Functional analysis, overexpression, and kinetic characterization of pyruvate kinase from Plasmodium falciparum

Are Allosteric Mechanisms Conserved Among Homologues?

Conservation of allosteric mechanisms among homologues is often assumed but seldom tested. This assumption underpins key concepts like coevolution of residues involved in allosteric mechanisms and the comparison of structures of two different homologues to gain insights into allosteric mechanisms. As an initial assessment of whether allosteric mechanisms are conserved among homologues, this work reviews what is known about the allosteric mechanisms of liver pyruvate kinase (LPYK) vs. skeletal muscle pyruvate kinase (M1PYK), framed within a two-ligand allosteric energy cycle description of allosteric regulation. Selective observations from other PYK homologues are included when relevant. The primary focus of this review is on functional data, while expressing caution regarding the interpretation of allosteric mechanisms based solely on available X-ray crystallographic structures. Additionally, this review considers types of data that are currently lacking for these two PYK homologues, highlighting potential techniques that could be valuable for evaluating the conservation of allosteric mechanisms among homologues. In particular, a hybrid tetramer technique that has been used to study bacterial phosphofructokinases 1 is summarized. Interestingly, despite a high degree of similarity (66.5% sequence identity) between the LPYK and rM1PYK proteins, the available functional comparisons do not provide strong evidence for conserved allosteric mechanisms. Lastly, we consider whether insights into native allosteric mechanisms are relevant to allosteric mechanisms associated with allosteric drug designs.

Proteomic analysis of mouse liver lesions at all three stages of Echinococcus granulosus infection

Echinococcus granulosus, a zoonotic parasite, can severely damage host health or even lead to host death. In humans, early diagnosis of E. granulosus infection is difficult because the initial stages of the infection tend to be asymptomatic, this delays treatment and worsens prognosis in most patients. Herein, we present a comprehensive, temporal proteomic atlas of the liver at three stages of E. granulosus infection and analyze the changes in the proteome of host focal lesions; this atlas may provide an overview of the effects of E. granulosus in the host, as well as the interactions between them. We identified 3,197 proteins from mice model at 1, 3, and 6 months after E. granulosus infection; of these proteins, 760 were differentially expressed (520 upregulated; 240 downregulated). Moreover, 228 differentially expressed proteins were screened through cluster analysis and classified into four clusters according to their changing trends. Subsequently, candidate molecules related to cyst invasion, growth, candidate pathways and proteins related to angiogenesis were noted to demonstrate important value in mouse liver. Next, we used western blotting to verify the presence of the aforementioned proteins in mouse liver. In the later stages, E. granulosus infection was noted to result in significant enrichment of crucial proteins facilitating protoscoleces growth and development and inhibition of amino acid and lipid metabolic enzyme expression in mouse liver; it was also noted to transform host metabolism by weakening oxidative phosphorylation and enhancing glycolysis. In conclusion, we explored the molecular mechanisms underlying the parasitic processes of E. granulosus through proteomic analysis. Our results provide evidence that may enable the exploration of core regulatory targets for early and effective diagnosis and immunotherapy of E. granulosus infection, as well as parasite-host interactions involved in cystic echinococcosis development.

Prominent role of cysteine residues C49 and C343 in regulating Plasmodiumfalciparum pyruvate kinase activity

The protozoan parasite Plasmodium falciparum causes the most severe form of malaria and is highly dependent on glycolysis. Glycolytic enzymes were shown to be massively redox regulated, inter alia via oxidative post-translational modifications (oxPTMs) of their cysteine residues. In this study, we identified P. falciparum pyruvate kinase (PfPK) C49 and C343 as amino acid residues essentially involved in maintaining structural and functional integrity of the enzyme. The mutation of these cysteines resulted in an altered substrate affinity, lower enzymatic activities, and, as studied by X-ray crystallography, conformational changes within the A-domain where the substrate binding site is located. Although the loss of a cysteine evoked an impaired catalysis in both mutants, the effects observed for mutant C49A were more severe: multiple conformational changes, caused by the loss of two hydrogen bonds, impeded proper substrate binding and thus the transfer of phosphate upon catalysis.

Kinetic Characterization and Inhibitor Screening of Pyruvate Kinase I From Babesia microti

The apicomplexan Babesia microti is a main pathogenic parasite causing human babesiosis, which is one of the most widely distributed tick-borne diseases in humans. Pyruvate kinase (PYK) plays a central metabolic regulatory role in most living organisms and catalyzes the essentially irreversible step in glycolysis that converts phosphoenolpyruvate (PEP) to pyruvate. Hence, PYK is recognized as an attractive therapeutic target in cancer and human pathogens such as apicomplexans. In this study, we cloned, expressed, and purified B. microti PYK I (BmPYKI). Western blotting illustrated that anti-rBmPYKI antibody could specifically recognize the native BmPYKI protein in the lysate of B. microti with a 54-kDa band, which is consistent with the predicted size. In addition, the enzymatic activity of the purified recombinant PYKI (rPYKI) was tested under a range of pH values. The results showed that the maximum catalytic activity could be achieved at pH 7.0. The saturation curves for substrates demonstrated that the K_m value for PEP was 0.655 ± 0.117 mM and that for ADP was 0.388 ± 0.087 mM. We further investigated the effect of 13 compounds on rBmPYKI. Kinetic analysis indicated that six inhibitors (tannic acid, shikonin, apigenin, PKM2 inhibitor, rosiglitazone, and pioglitazone) could significantly inhibit the catalytic activity of PYKI, among which tannic acid is the most efficient inhibitor with an IC₅₀ value 0.49 μM. Besides, four inhibitors (tannic acid, apigenin, shikonin, and PKM2 inhibitor) could significantly decrease the growth of in vitro-cultured B. microti with IC₅₀ values of 0.77, 2.10, 1.73, and 1.15 μM. Overall, the present study provides a theoretical basis for the design and development of new anti-Babesia drugs.

Structural and kinetic characterization of Trypanosoma congolense pyruvate kinase

Trypanosoma are blood-borne parasites and are the causative agents of neglected tropical diseases (NTDs) affecting both humans and animals. These parasites mainly rely on glycolysis for their energy production within the mammalian host, which is why trypanosomal glycolytic enzymes have been pursued as interesting targets for the development of trypanocidal drugs. The structure-function relationships of pyruvate kinases (PYKs) from trypanosomatids (Trypanosoma and Leishmania) have been well-studied within this context. In this paper, we describe the structural and enzymatic characterization of PYK from T. congolense (TcoPYK), the main causative agent of Animal African Trypanosomosis (AAT), by employing a combination of enzymatic assays, thermal unfolding studies and X-ray crystallography.

Distinctive regulatory properties of pyruvate kinase 1 from Aedes aegypti mosquitoes

Female Aedes aegypti mosquitoes are vectors of arboviruses that cause diseases of public health significance. The discovery of new metabolic targets is crucial for improving mosquito control strategies. We recently demonstrated that glucose oxidation supports ammonia detoxification in A. aegypti. Pyruvate kinase (PK, EC 2.7.1.40) catalyzes the last step of the glycolytic pathway. In most organisms, one or more allosteric effectors control PK activity. However, the kinetic properties and structure of PK in mosquitoes have not been previously reported. In this study, two alternatively spliced mRNA variants (AaPK1 and AaPK2) that code for PKs were identified in the A. aegypti genome. The AaPK1 mRNA variant, which encodes a 529 amino acid protein with an estimated molecular weight of ∼57 kDa, was cloned. The protein was expressed in Escherichia coli and purified. The AaPK1 kinetic properties were identified. The recombinant protein was also crystallized and its 3D structure determined. We found that alanine, glutamine, proline, serine and fructose-1-phosphate displayed a classic allosteric activation on AaPK1. Ribulose-5-phosphate acted as an allosteric inhibitor of AaPK1 but its inhibitory effect was reversed by alanine, glutamine, proline and serine. Additionally, the allosteric activation of AaPK1 by amino acids was weakened by fructose-1,6-bisphosphate, whereas the allosteric activation of AaPK1 by alanine and serine was diminished by glucose-6-phosphate. The AaPK1 structure shows the presence of fructose-1,6-bisphosphate in the allosteric site. Together, our results reveal that specific amino acids and phosphorylated sugars tightly regulate conformational dynamics and catalytic changes of AaPK1. The distinctive AaPK1 allosteric properties support a complex role for this enzyme within mosquito metabolism.

Sequence analysis and characterization of pyruvate kinase from Clonorchis sinensis, a 53.1-kDa homopentamer, implicated immune protective efficacy against clonorchiasis

BACKGROUND

Clonorchis sinensis, the causative agent of clonorchiasis, is classified as one of the most neglected tropical diseases and affects more than 15 million people globally. This hepatobiliary disease is highly associated with cholangiocarcinoma. As key molecules in the infectivity and subsistence of trematodes, glycolytic enzymes have been targets for drug and vaccine development. Clonorchis sinensis pyruvate kinase (CsPK), a crucial glycolytic enzyme, was characterized in this research.

RESULTS

Differences were observed in the sequences and spatial structures of CsPK and PKs from humans, rats, mice and rabbits. CsPK possessed a characteristic active site signature (IKLIAKIENHEGV) and some unique sites but lacked the N-terminal domain. The predicted subunit molecular mass (Mr) of CsPK was 53.1 kDa. Recombinant CsPK (rCsPK) was a homopentamer with a Mr. of approximately 290 kDa by both native PAGE and gel filtration chromatography. Significant differences in the protein and mRNA levels of CsPK were observed among four life stages of C. sinensis (egg, adult worm, excysted metacercaria and metacercaria), suggesting that these developmental stages may be associated with diverse energy demands. CsPK was widely distributed in adult worms. Moreover, an intense Th1-biased immune response was persistently elicited in rats immunized with rCsPK. Also, rat anti-rCsPK sera suppressed C. sinensis adult subsistence both in vivo and in vitro.

CONCLUSIONS

The sequences and spatial structures, molecular mass, and expression profile of CsPK have been characterized. rCsPK was indicated to be a homopentamer. Rat anti-rCsPK sera suppressed C. sinensis adult subsistence both in vivo and in vitro. CsPK is worthy of further study as a promising target for drug and vaccine development.

Stage-Specific Changes in Plasmodium Metabolism Required for Differentiation and Adaptation to Different Host and Vector Environments

Malaria parasites (Plasmodium spp.) encounter markedly different (nutritional) environments during their complex life cycles in the mosquito and human hosts. Adaptation to these different host niches is associated with a dramatic rewiring of metabolism, from a highly glycolytic metabolism in the asexual blood stages to increased dependence on tricarboxylic acid (TCA) metabolism in mosquito stages. Here we have used stable isotope labelling, targeted metabolomics and reverse genetics to map stage-specific changes in Plasmodium berghei carbon metabolism and determine the functional significance of these changes on parasite survival in the blood and mosquito stages. We show that glutamine serves as the predominant input into TCA metabolism in both asexual and sexual blood stages and is important for complete male gametogenesis. Glutamine catabolism, as well as key reactions in intermediary metabolism and CoA synthesis are also essential for ookinete to oocyst transition in the mosquito. These data extend our knowledge of Plasmodium metabolism and point towards possible targets for transmission-blocking intervention strategies. Furthermore, they highlight significant metabolic differences between Plasmodium species which are not easily anticipated based on genomics or transcriptomics studies and underline the importance of integration of metabolomics data with other platforms in order to better inform drug discovery and design.

Malaria kills almost half a million people worldwide every year and more than two hundred million people are diagnosed with this deadly disease annually. It is caused by the protozoan parasite Plasmodium spp., mostly in sub-Saharan Africa and Asia and is transmitted by bites of infected female Anopheles mosquitoes. Due to an increase in resistance to existing drugs and lack of an effective vaccine, new intervention strategies which target development of parasite in human host and transmission through the mosquito vector are urgently needed. In this study, we explored the metabolic capacity of different developmental stages of the malaria parasite to determine carbon source utilization in different host niches and whether any stage-specific switches in metabolism could be exploited in new therapies aimed at eradicating malaria. Using stable isotope labelling and metabolomics, we have identified considerable nutritional adaptability of malaria parasites between the mammalian host and the mosquito vector. Gene disruption in the rodent malaria parasite P. berghei was used to identify the metabolic pathways which are crucial to the survival and development of the parasite. Our data also point at key metabolic differences in different Plasmodium species highlighting the importance of integrating metabolomics analyses with molecular tools and identifies possible transmission blocking candidates for malaria intervention.

The influence of host genetics on erythrocytes and malaria infection: is there therapeutic potential?

As parasites, Plasmodium species depend upon their host for survival. During the blood stage of their life-cycle parasites invade and reside within erythrocytes, commandeering host proteins and resources towards their own ends, and dramatically transforming the host cell. Parasites aptly avoid immune detection by minimizing the exposure of parasite proteins and removing themselves from circulation through cytoadherence. Erythrocytic disorders brought on by host genetic mutations can interfere with one or more of these processes, thereby providing a measure of protection against malaria to the host. This review summarizes recent findings regarding the mechanistic aspects of this protection, as mediated through the parasites interaction with abnormal erythrocytes. These novel findings include the reliance of the parasite on the host enzyme ferrochelatase, and the discovery of basigin and CD55 as obligate erythrocyte receptors for parasite invasion. The elucidation of these naturally occurring malaria resistance mechanisms is increasing the understanding of the host-parasite interaction, and as discussed below, is providing new insights into the development of therapies to prevent this disease.

Construction and validation of a detailed kinetic model of glycolysis in Plasmodium falciparum

The enzymes in the Embden-Meyerhof-Parnas pathway of Plasmodium falciparum trophozoites were kinetically characterized and their integrated activities analyzed in a mathematical model. For validation of the model, we compared model predictions for steady-state fluxes and metabolite concentrations of the hexose phosphates with experimental values for intact parasites. The model, which is completely based on kinetic parameters that were measured for the individual enzymes, gives an accurate prediction of the steady-state fluxes and intermediate concentrations. This is the first detailed kinetic model for glucose metabolism in P. falciparum, one of the most prolific malaria-causing protozoa, and the high predictive power of the model makes it a strong tool for future drug target identification studies. The modelling workflow is transparent and reproducible, and completely documented in the SEEK platform, where all experimental data and model files are available for download.

DATABASE

The mathematical models described in the present study have been submitted to the JWS Online Cellular Systems Modelling Database (http://jjj.bio.vu.nl/database/penkler). The investigation and complete experimental data set is available on SEEK (10.15490/seek.1.

INVESTIGATION

56).

The proliferating cell hypothesis: a metabolic framework for Plasmodium growth and development

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The hypothesis offers a framework to explain the atypical features of parasite metabolism.

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Aerobic glycolysis is hypothesised to meet the biosynthetic demands of rapid proliferation.

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Differentiation may be epigenetically regulated in response to nutrient-linked metabolism.

We hypothesise that intraerythrocytic malaria parasite metabolism is not merely fulfilling the need for ATP generation, but is evolved to support rapid proliferation, similar to that seen in other rapidly proliferating cells such as cancer cells. Deregulated glycolytic activity coupled with impaired mitochondrial metabolism is a metabolic strategy to generate glycolytic intermediates essential for rapid biomass generation for schizogony. Further, we discuss the possibility that Plasmodium metabolism is not only a functional consequence of the ‘hard-wired’ genome and argue that metabolism may also have a causal role in triggering the cascade of events that leads to developmental stage transitions. This hypothesis offers a framework to rationalise the observations of aerobic glycolysis, atypical mitochondrial metabolism, and metabolic switching in nonproliferating stages.

Protein S-glutathionylation in malaria parasites

AIMS

Protein S-glutathionylation is a widely distributed post-translational modification of thiol groups with glutathione that can function as a redox-sensitive switch to mediate redox regulation and signal transduction. The malaria parasite Plasmodium falciparum is exposed to intense oxidative stress and possesses the enzymatic system required to regulate protein S-glutathionylation, but despite its potential importance, protein S-glutathionylation has not yet been studied in malaria parasites. In this work we applied a method based on enzymatic deglutathionylation, affinity purification of biotin-maleimide-tagged proteins, and proteomic analyses to characterize the Plasmodium glutathionylome.

RESULTS

We identified 493 targets of protein S-glutathionylation in Plasmodium. Functional profiles revealed that the targets are components of central metabolic pathways, such as nitrogen compound metabolism and protein metabolism. Fifteen identified proteins with important functions in metabolic pathways (thioredoxin reductase, thioredoxin, thioredoxin peroxidase 1, glutathione reductase, glutathione S-transferase, plasmoredoxin, mitochondrial dihydrolipoamide dehydrogenase, glutamate dehydrogenase 1, glyoxalase I and II, ornithine δ-aminotransferase, lactate dehydrogenase, glyceraldehyde 3-phosphate dehydrogenase [GAPDH], pyruvate kinase [PK], and phosphoglycerate mutase) were further analyzed to study their ability to form mixed disulfides with glutathione. We demonstrate that P. falciparum GAPDH, PK, and ornithine δ-aminotransferase are reversibly inhibited by S-glutathionylation. Further, we provide evidence that not only P. falciparum glutaredoxin 1, but also thioredoxin 1 and plasmoredoxin are able to efficiently catalyze protein deglutathionylation.

INNOVATION

We used an affinity-purification based proteomic approach to characterize the Plasmodium glutathionylome.

CONCLUSION

Our results indicate a wide regulative use of S-glutathionylation in the malaria parasite and contribute to our understanding of redox-regulatory processes in this pathogen.

Mechanisms of genetically-based resistance to malaria

Malaria remains one of the most prevalent parasitoses worldwide. About 350 to 500 million febrile episodes are observed yearly in African children alone and more than 1 million people die because of malaria each year. Multiple factors have hampered the effective control of this disease, some of which include the complex biology of the Plasmodium parasites, their high polymorphism and their increasingly high resistance to antimalarial drugs, mainly in endemic regions. The ancient interaction between malarial parasites and humans has led to the fixation in the population of several inherited alterations conferring protection against malaria. Some of the mechanisms underlying protection against this disease are described in this review for hemoglobin-inherited disorders (thalassemia, sickle-cell trait, HbC and HbE), erythrocyte polymorphisms (ovalocytosis and Duffy blood group), enzymopathies (G6PD deficiency and PK deficiency) and immunogenetic variants (HLA alleles, complement receptor 1, NOS2, tumor necrosis factor-α promoter and chromosome 5q31-q33 polymorphisms).

Plant-like phosphofructokinase from Plasmodium falciparum belongs to a novel class of ATP-dependent enzymes

Malaria parasite-infected erythrocytes exhibit enhanced glucose utilisation and 6-phospho-1-fructokinase (PFK) is a key enzyme in glycolysis. Here we present the characterisation of PFK from the human malaria parasite Plasmodium falciparum. Of the two putative PFK genes on chromosome 9 (PfPFK9) and 11 (PfPFK11), only the PfPFK9 gene appeared to possess all the catalytic features appropriate for PFK activity. The deduced PfPFK proteins contain domains homologous to the plant-like pyrophosphate (PPi)-dependent PFK beta and alpha subunits, which are quite different from the human erythrocyte PFK protein. The PfPFK9 gene beta and alpha regions were cloned and expressed as His(6)- and GST-tagged proteins in Escherichia coli. Complementation of PFK-deficient E. coli and activity analysis of purified recombinant proteins confirmed that PfPFK9beta possessed catalytic activity. Monoclonal antibodies against the recombinant beta protein confirmed that the PfPFK9 protein has beta and alpha domains fused into a 200 kDa protein, as opposed to the independent subunits found in plants. Despite an overall structural similarity to plant PPi-PFKs, the recombinant protein and the parasite extract exhibited only ATP-dependent enzyme activity, and none with PPi. Unlike host PFK, the Plasmodium PFK was insensitive to fructose-2,6-bisphosphate (F-2,6-bP), phosphoenolpyruvate (PEP) and citrate. A comparison of the deduced PFK proteins from several protozoan PFK genome databases implicates a unique class of ATP-dependent PFK present amongst the apicomplexan protozoans.

Pyruvate kinase type-II isozyme in Plasmodium falciparum localizes to the apicoplast

Bioinformatics research on Plasmodium falciparum revealed two isoforms of pyruvate kinase: type-I and type-II enzymes. The type-I enzyme shows typical glycolytic properties, while type-II enzyme is involved in fatty acid type-II biosynthesis and has been predicted to localize to the apicoplast with the targeting signal in its N-terminus. The type-I and type-II isoforms have the same evolutionary origin as Toxoplasma gondii isozymes, TgPyKI and TgPyKII, respectively; however, TgPyKII localizes to both the mitochondrion and the apicoplast. Accordingly, we made a recombinant full length of P. falciparum pyruvate kinase type-II protein using a wheat germ cell-free expression system and obtained a specific antibody against the type-II protein. Fluorescent microscopic analysis revealed that P. falciparum type-II enzyme was localized only to the apicoplast, not to the mitochondrion. The data suggest differences in localization and metabolic pathways between P. falciparum and T. gondii pyruvate kinase isoforms.

Eimeria tenella enolase and pyruvate kinase: A likely role in glycolysis and in others functions

Two cDNA codings for glycolytic enzymes were cloned from a cDNA library constructed from the schizont stage of the avian parasite Eimeria tenella. Enolase and pyruvate kinase cDNA were fully sequenced and compared with sequences of enzymes from other organisms. Although these enzymes were already detected in the sporozoite stage, their expression was enhanced during the first schizogony in accordance with the anaerobic conditions of this part of the life cycle of the parasite. Under activating conditions, microscopic observations suggest that these glycolytic enzymes were relocalised inside sporozoites and moreover were in part secreted. The enzymes were also localised at the apex of the first generation of merozoites. Enolase was partly observed inside the nucleus of sporozoites and schizonts. Taken together, these results suggest that glycolytic enzymes not only have a function in glycolysis during anaerobic intracellular stages but may also participate in the invasion process and, for enolase, in the control of gene regulation.

Pfnek3: an atypical activator of a MAP kinase in Plasmodium falciparum

The canonical mitogen-activated protein kinase (MAPK) signal cascade was previously suggested to be atypical in the malaria parasite. This raises queries on the existence of alternative mediators of plasmodial MAPK pathways. This study describes, Pfnek3, a malarial protein kinase belonging to the NIMA (Never in Mitosis, Aspergillus) family. Endogenous Pfnek3 is expressed during late asexual to gametocyte stages and lacks some classical protein kinase sequence motifs. Moreover, Pfnek3 is phylogenetically distant from mammalian NIMA-kinases. Recombinant Pfnek3 was able to phosphorylate and stimulate a malarial MAPK (Pfmap2). Contrastingly, this was not observed with two other kinases, Pfmap1 and human MAPK1, suggesting that the Pfnek3-Pfmap2 interaction may be specific for Pfmap2 regulation. In summary, our data reveal a malarial NIMA-kinase with the potential to regulate a MAPK. Possessing biochemical properties divergent from classical mammalian NIMA-kinases, Pfnek3 could potentially be an attractive target for parasite-selective anti-malarials.

A relevant in vitro eukaryotic live-cell system for the evaluation of plasmodial protein localization

Understanding the functional genomics and proteomics of plasmodia underpins the development of new approaches to antimalarial chemotherapy. Although genome databanks (e.g. PlasmoDB) and biocomputing tools (e.g. PlasMit, PlasmoAP, PATS) are useful in providing a global albeit predictive view of the myriad of about 5000 genes, only 40% are annotated, with few cases of endorsed subcellular localizations of the corresponding proteins in animal models. Progress in plasmodial protein trafficking has been hampered by the lack of a simple yet reliable method for studying subcellular localization of plasmodial proteins. In this study, we have used a combination of fluorescent markers, organelle-specific probes, phase contrast microscopy, and confocal microscopy to locate a selection of signal peptides from 10 plasmodial proteins in CHO-K1 cells. These eukaryotic cells serve as an in vitro living system for studying the cellular destinations of four mitochondrial-targeted TCA cycle proteins (citrate synthase, CS; isocitrate dehydrogenase, ICDH; branched chain alpha-keto-acid dehydrogenase E1alpha subunit, BCKDH; succinate dehydrogenase flavoprotein-subunit, SDH), two nuclear-targeted proteins (histone deacetylase, HDAC; RNA polymerase, RPOL), two apicoplast-targeted proteins (pyruvate kinase 2, PK2; glutamate dehydrogenase, GDH), and two cytoplasmic resident proteins (malate dehydrogenase, MDH; glycerol kinase, GK). The respective localizations of these malarial proteins have complied with the selected molecular targets, viz. mitochondrial, nuclear and cytoplasmic. Interestingly, MDH that is widely known to be resident in eukaryotic mitochondria was found to be cytoplasmic, probably due to the absence of molecular target sequences. Since the localization of plasmodial proteins is central to the authentication of their pathophysiological roles, this experimental system will serve as a useful a priori approach.

Plasmodium falciparum pyruvate kinase as a novel target for antimalarial drug-screening

BACKGROUND

Global travellers are increasingly at risk of contracting malaria. The increasing occurrence of drug-resistance in many endemic areas emphasizes the need for novel drug targets for antimalarial-screening. In this study, the use of pyruvate kinase as a drug-target is evaluated. The functional validation of a gene encoding pyruvate kinase (designated PK1) has previously been reported. However, alternative copies of this enzyme encoded by Plasmodium falciparum could also circumvent the role of PK1. A survey of genome data revealed a putative ORF seemingly coding for another pyruvate kinase (designated PK2).

METHODS

The expression of PK1 and PK2 in in vitro cultures were investigated by RT-PCR. Biocomputational analysis was carried out to identify structural differences between the P. falciparum pyruvate kinases and the corresponding enzymes from its human host.

RESULTS

Both PK1 and PK2 were indeed actively transcribed during the intraerythrocytic stages, suggesting the involvement of both enzymes during infection. A comparison of amino acid residues at the effector binding sites of PK1 and PK2, to those of the human pyruvate kinases revealed some significant differences that could serve as targets for selective inhibitors to be designed against parasitic pyruvate kinases.

CONCLUSION

Experimental evidence for the expression of both PK1 and PK2 during the blood stages of malaria infection was provided. Interestingly, phylogenetic analysis revealed that the "PK2" type of enzyme appears to be confined to Apicomplexans, an important observation with respect to the assessment of PK2 as a drug-target.

Malaria parasite-infected erythrocytes inhibit glucose utilization in uninfected red cells

The erythrocytic stages of the malaria parasite depend on anaerobic glycolysis for energy. Using [2-13C]glucose and nuclear magnetic resonance, the glucose utilization rate and 2,3-diphosphoglycerate (2,3-DPG) level produced in normal RBCs and Plasmodium falciparum infected red blood cell populations (IRBCs, with <4% parasite infected red cells), were measured. The glucose flux in IRBCs was several-folds greater, was proportional to parasitemia, and maximal at trophozoite stage. The 2,3-DPG levels were disproportionately lower in IRBCs, indicating a downregulation of 2,3-DPG flux in non-parasitized RBCs. This may be due to lowered pH leading to selective differential inhibition of the regulatory glycolytic enzyme phosphofructokinase. This downregulation of the glucose utilization rate in the majority (>96%) of uninfected RBCs in an IRBC population may have physiological implications in malaria patients.