Identification and purification of glucose phosphate isomerase of Plasmodium falciparum

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).

Exploiting Unique Structural and Functional Properties of Malarial Glycolytic Enzymes for Antimalarial Drug Development

Metabolic enzymes have been known to carry out a variety of functions besides their normal housekeeping roles known as “moonlighting functions.” These functionalities arise from structural changes induced by posttranslational modifications and/or binding of interacting proteins. Glycolysis is the sole source of energy generation for malaria parasite Plasmodium falciparum, hence a potential pathway for therapeutic intervention. Crystal structures of several P. falciparum glycolytic enzymes have been solved, revealing that they exhibit unique structural differences from the respective host enzymes, which could be exploited for their selective targeting. In addition, these enzymes carry out many parasite-specific functions, which could be of potential interest to control parasite development and transmission. This review focuses on the moonlighting functions of P. falciparum glycolytic enzymes and unique structural differences and functional features of the parasite enzymes, which could be exploited for therapeutic and transmission blocking interventions against malaria.

DETECT--a density estimation tool for enzyme classification and its application to Plasmodium falciparum

MOTIVATION

A major challenge in genomics is the accurate annotation of component genes. Enzymes are typically predicted using homology-based search methods, where the membership of a protein to an enzyme family is based on single-sequence comparisons. As such, these methods are often error-prone and lack useful measures of reliability for the prediction.

RESULTS

Here, we present DETECT, a probabilistic method for enzyme prediction that accounts for the sequence diversity across enzyme families. By comparing the global alignment scores of an unknown protein to those of all known enzymes, an integrated likelihood score can be readily calculated, ranking the reaction classes relevant for that protein. Comparisons to BLAST reveal significant improvements in enzyme annotation accuracy. Applied to Plasmodium falciparum, we identify potential annotation errors and predict novel enzymes of therapeutic interest.

AVAILABILITY

A standalone application is available from the website: http://www.compsysbio.org/projects/DETECT/

Why is the Plasmodium falciparum hexose transporter a promising new drug target?

Chemotherapy of malaria parasites is limited by established drug resistance and lack of novel treatment options. Intraerythrocytic stages of Plasmodium falciparum, the causative agent of severe malaria, are wholly dependent upon host glucose for energy. A facilitative hexose transporter (PfHT), encoded by a single-copy gene, mediates glucose uptake and is therefore an attractive potential target. The authors first established heterologous expression in Xenopus laevis to allow functional characterisation of PfHT. They then used this expression system to compare the interaction of substrates with PfHT and mammalian Gluts (hexose transporters) and identified important differences between host and parasite transporters. Certain Omethyl derivatives of glucose proved to be particularly useful discriminators between mammalian transporters and PfHT. The authors exploited this selectivity and synthesised an O-3 hexose derivative that potently inhibits PfHT expressed in oocytes. This O-3 derivative (compound 3361) also kills cultured P. falciparum with comparable potency. Compound 3361 acts with reasonable specificity against PfHT orthologues encoded by other parasites such as Plasmodium vivax, Plasmodium yoelii and Plasmodium knowlesi. Multiplication of Plasmodium berghei in a mouse model is also significantly impeded by this compound. These findings validate PfHT as a novel target.

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

The important role of pyruvate kinase during malarial infection has prompted the cloning of a cDNA encoding Plasmodium falciparum pyruvate kinase (pfPyrK), using mRNA from intraerythrocytic-stage malaria parasites. The full-length cDNA encodes a protein with a computed molecular weight of 55.6 kDa and an isoelectric point of 7.5. The purified recombinant pfPyrK is enzymatically active and exists as a homotetramer in its active form. The enzyme exhibits hyperbolic kinetics with respect to phosphoenolpyruvate and ADP, with K(m) of 0.19 and 0.12 mM, respectively. pfPyrK is not affected by fructose-1,6-bisphosphate, a general activating factor of pyruvate kinase for most species. Glucose-6-phosphate, an activator of the Toxoplasma gondii enzyme, does not affect pfPyrK activity. Similar to rabbit pyruvate kinase, pfPyrK is susceptible to inactivation by 1mM pyridoxal-5'-phosphate, but to a lesser extent. A screen for inhibitors to pfPyrK revealed that it is markedly inhibited by ATP and citrate. Detailed kinetic analysis revealed a transition from hyperbolic to sigmoidal kinetics for PEP in the presence of citrate, as well as competitive inhibitory behavior for ATP with respect to PEP. Citrate exhibits non-competitive inhibition with respect to ADP with a K(i) of 0.8mM. In conclusion, P. falciparum expresses an active pyruvate kinase during the intraerythrocytic-stage of its developmental cycle that may play important metabolic roles during infection.

Excess hemoglobin digestion and the osmotic stability of Plasmodium falciparum-infected red blood cells

During their asexual reproduction cycle (about 48 hours) in human red cells, Plasmodium falciparum parasites consume most of the host cell hemoglobin, far more than they require for protein biosynthesis. They also induce a large increase in the permeability of the host cell plasma membrane to allow for an increased traffic of nutrients and waste products. Why do the parasites digest hemoglobin in such excess? And how can infected red cells retain their integrity for the duration of the asexual cycle when comparably permeabilized uninfected cells hemolyse earlier? To address these questions we encoded the multiplicity of factors known to influence host cell volume in a mathematical model of the homeostasis of a parasitized red cell. The predicted volume changes were subjected to thorough experimental tests by monitoring the stage-related changes in the osmotic fragility of infected red cell populations. The results supported the model predictions of biphasic volume changes comprising transient shrinkage of infected cells with young trophozoites followed by continuous volume increase to about 10% lower than the critical hemolytic volume of approximately 150 fL by the end of the asexual cycle. Analysis of these results and of additional model predictions demonstrated that the osmotic stability of infected red cells can be preserved only by a large reduction in impermeant solute concentration within the host cell compartment. Thus, excess hemoglobin consumption represents an essential evolutionary strategy to prevent the premature hemolysis of the highly permeabilized infected red cell.

Diagnosis of malaria by detection of plasmodial lactate dehydrogenase with an immunodot enzyme assay

We have previously demonstrated, using polyclonal and monoclonal antibodies, that the lactate dehydrogenase (LDH) of malaria parasites is immunologically distinct from the host enzyme. The polyclonal antibodies, produced against the affinity purified plasmodial LDH (pLDH) in rabbits, showed specificity to LDH of malaria parasites. In the present study, these anti-pLDH polyclonal antibodies were used to develop an immunodiagnostic test (immunodot enzyme assay of plasmodial LDH) based on the detection of parasite LDH in patient blood. The immunodot enzyme assay of plasmodial LDH was evaluated using blood samples from patients with malaria or other infections. Out of 502 microscopically positive malaria blood samples, 497 blood samples showed positive immunodot assays of pLDH while all the 423 microscopically negative cases were found negative by our test. The blood samples from other infections and non-endemic controls were negative by the immunodot enzyme assay of pLDH. This LDH based test was also found negative in blood samples of cured patients 7 days after chloroquine treatment. The test is simple to perform, can be read visually, econimal, highly specific with a sensitivity of approximately 99% and is thus suitable for accurate diagnosis of malaria in field conditions.

Antisense oligonucleotides targeting malarial aldolase inhibit the asexual erythrocytic stages of Plasmodium falciparum

A major obstacle in the global effort to control malaria is the paucity of anti-malarial drugs. This is compounded by the continuing emergence and spread of resistance to old and new anti-malarial drugs in the malarial parasites. Here we describe the anti-malarial effect of phosphorothioate antisense (AS) oligodeoxynucleotides (ODNs) targeting the aldolase enzyme of Plasmodium falciparum, using the asexual blood stages of the parasite grown in vitro. The blood stages of P. falciparum depend almost entirely on the energy produced by their own glycolysis. Aldolase, the fourth enzyme of the glycolytic pathway, is highly upregulated during the malarial 48-h life cycle. We found that the mRNA of this enzyme can be inhibited, in a sequence specific manner, using AS-ODN to the splice sites on the pre-mRNA of malarial aldolase. At the enzyme level, both specific AS-ODNs for the splice sites, as well as for the translation initiation site on mature mRNA, can inhibit aldolase enzyme activity within the trophozoites of P. falciparum. Furthermore, this downregulation of the malarial aldolase results in a reduction in the production of ATP within the parasite. Finally, the treatment reduces parasitemia. In summary, AS-ODNs targeting the aldolase gene of P. falciparum can interfere with the blood-stage life cycle of this parasite in vitro by inhibiting the expression of the enzyme aldolase which results in decreased malarial glycolysis and energy production. Thus, we conclude that blockade of the expression of malarial glycolytic enzymes using specific AS-ODNs has the potential of a new anti-malarial strategy.

Competitive inhibitors of yeast phosphoglucose isomerase: synthesis and evaluation of new types of phosphorylated sugars from the synthon D-arabinolactone-5-phosphate

Designed as competitive inhibitors of the isomerization reaction catalyzed by the potential chemotherapeutic target phosphoglucose isomerases (PGI), D-arabinonamide-5-phosphate and D-arabinohydrazine-5-phosphate were synthesized and fully characterized. These new types of phosphorylated sugar derivatives were easily and efficiently obtained in a one-step procedure from the promising synthon D-arabinono-1,4-lactone 5-phosphate. These two compounds proved to be new good competitive inhibitors of yeast PGI with the substrate D-fructose-6-phosphate, though not as strong as D-arabinohydroxamic acid-5-phosphate. Overall, our results are in accord with the postulated 1,2-cis-enediolate species as a probable high-energy intermediate of the PGI-catalyzed reaction.

Metabolic changes of the malaria parasite during the transition from the human to the mosquito host

Plasmodium falciparum is an obligate human parasite that is the causative agent of the most lethal form of human malaria. Transmission of P. falciparum to a new human host requires a mosquito vector within which sexual replication occurs. P. falciparum replicates as an intracellular parasite in man and as an extracellular parasite in the mosquito, and it undergoes multiple developmental changes in both hosts. Changes in the environment and the activities of parasites in these various life-cycle stages are likely to be reflected in changes in the metabolic needs and capabilities of the parasite. Most of our knowledge of the metabolic capabilities of P. falciparum is derived from studies of the asexual erythrocytic cycle of the parasite, the portion of the parasite life cycle found in infected humans that is responsible for malarial symptoms. Efforts to control transmission and to understand the sometimes unique biology of this parasite have led to information about the metabolic capabilities of sexual and/or sporogonic stages of these parasites. This review focuses on comparing and contrasting the carbohydrate, nucleic acid, and protein synthetic capabilities of asexual erythrocytic stages and sexual stages of P. falciparum.

Triosephosphate isomerase from Plasmodium falciparum: the crystal structure provides insights into antimalarial drug design

BACKGROUND

Malaria caused by the parasite Plasmodium falciparum is a major public health concern. The parasite lacks a functional tricarboxylic acid cycle, making glycolysis its sole energy source. Although parasite enzymes have been considered as potential antimalarial drug targets, little is known about their structural biology. Here we report the crystal structure of triosephosphate isomerase (TIM) from P. falciparum at 2.2 A resolution.

RESULTS

The crystal structure of P. falciparum TIM (PfTIM), expressed in Escherichia coli, was determined by the molecular replacement method using the structure of trypanosomal TIM as the starting model. Comparison of the PfTIM structure with other TIM structures, particularly human TIM, revealed several differences. In most TIMs the residue at position 183 is a glutamate but in PfTIM it is a leucine. This leucine residue is completely exposed and together with the surrounding positively charged patch, may be responsible for binding TIM to the erythrocyte membrane. Another interesting feature is the occurrence of a cysteine residue at the dimer interface of PfTIM (Cys13), in contrast to human TIM where this residue is a methionine. Finally, residue 96 of human TIM (Ser96), which occurs near the active site, has been replaced by phenylalanine in PfTIM.

CONCLUSIONS

Although the human and Plasmodium enzymes share 42% amino acid sequence identity, several key differences suggest that PfTIM may turn out to be a potential drug target. We have identified a region which may be responsible for binding PfTIM to cytoskeletal elements or the band 3 protein of erythrocytes; attachment to the erythrocyte membrane may subsequently lead to the extracellular exposure of parts of the protein. This feature may be important in view of a recent report that patients suffering from P. falciparum malaria mount an antibody response to TIM leading to prolonged hemolysis. A second approach to drug design may be provided by the mutation of the largely conserved residue (Ser96) to phenylalanine in PfTIM. This difference may be of importance in designing specific active-site inhibitors against the enzyme. Finally, specific inhibition of PfTIM subunit assembly might be possible by targeting Cys13 at the dimer interface. The crystal structure of PfTIM provides a framework for new therapeutic leads.

Molecular analysis of Plasmodium falciparum hexokinase

Hexokinase, a key glycolytic enzyme, is involved in the initial phosphorylation reaction of imported glucose and specific blocking of this activity may therefore arrest the development of malaria parasites. We describe here the cloning of a single copy hexokinase gene of Plasmodium falciparum (PfHK) from cDNA or genomic DNA libraries. The deduced amino acid sequence of PfHK has 26% identity with human hexokinase I and its predicted molecular mass assigns it as an invertebrate type isoenzyme of hexokinase. A single 1.5-kb exon is translated from a 3-kb mRNA in asexual stages of the parasite. In contrast to aldolase and GPI, the gene for this glycolytic enzyme is located on chromosome 8. Poly- and monoclonal antibodies against recombinant PfHK support our cloning results at the protein level as they detect a protein of the predicted size and isoelectric point by Western blotting in parasite protein samples. Moreover, polyclonal rabbit IgG against recombinant PfHK partially inhibits the hexokinase activity of a P. falciparum lysate which provides direct proof that the gene cloned encodes hexokinase of the parasite.