-resistance in soybean agriculture, we report here the biochemical and structural characterization of the P130R and N358Y single variants to elucidate their individual effects on soybean SHMT8. We find that both single variants have reduced THF-dependent catalytic activity relative to Essex SHMT8 (10- to 50-fold decrease in kcat /Km ) but are significantly more active than the P130R/N368Y double variant. The kinetic data also show that the single variants lack THF-substrate inhibition as found in Essex SHMT8, an observation with implications for regulation of the folate cycle. Five crystal structures of the P130R and N358Y variants in complex with various ligands (resolutions from 1.49 to 2.30 Å) reveal distinct structural impacts of the mutations and provide new insights into allosterism. Our results support the notion that the P130R/N358Y double variant in Forrest SHMT8 produces unique and unexpected effects on the enzyme, which cannot be easily predicted from the behavior of the individual variants.

In Silico and In Vitro Potential of FDA-Approved Drugs for Antimalarial Drug Repurposing against Plasmodium Serine Hydroxymethyltransferases

Malaria has spread in many countries, with a 12% increase in deaths after the coronavirus disease 2019 pandemic. Malaria is one of the most concerning diseases in the Greater Mekong subregion, showing increased drug-resistant rates. Serine hydroxymethyltransferase (SHMT), a key enzyme in the deoxythymidylate synthesis pathway, has been identified as a promising antimalarial drug target due to its conserved folate binding pocket. This study used a molecular docking approach to screen 2509 US Food and Drug Administration (FDA)-approved drugs against seven Plasmodium SHMT structures. Eight compounds had significantly lower binding energies than the known SHMT inhibitors pyrazolopyran(+)-86, tetrahydrofolate, and antimalarial drugs, ranging from 4 to 10 kcal/mol. Inhibition assays testing the eight compounds against Plasmodium falciparum SHMT (PfSHMT) showed that amphotericin B was a competitive inhibitor of PfSHMT with a half-maximal inhibitory concentration (IC50) of 106 ± 1 μM. Therefore, a 500 ns molecular dynamics simulation of PfSHMT/PLS/amphotericin B was performed. The backbone root-mean-square deviation of the protein-ligand complex indicated the high complex stability during simulations, supported by its radius of gyration, hydrogen-bond interactions, and number of atom contacts. The appreciable binding affinity of amphotericin B for PfSHMT was indicated by their solvated interaction energy (-11.15 ± 0.09 kcal/mol) and supported by strong ligand-protein interactions (≥80% occurrences) with its essential residues (i.e., Y78, K151, N262, F266, and V365) predicted by pharmacophore modeling and per-residue decomposition free energy methods. Therefore, our findings identify a promising new PfSHMT inhibitor, albeit with less inhibitory activity, and suggest a core structure that differs from that of previous SHMT inhibitors, thus being a rational approach for novel antimalarial drug design.

Identification and dynamics of novel scaffolds against Enterococcus faecium serine hydroxymethyltransferase enzyme: a potential target for antibiotics development

Serine hydroxymethyltransferase enzyme is a significant player in purine, thymidylate, and L-serine biosynthesis and has been tagged as a potential target for cancer, viruses, and parasites. However, this enzyme as an anti-bacterial druggable target has not been explored much. Herein, in this work, different computational chemistry and biophysics techniques were applied to identify potential computational predicted inhibitory molecules against Enterococcus faecium serine hydroxymethyltransferase enzyme. By structure based virtual screening process of ASINEX antibacterial library against the enzyme two main compounds: Top-1_BDC_21204033 and Top-2_BDC_20700155 were reported as best binding molecules. The Top-1_BDC_21204033 and Top-2_BDC_20700155 binding energy value is -9.3 and -8.9 kcal/mol, respectively. The control molecule binding energy score is -6.55 kcal/mol. The mean RMSD of Top-1-BDC_21204033, Top-2-BDC_20700155 and control is 3.7 Å (maximum 5.03 Å), 1.7 Å (maximum 3.05 Å), and 3.84 Å (maximum of 6.7 Å), respectively. During the simulation time, the intermolecular docked conformation and interactions were seen stable despite of few small jumps by the compounds/control, responsible for high RMSD in some frames. The MM/GBSA and MM/PBSA binding free energy of lead Top-2-BDC_20700155 complex is -79.52 and -82.63 kcal/mol, respectively. This complex was seen as the most stable compared to the control. Furthermore, the lead molecules and control showed good druglikeness and pharmacokinetics profile. The lead molecules were non-toxic and non-mutagenic. In short, the compounds are promising in terms of binding to the serine hydroxymethyltransferase enzyme and need to be subjected to experimental studies.Communicated by Ramaswamy H. Sarma.

Mangiferin is a new potential antimalarial and anticancer drug for targeting serine hydroxymethyltransferase

Mangiferin, a polyphenolic xanthone glycoside found in various botanical sources, including mango (Mangifera indica L.) leaves, can exhibit a variety of bioactivities. Although mangiferin has been reported to inhibit many targets, none of the studies have investigated the inhibition of serine hydroxymethyltransferase (SHMT), an attractive target for antimalarial and anticancer drugs. SHMT, one of the key enzymes in the deoxythymidylate synthesis cycle, catalyzes the reversible conversion of l-serine and (6S)-tetrahydrofolate (THF) into glycine and 5,10-methylene THF. Here, in vitro and in silico studies were used to probe how mangiferin isolated from mango leaves inhibits Plasmodium falciparum and human cytosolic SHMTs. The inhibition kinetics at pH 7.5 revealed that mangiferin is a competitive inhibitor against THF for enzymes from both organisms. Molecular docking and molecular dynamic (MD) simulations demonstrated the inhibitory effects of the deprotonated forms of mangiferin, specifically the C6-O- species and its resonance C9-O- species appearing at pH 7.5, combined with two docked poses, either a xanthone or glucose moiety, placed inside the THF-binding pocket. The MD analysis revealed that both C6-O- and its resonance-stabilized C9-O- species can favorably bind to SHMT in a similar fashion to THF, supporting the THF competitive inhibition of mangiferin. In addition, characterization of the proton dissociation equilibria of isolated mangiferin revealed that only three hydroxy groups of the xanthone moiety, C6-OH, C3-OH, and C7-OH, underwent varying degrees of deprotonation with pKa values of 6.38 ± 0.11, 8.21 ± 0.35, and 12.37 ± 0.30, respectively, while C1-OH remained protonated. Altogether, our findings demonstrate a new bioactivity of mangiferin and provide the basis for the future development of mangiferin as a potent antimalarial and anticancer drug.

Characterisation of the Theileria orientalis Piroplasm Proteome across Three Common Genotypes

Theileria orientalis is an emerging apicomplexan pathogen of cattle occurring in areas populated by the principal vector tick, Haemaphysalis longicornis. Unlike transforming Theileria spp. that induce cancer-like proliferation of lymphocytes via their schizont stage, T. orientalis destroys host erythrocytes during its piroplasm phase resulting in anaemia. The underlying pathogenic processes of T. orientalis infection are poorly understood; consequently, there are no vaccines for prevention of T. orientalis infection and chemotherapeutic options are limited. To identify antigens expressed during the piroplasm phase of T. orientalis, including those which may be useful targets for future therapeutic development, we examined the proteome across three common genotypes of the parasite (Ikeda, Chitose and Buffeli) using preparations of piroplasms purified from bovine blood. A combination of Triton X-114 extraction, one-dimensional electrophoresis and LC-MS/MS identified a total of 1113 proteins across all genotypes, with less than 3% of these representing host-derived proteins. Just over three quarters of T. orientalis proteins (78%) identified were from the aqueous phase of the TX-114 extraction representing cytosolic proteins, with the remaining 22% from the detergent phase, representing membrane-associated proteins. All enzymes involved in glycolysis were expressed, suggesting that this is the major metabolic pathway used during the T. orientalis piroplasm phase. Proteins involved in binding and breakdown of haemoglobin were also identified, suggesting that T. orientalis uses haemoglobin as a source of amino acids. A number of proteins involved in host cell interaction were also identified which may be suitable targets for the development of chemotherapeutics or vaccines.

Dihydrofolate reductase, thymidylate synthase, and serine hydroxy methyltransferase: successful targets against some infectious diseases

Parasitic diseases have a serious impact on the world in terms of health and economics and are responsible for worldwide mortality and morbidity. The present review features the hybrid targeting involving three main enzymes for the treatment of different parasitic diseases. The enzymes Dihydrofolate reductase, thymidylate synthase, and Serine hydroxy methyltransferase play an essential role in the folate pathway. The present review focuses on these enzymes, which can be targeted against several diseases. It shed light on the past, present, and future of these targets, and it can be assessed that these targets can play a significant role against several infectious diseases. For combating viral and protozoal infectious diseases, these targets in combination should be addressed.

Characterization of Apicomplexan Amino Acid Transporters (ApiATs) in the Malaria Parasite Plasmodium falciparum

During the symptomatic human blood phase, malaria parasites replicate within red blood cells. Parasite proliferation relies on the uptake of nutrients, such as amino acids, from the host cell and blood plasma, requiring transport across multiple membranes. Amino acids are delivered to the parasite through the parasite-surrounding vacuolar compartment by specialized nutrient-permeable channels of the erythrocyte membrane and the parasitophorous vacuole membrane (PVM). However, further transport of amino acids across the parasite plasma membrane (PPM) is currently not well characterized. In this study, we focused on a family of Apicomplexan amino acid transporters (ApiATs) that comprises five members in Plasmodium falciparum. First, we localized four of the P. falciparum ApiATs (PfApiATs) at the PPM using endogenous green fluorescent protein (GFP) tagging. Next, we applied reverse genetic approaches to probe into their essentiality during asexual replication and gametocytogenesis. Upon inducible knockdown and targeted gene disruption, a reduced asexual parasite proliferation was detected for PfApiAT2 and PfApiAT4. Functional inactivation of individual PfApiATs targeted in this study had no effect on gametocyte development. Our data suggest that individual PfApiATs are partially redundant during asexual in vitro proliferation and fully redundant during gametocytogenesis of P. falciparum parasites.

IMPORTANCE Malaria parasites live and multiply inside cells. To facilitate their extremely fast intracellular proliferation, they hijack and transform their host cells. This also requires the active uptake of nutrients, such as amino acids, from the host cell and the surrounding environment through various membranes that are the consequence of the parasite’s intracellular lifestyle. In this paper, we focus on a family of putative amino acid transporters termed ApiAT. We show expression and localization of four transporters in the parasite plasma membrane of Plasmodium falciparum-infected erythrocytes that represent one interface of the pathogen to its host cell. We probed into the impact of functional inactivation of individual transporters on parasite growth in asexual and sexual blood stages of P. falciparum and reveal that only two of them show a modest but significant reduction in parasite proliferation but no impact on gametocytogenesis, pointing toward dispensability within this transporter family.

Plasmodium pre-erythrocytic vaccine antigens enhance sterile protection in mice induced by circumsporozoite protein

Pre-erythrocytic vaccines prevent malaria by targeting parasites in the clinically silent sporozoite and liver stages and preventing progression to the virulent blood stages. The leading pre-erythrocytic vaccine RTS,S/AS01E (Mosquirix®) entered implementation programs in 2019 and targets the major sporozoite surface antigen called circumsporozoite protein or CSP. However, in phase III clinical trials, RTS,S conferred partial protection with limited durability, indicating a need to improve CSP-based vaccination. Previously, we identified highly expressed liver stage proteins that could potentially be used in combination with CSP and are referred to as pre-erythrocytic vaccine antigens (PEVA). Here, we developed heterologous prime-boost CSP vaccination models to confer partial sterilizing immunity against Plasmodium yoelii (Py)(protein prime/adenovirus 5 (Ad5) boost) and P. berghei (Pb) (DNA prime/Ad5 boost) in mice. When combined as individual antigens with PyCSP, 3 of 8 PyPEVA significantly enhanced sterile protection against sporozoite challenge, compared to PyCSP alone. Similar results were obtained when 3 PbPEVA and PbCSP were combined in a single vaccine regimen. In general, PyCSP antibody responses were similar after CSP alone versus CSP+PEVA vaccinations. Both Py and Pb CSP+PEVA combination vaccines induced robust CD8⁺ T cell responses including signature IFN-γ increases. In the Pb model system, IFN-γ responses were significantly higher in hepatic than splenic CD8⁺ T cells. The addition of novel antigens may enhance the degree and duration of sterile protective immunity conferred by a human vaccine such as RTS,S.

Amino Acid Metabolism in Apicomplexan Parasites

Obligate intracellular pathogens have coevolved with their host, leading to clever strategies to access nutrients, to combat the host's immune response, and to establish a safe niche for intracellular replication. The host, on the other hand, has also developed ways to restrict the replication of invaders by limiting access to nutrients required for pathogen survival. In this review, we describe the recent advancements in both computational methods and high-throughput -omics techniques that have been used to study and interrogate metabolic functions in the context of intracellular parasitism. Specifically, we cover the current knowledge on the presence of amino acid biosynthesis and uptake within the Apicomplexa phylum, focusing on human-infecting pathogens: Toxoplasma gondii and Plasmodium falciparum. Given the complex multi-host lifecycle of these pathogens, we hypothesize that amino acids are made, rather than acquired, depending on the host niche. We summarize the stage specificities of enzymes revealed through transcriptomics data, the relevance of amino acids for parasite pathogenesis in vivo, and the role of their transporters. Targeting one or more of these pathways may lead to a deeper understanding of the specific contributions of biosynthesis versus acquisition of amino acids and to design better intervention strategies against the apicomplexan parasites.

Impaired folate binding of serine hydroxymethyltransferase 8 from soybean underlies resistance to the soybean cyst nematode

Management of the agricultural pathogen soybean cyst nematode (SCN) relies on the use of SCN-resistant soybean cultivars, a strategy that has been failing in recent years. An underutilized source of resistance in the soybean genotype Peking is linked to two polymorphisms in serine hydroxy-methyltransferase 8 (SHMT8). SHMT is a pyridoxal 5'-phosphate-dependent enzyme that converts l-serine and (6S)-tetrahydrofolate to glycine and 5,10-methylenetetrahydrofolate. Here, we determined five crystal structures of the 1884-residue SHMT8 tetramers from the SCN-susceptible cultivar (cv.) Essex and the SCN-resistant cv. Forrest (whose resistance is derived from the SHMT8 polymorphisms in Peking); the crystal structures were determined in complex with various ligands at 1.4-2.35 Å resolutions. We find that the two Forrest-specific polymorphic substitutions (P130R and N358Y) impact the mobility of a loop near the entrance of the (6S)-tetrahydrofolate-binding site. Ligand-binding and kinetic studies indicate severely reduced affinity for folate and dramatically impaired enzyme activity in Forrest SHMT8. These findings imply widespread effects on folate metabolism in soybean cv. Forrest that have implications for combating the widespread increase in virulent SCN.

Theoretical Evaluation of the Reaction Mechanism of Serine Hydroxymethyltransferase

Serine hydroxymethyltransferase (SHMT) is a pyridoxal phosphate (PLP)-dependent enzyme that catalyzes the reversible conversion of serine and tetrahydrofolate (THF) to glycine and 5,10-methylene THF. SHMT is a folate pathway enzyme and is therefore of considerable medical interest due to its role as an important intervention point for antimalarial, anticancer, and antibacterial treatments. Despite considerable experimental effort, the precise reaction mechanism of SHMT remains unclear. In this study, we explore the mechanism of SHMT to determine the roles of active site residues and the nature and the sequence of chemical steps. Molecular dynamics (MD) methods were employed to generate a suitable starting structure which then underwent analysis using hybrid quantum mechanical/molecular mechanical (QM/MM) simulations. The QM region consisted of 12 key residues, two substrates, and explicit solvent. Our results show that the catalytic reaction proceeds according to a retro-aldol synthetic process with His129 acting as the general base in the reaction. The rate-determining step involves the cleavage of the PLP-serine aldimine C_α-C_β bond and the formation of formaldehyde in line with experimental evidence. The pyridyl ring of the PLP-serine aldimine substrate exists in deprotonated form, being stabilized directly by Asp208 via a strong H-bond, as well as through interactions with Arg371, Lys237, and His211, and with the surrounding protein which was electrostatically embedded. This knowledge has the potential to impact the design and development of new inhibitors.

Elucidation of the catalytic mechanism of 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase using QM/MM calculations

This account describes the application of QM/MM calculations to understand the reaction mechanism of HPPK, an important pharmacological target on the folate pathway for the treatment of diseases including anti-microbial resistance, malaria and cancer.

Human and Plasmodium serine hydroxymethyltransferases differ in rate-limiting steps and pH-dependent substrate inhibition behavior

Serine hydroxymethyltransferase (SHMT), an essential enzyme for cell growth and development, catalyzes the transfer of -CH₂OH from l-serine to tetrahydrofolate (THF) to form glycine and 5,10-methylenetetrahydrofolate (MTHF) which is used for nucleotide synthesis. Insights into the ligand binding and inhibition properties of human cytosolic SHMT (hcSHMT) and Plasmodium SHMT (PvSHMT) are crucial for designing specific drugs against malaria and cancer. The results presented here revealed strong and pH-dependent THF inhibition of hcSHMT. In contrast, in PvSHMT, THF inhibition and the influence of pH were not as pronounced. Ligand binding experiments performed at various pH values indicated that the hcSHMT:Gly complex binds THF more tightly at lower pH conditions, while the binding affinity of the PvSHMT:Gly complex for THF is not pH-dependent. Pre-steady state kinetic (rapid-quench) analysis of hcSHMT showed burst kinetics, indicating that glycine formation occurs fastest in the first turnover relative to the subsequent turnovers i.e. glycine release is the rate-limiting step in the hcSHMT reaction. All data suggest that excess THF likely binds E:Gly binary complex and forms the E:Gly:THF dead-end complex before glycine is released. A unique flap motif found in the structure of hcSHMT may be the key structural feature that imparts these described characteristics of hcSHMT.

Kinetic mechanism and the rate-limiting step of Plasmodium vivax serine hydroxymethyltransferase

Serine hydroxymethyltransferase (SHMT) is a pyridoxal 5'-phosphate (PLP)-dependent enzyme that catalyzes a hydroxymethyl group transfer from L-serine to tetrahydrofolate (H4folate) to yield glycine and 5,10-methylenetetrahydrofolate (CH2-H4folate). SHMT is crucial for deoxythymidylate biosynthesis and a target for antimalarial drug development. Our previous studies indicate that PvSHMT catalyzes the reaction via a ternary complex mechanism. To define the kinetic mechanism of this catalysis, we explored the PvSHMT reaction by employing various methodologies including ligand binding, transient, and steady-state kinetics as well as product analysis by rapid-quench and HPLC/MS techniques. The results indicate that PvSHMT can bind first to either L-serine or H4folate. The dissociation constants for the enzyme·L-serine and enzyme·H4folate complexes were determined as 0.18 ± 0.08 and 0.35 ± 0.06 mM, respectively. The amounts of glycine formed after single turnovers of different preformed binary complexes were similar, indicating that the reaction proceeds via a random-order binding mechanism. In addition, the rate constant of glycine formation measured by rapid-quench and HPLC/MS analysis is similar to the kcat value (1.09 ± 0.05 s(-1)) obtained from the steady-state kinetics, indicating that glycine formation is the rate-limiting step of SHMT catalysis. This information will serve as a basis for future investigation on species-specific inhibition of SHMT for antimalarial drug development.

Structures of Plasmodium vivax serine hydroxymethyltransferase: implications for ligand-binding specificity and functional control

Plasmodium parasites, the causative agent of malaria, rely heavily on de novo folate biosynthesis, and the enzymes in this pathway have therefore been explored extensively for antimalarial development. Serine hydroxymethyltransferase (SHMT) from Plasmodium spp., an enzyme involved in folate recycling and dTMP synthesis, has been shown to catalyze the conversion of L- and D-serine to glycine (Gly) in a THF-dependent reaction, the mechanism of which is not yet fully understood. Here, the crystal structures of P. vivax SHMT (PvSHMT) in a binary complex with L-serine and in a ternary complex with D-serine (D-Ser) and (6R)-5-formyltetrahydrofolate (5FTHF) provide clues to the mechanism underlying the control of enzyme activity. 5FTHF in the ternary-complex structure was found in the 6R form, thus differing from the previously reported structures of SHMT-Gly-(6S)-5FTHF from other organisms. This suggested that the presence of D-Ser in the active site can alter the folate-binding specificity. Investigation of binding in the presence of D-Ser and the (6R)- or (6S)-5FTHF enantiomers indicated that both forms of 5FTHF can bind to the enzyme but that only (6S)-5FTHF gives rise to a quinonoid intermediate. Likewise, a large surface area with a highly positively charged electrostatic potential surrounding the PvSHMT folate pocket suggested a preference for a polyglutamated folate substrate similar to the mammalian SHMTs. Furthermore, as in P. falciparum SHMT, a redox switch created from a cysteine pair (Cys125-Cys364) was observed. Overall, these results assert the importance of features such as stereoselectivity and redox status for control of the activity and specificity of PvSHMT.

Proteome scale comparative modeling for conserved drug and vaccine targets identification in Corynebacterium pseudotuberculosis

Corynebacterium pseudotuberculosis (Cp) is a pathogenic bacterium that causes caseous lymphadenitis (CLA), ulcerative lymphangitis, mastitis, and edematous to a broad spectrum of hosts, including ruminants, thereby threatening economic and dairy industries worldwide. Currently there is no effective drug or vaccine available against Cp. To identify new targets, we adopted a novel integrative strategy, which began with the prediction of the modelome (tridimensional protein structures for the proteome of an organism, generated through comparative modeling) for 15 previously sequenced C. pseudotuberculosis strains. This pan-modelomics approach identified a set of 331 conserved proteins having 95-100% intra-species sequence similarity. Next, we combined subtractive proteomics and modelomics to reveal a set of 10 Cp proteins, which may be essential for the bacteria. Of these, 4 proteins (tcsR, mtrA, nrdI, and ispH) were essential and non-host homologs (considering man, horse, cow and sheep as hosts) and satisfied all criteria of being putative targets. Additionally, we subjected these 4 proteins to virtual screening of a drug-like compound library. In all cases, molecules predicted to form favorable interactions and which showed high complementarity to the target were found among the top ranking compounds. The remaining 6 essential proteins (adk, gapA, glyA, fumC, gnd, and aspA) have homologs in the host proteomes. Their active site cavities were compared to the respective cavities in host proteins. We propose that some of these proteins can be selectively targeted using structure-based drug design approaches (SBDD). Our results facilitate the selection of C. pseudotuberculosis putative proteins for developing broad-spectrum novel drugs and vaccines. A few of the targets identified here have been validated in other microorganisms, suggesting that our modelome strategy is effective and can also be applicable to other pathogens.

The structure of Plasmodium falciparum serine hydroxymethyltransferase reveals a novel redox switch that regulates its activities

Plasmodium falciparum serine hydroxymethyltransferase (PfSHMT), an enzyme in the dTMP synthesis cycle, is an antimalarial target because inhibition of its expression or function has been shown to be lethal to the parasite. As the wild-type enzyme could not be crystallized, protein engineering of residues on the surface was carried out. The surface-engineered mutant PfSHMT-F292E was successfully crystallized and its structure was determined at 3 Å resolution. The PfSHMT-F292E structure is a good representation of PfSHMT as this variant revealed biochemical properties similar to those of the wild type. Although the overall structure of PfSHMT is similar to those of other SHMTs, unique features including the presence of two loops and a distinctive cysteine pair formed by Cys125 and Cys364 in the tetrahydrofolate (THF) substrate binding pocket were identified. These structural characteristics have never been reported in other SHMTs. Biochemical characterization and mutation analysis of these two residues confirm that they act as a disulfide/sulfhydryl switch to regulate the THF-dependent catalytic function of the enzyme. This redox switch is not present in the human enzyme, in which the cysteine pair is absent. The data reported here can be further exploited as a new strategy to specifically disrupt the activity of the parasite enzyme without interfering with the function of the human enzyme.

The folate metabolic network of Falciparum malaria

The targeting of key enzymes in the folate pathway continues to be an effective chemotherapeutic approach that has earned antifolate drugs a valuable position in the medical pharmacopoeia. The successful therapeutic use of antifolates as antimalarials has been a catalyst for ongoing research into the biochemistry of folate and pterin biosynthesis in malaria parasites. However, our understanding of the parasites folate metabolism remains partial and patchy, especially in relation to the shikimate pathway, the folate cycle, and folate salvage. A sizeable number of potential folate targets remain to be characterised. Recent reports on the parasite specific transport of folate precursors that would normally be present in the human host awaken previous hypotheses on the salvage of folate precursors or by-products. As the parasite progresses through its life-cycle it encounters very contrasting host cell environments that present radically different metabolic milieus and biochemical challenges. It would seem probable that as the parasite encounters differing environments it would need to modify its biochemistry. This would be reflected in the folate homeostasis in Plasmodium. Recent drug screening efforts and insights into folate membrane transport substantiate the argument that folate metabolism may still offer unexplored opportunities for therapeutic attack.

Plasmodium serine hydroxymethyltransferase: indispensability and display of distinct localization

Background

Serine hydroxymethyltransferase (SHMT), a pyridoxal phosphate-dependent enzyme, plays a vital role in the de novo pyrimidine biosynthesis pathway in malaria parasites. Two genes have been identified in Plasmodium spp. encoding a cytosolic SHMT (cSHMT) and putative mitochondria SHMT (mSHMT), but their roles have not been fully investigated.

Methods

The presence of Plasmodium SHMT isoforms in the intra-erythrocytic stage was assessed based on their gene expression using reverse transcription PCR (RT-PCR). Localization studies of Plasmodium SHMT isoforms were performed by transfection of fluorescent-tagged gene constructs into P. falciparum and expressions of fluorescent fusion proteins in parasites were observed using a laser scanning confocal microscope. Genetic targeting through homologous recombination was used to study the essentiality of SHMT in Plasmodium spp.

Results

Semi-quantitative RT-PCR revealed the expression of these two genes throughout intra-erythrocytic development. Localization studies using P. falciparum expressing fluorescent-tagged SHMT showed that PfcSHMT-red fluorescent fusion protein (PfcSHMT-DsRed) is localized in the cytoplasm, while PfmSHMT-green fluorescent fusion protein (PfmSHMT-GFP) co-localized with Mitotracker™-labelled mitochondria as predicted. The essentiality of plasmodial cSHMT was inferred from transfection experiments where recovery of viable knock-out parasites was not achieved, unless complemented with a functional equivalent copy of shmt.

Conclusions

Distinct compartment localizations of PfSHMT were observed between cytoplasmic and mitochondrial isoforms, and evidence was provided for the indispensable role of plasmodial cSHMT indicating it as a valid target for development of novel anti-malarials.