Subcellular compartmentation of glycolytic intermediates in Trypanosoma brucei

Biogenesis and metabolic homeostasis of trypanosomatid glycosomes: new insights and new questions

Kinetoplastea and Diplonemea possess peroxisome-related organelles that, uniquely, contain most of the enzymes of the glycolytic pathway and are hence called glycosomes. Enzymes of several other core metabolic pathways have also been located in glycosomes, in addition to some characteristic peroxisomal systems such as pathways of lipid metabolism. A considerable amount of research has been performed on glycosomes of trypanosomes since their discovery four decades ago. Not only the role of the glycosomal enzyme systems in the overall cell metabolism appeared to be unique, but the organelles display also remarkable features regarding their biogenesis and structural properties. These features are similar to those of the well-studied peroxisomes of mammalian and plant cells and yeasts yet exhibit also differences reflecting the large evolutionary distance between these protists and the representatives of other major eukaryotic lineages. Despite all research performed, many questions remain about various properties and the biological roles of glycosomes and peroxisomes. Here we review the current knowledge about glycosomes, often comparing it with information about peroxisomes. Furthermore, we highlight particularly many questions that remain about the biogenesis, and the heterogeneity in structure and content of these enigmatic organelles, and the properties of their boundary membrane.

Carbohydrate metabolism in trypanosomatids: New insights revealing novel complexity, diversity and species-unique features

The human pathogenic trypanosomatid species collectively called the "TriTryp parasites" - Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp. - have complex life cycles, with each of these parasitic protists residing in a different niche during their successive developmental stages where they encounter diverse nutrients. Consequently, they adapt their metabolic network accordingly. Yet, throughout the life cycles, carbohydrate metabolism - involving the glycolytic, gluconeogenic and pentose-phosphate pathways - always plays a central role in the biology of these parasites, whether the available carbon and free energy sources are saccharides, amino acids or lipids. In this paper, we provide an updated review of the carbohydrate metabolism of the TriTryps, highlighting new data about this metabolic network, the interconnection of its pathways and the compartmentalisation of its enzymes within glycosomes, cytosol and mitochondrion. Differences in the expression of the branches of the metabolic network between the successive life-cycle stages of each of these parasitic trypanosomatids are discussed, as well as differences between them. Recent structural and kinetic studies have revealed unique regulatory mechanisms for some of the network's key enzymes with important species-specific variations. Furthermore, reports of multiple post-translational modifications of trypanosomal glycolytic enzymes suggest that additional mechanisms for stage- and/or environmental cues that regulate activity are operational in the parasites. The detailed comparison of the carbohydrate metabolism of the TriTryps has thus revealed multiple differences and a greater complexity, including for the reduced metabolic network in bloodstream-form T. brucei, than previously appreciated. Although these parasites are related, share many cytological and metabolic features and are grouped within a single taxonomic family, the differences highlighted in this review reflect their separate evolutionary tracks from a common ancestor to the extant organisms. These differences are indicative of their adaptation to the different insect vectors and niches occupied in their mammalian hosts.

Trypanosoma evansi is alike to Trypanosoma brucei brucei in the subcellular localisation of glycolytic enzymes

Trypanosoma evansi, which causes surra, is descended from Trypanosoma brucei brucei, which causes nagana. Although both parasites are presumed to be metabolically similar, insufficient knowledge of T. evansi precludes a full comparison. Herein, we provide the first report on the subcellular localisation of the glycolytic enzymes in T. evansi, which is a alike to that of the bloodstream form (BSF) of T. b. brucei: (i) fructose-bisphosphate aldolase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), hexokinase, phosphofructokinase, glucose-6-phosphate isomerase, phosphoglycerate kinase, triosephosphate isomerase (glycolytic enzymes) and glycerol-3-phosphate dehydrogenase (a glycolysis-auxiliary enzyme) in glycosomes, (ii) enolase, phosphoglycerate mutase, pyruvate kinase (glycolytic enzymes) and a GAPDH isoenzyme in the cytosol, (iii) malate dehydrogenase in cytosol and (iv) glucose-6-phosphate dehydrogenase in both glycosomes and the cytosol. Specific enzymatic activities also suggest that T. evansi is alike to the BSF of T. b. brucei in glycolytic flux, which is much faster than the pentose phosphate pathway flux, and in the involvement of cytosolic GAPDH in the NAD+/NADH balance. These similarities were expected based on the close phylogenetic relationship of both parasites.

Broad distribution of TPI-GAPDH fusion proteins among eukaryotes: evidence for glycolytic reactions in the mitochondrion?

Glycolysis is a central metabolic pathway in eukaryotic and prokaryotic cells. In eukaryotes, the textbook view is that glycolysis occurs in the cytosol. However, fusion proteins comprised of two glycolytic enzymes, triosephosphate isomerase (TPI) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), were found in members of the stramenopiles (diatoms and oomycetes) and shown to possess amino-terminal mitochondrial targeting signals. Here we show that mitochondrial TPI-GAPDH fusion protein genes are widely spread across the known diversity of stramenopiles, including non-photosynthetic species (Bicosoeca sp. and Blastocystis hominis). We also show that TPI-GAPDH fusion genes exist in three cercozoan taxa (Paulinella chromatophora, Thaumatomastix sp. and Mataza hastifera) and an apusozoan protist, Thecamonas trahens. Interestingly, subcellular localization predictions for other glycolytic enzymes in stramenopiles and a cercozoan show that a significant fraction of the glycolytic enzymes in these species have mitochondrial-targeted isoforms. These results suggest that part of the glycolytic pathway occurs inside mitochondria in these organisms, broadening our knowledge of the diversity of mitochondrial metabolism of protists.

Role of cytosolic glyceraldehyde-3-phosphate dehydrogenase in visceral organ infection by Leishmania donovani

The initial 7 steps of the glycolytic pathway from glucose to 3-phosphoglycerate are localized in the glycosomes in Leishmania, including step 6, catalyzed by the enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). In L. donovani and L. mexicana, there exists a second GAPDH enzyme present in the cytosol that is absent in L. braziliensis and that has become a pseudogene in L. major. To investigate the role of the cytosolic GAPDH (cGAPDH), an L. donovani cGAPDH-null mutant was generated, and conversely, the functional L. donovani cGAPDH was introduced into L. major and the resulting engineered parasites were characterized. The L. donovani cGAPDH-null mutant was able to proliferate at the same rate as the wild-type parasite in glucose-deficient medium. However, in the presence of glucose, the L. donovani cGAPDH-null mutant consumed less glucose and proliferated more slowly than the wild-type parasite and displayed reduced infectivity in visceral organs of experimentally infected mice. This demonstrates that cGAPDH is functional in L. donovani and is required for survival in visceral organs. Restoration of cGAPDH activity in L. major, in contrast, had an adverse effect on L. major proliferation in glucose-containing medium, providing a possible explanation of why it has evolved into a pseudogene in L. major. This study indicates that there is a difference in glucose metabolism between L. donovani and L. major, and this may represent an important factor in the ability of L. donovani to cause visceral disease.

Translocation of solutes and proteins across the glycosomal membrane of trypanosomes; possibilities and limitations for targeting with trypanocidal drugs

Glycosomes are specialized peroxisomes found in all kinetoplastid organisms. The organelles are unique in harbouring most enzymes of the glycolytic pathway. Matrix proteins, synthesized in the cytosol, cofactors and metabolites have to be transported across the membrane. Recent research on Trypanosoma brucei has provided insight into how these translocations across the membrane occur, although many details remain to be elucidated. Proteins are imported by a cascade of reactions performed by specialized proteins, called peroxins, in which a cytosolic receptor with bound matrix protein inserts itself in the membrane to deliver its cargo into the organelle and is subsequently retrieved from the glycosome to perform further rounds of import. Bulky solutes, such as cofactors and acyl-CoAs, seem to be translocated by specific transporter molecules, whereas smaller solutes such as glycolytic intermediates probably cross the membrane through pore-forming channels. The presence of such channels is in apparent contradiction with previous results that suggested a low permeability of the glycosomal membrane. We propose 3 possible, not mutually exclusive, solutions for this paradox. Glycosomal glycolytic enzymes have been validated as drug targets against trypanosomatid-borne diseases. We discuss the possible implications of the new data for the design of drugs to be delivered into glycosomes.

Channel-forming activities in the glycosomal fraction from the bloodstream form of Trypanosoma brucei

BACKGROUND

Glycosomes are a specialized form of peroxisomes (microbodies) present in unicellular eukaryotes that belong to the Kinetoplastea order, such as Trypanosoma and Leishmania species, parasitic protists causing severe diseases of livestock and humans in subtropical and tropical countries. The organelles harbour most enzymes of the glycolytic pathway that is responsible for substrate-level ATP production in the cell. Glycolysis is essential for bloodstream-form Trypanosoma brucei and enzymes comprising this pathway have been validated as drug targets. Glycosomes are surrounded by a single membrane. How glycolytic metabolites are transported across the glycosomal membrane is unclear.

METHODS/PRINCIPAL FINDINGS

We hypothesized that glycosomal membrane, similarly to membranes of yeast and mammalian peroxisomes, contains channel-forming proteins involved in the selective transfer of metabolites. To verify this prediction, we isolated a glycosomal fraction from bloodstream-form T. brucei and reconstituted solubilized membrane proteins into planar lipid bilayers. The electrophysiological characteristics of the channels were studied using multiple channel recording and single channel analysis. Three main channel-forming activities were detected with current amplitudes 70-80 pA, 20-25 pA, and 8-11 pA, respectively (holding potential +10 mV and 3.0 M KCl as an electrolyte). All channels were in fully open state in a range of voltages ±150 mV and showed no sub-conductance transitions. The channel with current amplitude 20-25 pA is anion-selective (P(K+)/P(Cl-)∼0.31), while the other two types of channels are slightly selective for cations (P(K+)/P(Cl-) ratios ∼1.15 and ∼1.27 for the high- and low-conductance channels, respectively). The anion-selective channel showed an intrinsic current rectification that may suggest a functional asymmetry of the channel's pore.

CONCLUSIONS/SIGNIFICANCE

These results indicate that the membrane of glycosomes apparently contains several types of pore-forming channels connecting the glycosomal lumen and the cytosol.

When, how and why glycolysis became compartmentalised in the Kinetoplastea. A new look at an ancient organelle

A characteristic, well-studied feature of the pathogenic protists belonging to the family Trypanosomatidae is the compartmentalisation of the major part of the glycolytic pathway in peroxisome-like organelles, hence designated glycosomes. Such organelles containing glycolytic enzymes appear to be present in all members of the Kinetoplastea studied, and have recently also been detected in a representative of the Diplonemida, but they are absent from the Euglenida. Glycosomes therefore probably originated in a free-living, common ancestor of the Kinetoplastea and Diplonemida. The initial sequestering of glycolytic enzymes inside peroxisomes may have been the result of a minor mistargeting of proteins, as generally observed in eukaryotic cells, followed by preservation and its further expansion due to the selective advantage of this specific form of metabolic compartmentalisation. This selective advantage may have been a largely increased metabolic flexibility, allowing the organisms to adapt more readily and efficiently to different environmental conditions. Further evolution of glycosomes involved, in different taxonomic lineages, the acquisition of additional enzymes and pathways - often participating in core metabolic processes - as well as the loss of others. The acquisitions may have been promoted by the sharing of cofactors and crucial metabolites between different pathways, thus coupling different redox processes and catabolic and anabolic pathways within the organelle. A notable loss from the Trypanosomatidae concerned a major part of the typical peroxisomal H(2)O(2)-linked metabolism. We propose that the compartmentalisation of major parts of the enzyme repertoire involved in energy, carbohydrate and lipid metabolism has contributed to the multiple development of parasitism, and its elaboration to complicated life cycles involving consecutive different hosts, in the protists of the Kinetoplastea clade.

Metabolomic analysis of trypanosomatid protozoa

Metabolomics aims to measure all low molecular weight chemicals within a given system in a manner analogous to transcriptomics, proteomics and genomics. In this review we highlight metabolomics approaches that are currently being applied to the kinetoplastid parasites, Trypanosoma brucei and Leishmania spp. The use of untargeted metabolomics approaches, made possible through advances in mass spectrometry and informatics, and stable isotope labelling has increased our understanding of the metabolism in these organisms beyond the views established using classical biochemical approaches. Set within the context of metabolic networks, predicted using genome-wide reconstructions of metabolism, new hypotheses on how to target aspects of metabolism to design new drugs against these protozoa are emerging.

Rewiring and regulation of cross-compartmentalized metabolism in protists

Plastid acquisition, endosymbiotic associations, lateral gene transfer, organelle degeneracy or even organelle loss influence metabolic capabilities in many different protists. Thus, metabolic diversity is sculpted through the gain of new metabolic functions and moderation or loss of pathways that are often essential in the majority of eukaryotes. What is perhaps less apparent to the casual observer is that the sub-compartmentalization of ubiquitous pathways has been repeatedly remodelled during eukaryotic evolution, and the textbook pictures of intermediary metabolism established for animals, yeast and plants are not conserved in many protists. Moreover, metabolic remodelling can strongly influence the regulatory mechanisms that control carbon flux through the major metabolic pathways. Here, we provide an overview of how core metabolism has been reorganized in various unicellular eukaryotes, focusing in particular on one near universal catabolic pathway (glycolysis) and one ancient anabolic pathway (isoprenoid biosynthesis). For the example of isoprenoid biosynthesis, the compartmentalization of this process in protists often appears to have been influenced by plastid acquisition and loss, whereas for glycolysis several unexpected modes of compartmentalization have emerged. Significantly, the example of trypanosomatid glycolysis illustrates nicely how mathematical modelling and systems biology can be used to uncover or understand novel modes of pathway regulation.

Biochemistry of mammalian peroxisomes revisited

In this review, we describe the current state of knowledge about the biochemistry of mammalian peroxisomes, especially human peroxisomes. The identification and characterization of yeast mutants defective either in the biogenesis of peroxisomes or in one of its metabolic functions, notably fatty acid beta-oxidation, combined with the recognition of a group of genetic diseases in man, wherein these processes are also defective, have provided new insights in all aspects of peroxisomes. As a result of these and other studies, the indispensable role of peroxisomes in multiple metabolic pathways has been clarified, and many of the enzymes involved in these pathways have been characterized, purified, and cloned. One aspect of peroxisomes, which has remained ill defined, is the transport of metabolites across the peroxisomal membrane. Although it is clear that mammalian peroxisomes under in vivo conditions are closed structures, which require the active presence of metabolite transporter proteins, much remains to be learned about the permeability properties of mammalian peroxisomes and the role of the four half ATP-binding cassette (ABC) transporters therein.

Metabolic functions of glycosomes in trypanosomatids

Protozoan Kinetoplastida, including the pathogenic trypanosomatids of the genera Trypanosoma and Leishmania, compartmentalize several important metabolic systems in their peroxisomes which are designated glycosomes. The enzymatic content of these organelles may vary considerably during the life-cycle of most trypanosomatid parasites which often are transmitted between their mammalian hosts by insects. The glycosomes of the Trypanosoma brucei form living in the mammalian bloodstream display the highest level of specialization; 90% of their protein content is made up of glycolytic enzymes. The compartmentation of glycolysis in these organelles appears essential for the regulation of this process and enables the cells to overcome short periods of anaerobiosis. Glycosomes of all other trypanosomatid forms studied contain an extended glycolytic pathway catalyzing the aerobic fermentation of glucose to succinate. In addition, these organelles contain enzymes for several other processes such as the pentose-phosphate pathway, beta-oxidation of fatty acids, purine salvage, and biosynthetic pathways for pyrimidines, ether-lipids and squalenes. The enzymatic content of glycosomes is rapidly changed during differentiation of mammalian bloodstream-form trypanosomes to the forms living in the insect midgut. Autophagy appears to play an important role in trypanosomatid differentiation, and several lines of evidence indicate that it is then also involved in the degradation of old glycosomes, while a population of new organelles containing different enzymes is synthesized. The compartmentation of environment-sensitive parts of the metabolic network within glycosomes would, through this way of organelle renewal, enable the parasites to adapt rapidly and efficiently to the new conditions.

Energy metabolism and its compartmentation in Trypanosoma brucei

African trypanosomes are parasitic protozoa of the order of Kinetoplastida, which cause sleeping sickness and nagana. Trypanosomes are not only of scientific interest because of their clinical importance, but also because these protozoa contain several very unusual biological features, such as their special energy metabolism. The energy metabolism of Trypanosoma brucei differs significantly from that of its host, not only because it comprises distinct enzymes and metabolic pathways, but also because some of the glycolytic enzymes are localized in organelles called glycosomes. Furthermore, the energy metabolism changes drastically during the complex life cycle of this parasite. This review will focus on the recent advances made in understanding the process of ATP production in T. brucei during its life cycle and the consequences of the special subcellular compartmentation.

Biogenesis of peroxisomes and glycosomes: trypanosomatid glycosome assembly is a promising new drug target

In trypanosomatids (Trypanosoma and Leishmania), protozoa responsible for serious diseases of mankind in tropical and subtropical countries, core carbohydrate metabolism including glycolysis is compartmentalized in peculiar peroxisomes called glycosomes. Proper biogenesis of these organelles and the correct sequestering of glycolytic enzymes are essential to these parasites. Biogenesis of glycosomes in trypanosomatids and that of peroxisomes in other eukaryotes, including the human host, occur via homologous processes involving proteins called peroxins, which exert their function through multiple, transient interactions with each other. Decreased expression of peroxins leads to death of trypanosomes. Peroxins show only a low level of sequence conservation. Therefore, it seems feasible to design compounds that will prevent interactions of proteins involved in biogenesis of trypanosomatid glycosomes without interfering with peroxisome formation in the human host cells. Such compounds would be suitable as lead drugs against trypanosomatid-borne diseases.

The Origin of Dihydroorotate Dehydrogenase Genes of Kinetoplastids, with Special Reference to Their Biological Significance and Adaptation to Anaerobic, Parasitic Conditions

Trypanosoma cruzi dihydroorotate dehydrogenase (DHOD), the fourth enzyme of the de novo pyrimidine biosynthetic pathway, is localized in the cytosol and utilizes fumarate as electron acceptor (fumarate reductase activity), while the enzyme from other various eukaryotes is mitochondrial membrane-linked. Here we report that DHOD-knockout T. cruzi did not express the enzyme protein and could not survive even in the presence of pyrimidine nucleosides, substrates for the potentially active salvage pathway, suggesting a vital role of fumarate reductase activity in the regulation of cellular redox balance. Cloning and phylogenetic analysis of euglenozoan DHOD genes showed that the euglenoid Euglena gracilis had a mitochondrial DHOD and that biflagellated bodonids, a sister group of trypanosomatids within kinetoplastids, harbor the cytosolic DHOD. Further, Bodo saliens, a bodonid, had an ACT/DHOD gene fusion encoding aspartate carbamoyltransferase (ACT), the second enzyme of the de novo pyrimidine pathway, and DHOD. This is the first report of this novel gene structure. These results are consistent with suggestions that an ancient common ancestor of Euglenozoa had a mitochondrial DHOD whose descendant exists in E. gracilis and that a common ancestor of kinetoplastids (bodonids and trypanosomatids) subsequently acquired a cytosolic DHOD by horizontal gene transfer. The cytosolic DHOD gene thus acquired may have contributed to adaptation to anaerobiosis in the kinetoplastid lineage and further contributed to the subsequent establishment of parasitism in a trypanosomatid ancestor. Different molecular strategies for anaerobic adaptation in pyrimidine biosynthesis, used by kinetoplastids and by euglenoids, are discussed. Evolutionary implications of the ACT/DHOD gene fusion are also discussed.

The glycosome membrane of Trypanosoma cruzi epimastigotes: protein and lipid composition

Highly purified glycosomes from Trypanosoma cruzi epimastigotes were obtained by differential centrifugation and isopycnic ultracentrifugation. Glycosomal membranes, produced by carbonate treatment of purified glycosomes, exhibited about eight main protein bands and eight minor ones. Essentially the same protein pattern was observed in the detergent-rich fraction of a Triton X-114 fractionation of whole glycosomes, indicating that most of the membrane-bound polypeptides were highly hydrophobic. The orientation of these proteins was studied by in situ labelling followed by limited pronase hydrolysis of intact glycosomes. Three glycosome membrane proteins were characterized as peripheral by comparing the protein bands patterns of membrane fractions obtained by different treatments. Noteworthy membrane polypeptides were: (1) a peripheral 75k Da membrane protein, oriented towards the cytosol, which was the most abundant glycosomal membrane protein in exponentially growing epimastigotes but was essentially absent in stationary phase cells; (2) a pair of integral membrane proteins with molecular masses in the range of 85-100 kDa, which were only present in stationary phase cells; (3) a heme-containing 36k Da protein, strongly associated to the membrane, present in both growth phases; (4) a very immunogenic 41k Da integral membrane polypeptide, oriented towards the cytosol. The lipid composition of the glycosomal membranes was also investigated. The distribution of phospholipid species in glycosomes and glycosomal membranes was very similar to that of whole cells, with phosphatidyl-ethanolamine, phosphatidyl-choline, and phosphatidyl-serine as main components and smaller proportions of sphingomyelin and with phosphatidyl-inositol. On the other hand, glycosomes were enriched in endogenous sterols (ergosterol, 24-ethyl-5,7,22-cholesta-trien-3beta-ol), and precursors, when compared with whole cells, a finding consistent with the proposal that these organelles are involved in the de novo biosynthesis of sterols in trypanosomatids.

More plastids in human parasites?

Trypanosomatid parasites are disease agents with an extraordinarily broad host range including humans, livestock and plants. Recent work has revealed that trypanosomatids harbour numerous genes sharing apparent common ancestry with plants and/or bacteria. Although there is no evidence of a plastid (chloroplast-like organelle) in trypanosomatids, the presence of such genes suggests lateral gene transfer from some photosynthetic organism(s) during trypanosomatid evolution. Remarkably, many products of these horizontally acquired genes now function in the glycosome, a highly modified peroxisome unique to trypanosomatids and their near relatives.

Evolution of energy metabolism and its compartmentation in Kinetoplastida

Kinetoplastida are protozoan organisms that probably diverged early in evolution from other eukaryotes. They are characterized by a number of unique features with respect to their energy and carbohydrate metabolism. These organisms possess peculiar peroxisomes, called glycosomes, which play a central role in this metabolism; the organelles harbour enzymes of several catabolic and anabolic routes, including major parts of the glycolytic and pentosephosphate pathways. The kinetoplastid mitochondrion is also unusual with regard to both its structural and functional properties.In this review, we describe the unique compartmentation of metabolism in Kinetoplastida and the metabolic properties resulting from this compartmentation. We discuss the evidence for our recently proposed hypothesis that a common ancestor of Kinetoplastida and Euglenida acquired a photosynthetic alga as an endosymbiont, contrary to the earlier notion that this event occurred at a later stage of evolution, in the Euglenida lineage alone. The endosymbiont was subsequently lost from the kinetoplastid lineage but, during that process, some of its pathways of energy and carbohydrate metabolism were sequestered in the kinetoplastid peroxisomes, which consequently became glycosomes. The evolution of the kinetoplastid glycosomes and the possible selective advantages of these organelles for Kinetoplastida are discussed. We propose that the possession of glycosomes provided metabolic flexibility that has been important for the organisms to adapt easily to changing environmental conditions. It is likely that metabolic flexibility has been an important selective advantage for many kinetoplastid species during their evolution into the highly successful parasites today found in many divergent taxonomic groups.Also addressed is the evolution of the kinetoplastid mitochondrion, from a supposedly pluripotent organelle, attributed to a single endosymbiotic event that resulted in all mitochondria and hydrogenosomes of extant eukaryotes. Furthermore, indications are presented that Kinetoplastida may have acquired other enzymes of energy and carbohydrate metabolism by various lateral gene transfer events different from those that involved the algal- and alpha-proteobacterial-like endosymbionts responsible for the respective formation of the glycosomes and mitochondria.

ATP generation in the Trypanosoma brucei procyclic form: cytosolic substrate level is essential, but not oxidative phosphorylation

Trypanosoma brucei is a parasitic protist responsible for sleeping sickness in humans. The procyclic form of this parasite, transmitted by tsetse flies, is considered to be dependent on oxidative phosphorylation for ATP production. Indeed, its respiration was 55% inhibited by oligomycin, which is the most specific inhibitor of the mitochondrial F0/F1-ATP synthase. However, a 10-fold excess of this compound did not significantly affect the intracellular ATP concentration and the doubling time of the parasite was only 1.5-fold increased, suggesting that oxidative phosphorylation is not essential for procyclic trypanosomes. To further investigate the sites of ATP production, we studied the role of two ATP producing enzymes, which are involved in the synthesis of pyruvate from phosphoenolpyruvate: the glycosomal pyruvate phosphate dikinase (PPDK) and the cytosolic pyruvate kinase (PYK). The parasite was not affected by PPDK gene knockout. In contrast, inhibition of PYK expression by RNA interference was lethal for these cells. In the absence of PYK activity, the intracellular ATP concentration was reduced by up to 2.3-fold, whereas the intracellular pyruvate concentration was not reduced. Furthermore, we show that this mutant cell line still excreted acetate from d-glucose metabolism, and both the wild type and mutant cell lines consumed pyruvate present in the growth medium with similar high rates, indicating that in the absence of PYK activity pyruvate is still present in the trypanosomes. We conclude that PYK is essential because of its ATP production, which implies that the cytosolic substrate level phosphorylation is essential for the growth of procyclic trypanosomes.

Metabolic aspects of glycosomes in trypanosomatidae - new data and views

The energy metabolism of Trypanosomatidae has been the subject of many reviews during the past decade. In recent years, however, new data have led to a more complete picture of trypanosomatid metabolism and a reappraisal of the role of some characteristic organelles in the energy supply of these parasites. For years, the glycosome was thought to be a peroxisome-like organelle that had evolved to allow the parasites to carry out glycolysis at a high rate using a relatively small amount of enzyme. However, the results of recent studies of trypanosomatid glycolysis and the detection of various other pathways and enzymes in the organelle necessitate a modification of this view. Here, Paul Michels, Véronique Hannaert and Frédéric Bringaud review the new data and discuss the possible implications for our view on the role of the glycosome.

Cloning, heterologous expression and kinetic analysis of glycerol kinase (TbGLK1) from Trypanosoma brucei

We have cloned and sequenced the gene for the glycerol kinase of Trypanosoma brucei (TbGLK1), obtained by RT-PCR. The corresponding mRNA is 2.3 kb in size and contains an ORF encoding a protein with high homology to known glycerol kinases of other organisms. It is 512 amino acids in length with a PTS1-like targeting sequence (AKL) at its C-terminus, suggesting glycosomal compartmentalization of this enzyme. Although Northern blot analysis revealed higher mRNA levels in slender bloodstream forms than in the procyclic insect forms, specific glycerol kinase activities were found to be virtually identical in both life stages. Southern blot analysis suggested a single copy gene, but we were able to clone two alleles utmost similar to each other. Heterologous expression of the trypanosomal glycerol kinase in E. coli enabled us to perform a kinetic analysis of this enzyme. In particular, we have been able to monitor ATP production from glycerol-3-phosphate and ADP, a reaction which, although thermodynamically very unfavorable, is regarded essential for the survival of Trypanosoma brucei under anoxic conditions. Since the unique spatial separation of glycolysis in the kinetoplastida imposes important consequences for the regulation of the energy metabolism in these organisms, we discuss the observed differences between TbGLK1 and glycerol kinases from other organisms in view of its physiological relevance.

Molecular characterization of the first two enzymes of the pentose-phosphate pathway of Trypanosoma brucei. Glucose-6-phosphate dehydrogenase and 6-phosphogluconolactonase

Trypanosomatids are parasitic protists that have part of their glycolytic pathway sequestered inside peroxisome-like organelles: the glycosomes. So far, at least one enzyme of the pentose-phosphate pathway has been found to be associated partially with glycosomes. Here, we describe how two genes from Trypanosoma brucei, coding for the first two enzymes of the pentose-phosphate pathway, i.e. glucose-6-phosphate dehydrogenase and 6-phosphogluconolactonase, were identified by in silico screening of trypanosome genome project data bases. These genes were cloned and sequenced. Analysis of the lactonase sequence revealed that it contained a C-terminal peroxisome targeting signal in agreement with its subcellular localization in the bloodstream form trypanosome (15% glycosomal and 85% cytosolic). However, the dehydrogenase sequence did not reveal any targeting signal, despite its localization inside glycosomes. The corresponding enzymes have been overexpressed in Escherichia coli and purified, and their biochemical characteristics have been determined.

Compartmentation protects trypanosomes from the dangerous design of glycolysis

Unlike in other organisms, in trypanosomes and other Kinetoplastida the larger part of glycolysis takes place in a specialized organelle, called the glycosome. At present it is impossible to remove the glycosome without changing much of the rest of the cell. It would seem impossible, therefore, to assess the metabolic consequences of this compartmentation. Therefore, we here develop a computer experimentation approach, which we call computational cell biology. A validated molecular kinetic computer replica was built of glycolysis in the parasite Trypanosoma brucei. Removing the glycosome membrane in that replica had little effect on the steady-state flux, which argues against the prevalent speculation that glycosomes serve to increase flux by concentrating the enzymes. Removal of the membrane did cause (i) the sugar phosphates to rise to unphysiologically high levels, which must have pathological effects, and (ii) a failure to recover from glucose deprivation. We explain these effects on the basis of the biochemical organization of the glycosome. We conclude (i) that the glycosome protects trypanosomes from the negative side effects of the "turbo" structure of glycolysis and (ii) that computer experimentation based on solid molecular data is a powerful tool to address questions that are not, or not yet, accessible to experimentation.

Metabolic control analysis of glycolysis in trypanosomes as an approach to improve selectivity and effectiveness of drugs

Glycolysis is the only ATP-generating process in bloodstream form trypanosomes and is therefore a promising drug target. Inhibitors which decrease significantly the glycolytic flux will kill the parasites. Both computer simulation and experimental studies of glycolysis in bloodstream form Trypanosoma brucei indicated that the control of the glycolytic flux is shared by several steps in the pathway. The results of these analyses provide quantitative information about the prospects of decreasing the flux by inhibition of any individual enzyme. The plasma membrane glucose transporter appears the most promising target from this perspective, followed by aldolase, glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase and glycerol-3-phosphate dehydrogenase. Non-competitive or irreversible inhibitors would be most effective, but it is argued that potent competitive inhibitors can be suitable, provided that the concentration of the competing substrate cannot increase unrestrictedly. Such is the case for inhibitors that compete with coenzymes or with blood glucose.

What controls glycolysis in bloodstream form Trypanosoma brucei?

On the basis of the experimentally determined kinetic properties of the trypanosomal enzymes, the question is addressed of which step limits the glycolytic flux in bloodstream form Trypanosoma brucei. There appeared to be no single answer; in the physiological range, control shifted between the glucose transporter on the one hand and aldolase (ALD), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoglycerate kinase (PGK), and glycerol-3-phosphate dehydrogenase (GDH) on the other hand. The other kinases, which are often thought to control glycolysis, exerted little control; so did the utilization of ATP. We identified potential targets for anti-trypanosomal drugs by calculating which steps need the least inhibition to achieve a certain inhibition of the glycolytic flux in these parasites. The glucose transporter appeared to be the most promising target, followed by ALD, GDH, GAPDH, and PGK. By contrast, in erythrocytes more than 95% deficiencies of PGK, GAPDH, or ALD did not cause any clinical symptoms (Schuster, R. and Holzhütter, H.-G. (1995) Eur. J. Biochem. 229, 403-418). Therefore, the selectivity of drugs inhibiting these enzymes may be much higher than expected from their molecular effects alone. Quite unexpectedly, trypanosomes seem to possess a substantial overcapacity of hexokinase, phosphofructokinase, and pyruvate kinase, making these "irreversible" enzymes mediocre drug targets.

Compartmentation of phosphoglycerate kinase in Trypanosoma brucei plays a critical role in parasite energy metabolism

African trypanosomes compartmentalize glycolysis in a microbody, the glycosome. When growing in the mammalian bloodstream, trypanosomes contain only a rudimentary mitochondrion, and the first seven glycolytic enzymes, including phosphoglycerate kinase, are located in the glycosome. Procyclic trypanosomes, growing in the gut of tsetse flies, possess a fully developed mitochondrion that is active in oxidative phosphorylation. The first six glycolytic enzymes are still glycosomal, but phosphoglycerate kinase is now found in the cytosol. We demonstrate here that bloodstream trypanosomes are killed by expression of cytosolic phosphoglycerate kinase. The toxicity depends on both enzyme activity and cytosolic location. One possible explanation is that cytosolic phosphoglycerate kinase creates an ATP-generating shunt in the cytosol, thus preventing full ATP regeneration in the glycosome and ultimately inhibiting the first, ATP-consuming, steps of glycolysis.

Lipid metabolism in peroxisomes in relation to human disease

Peroxisomes were long believed to play only a minor role in cellular metabolism but it is now clear that they catalyze a number of important functions. The importance of peroxisomes in humans is stressed by the existence of a group of genetic diseases in man in which one or more peroxisomal functions are impaired. Most of the functions known to take place in peroxisomes have to do with lipids. Indeed, peroxisomes are capable of 1. fatty acid beta-oxidation 2. fatty acid alpha-oxidation 3. synthesis of cholesterol and other isoprenoids 4. ether-phospholipid synthesis and 5. biosynthesis of polyunsaturated fatty acids. In Chapters 2-6 we will discuss the functional organization and enzymology of these pathways in detail. Furthermore, attention is paid to the permeability properties of peroxisomes with special emphasis on recent studies which suggest that peroxisomes are closed structures containing specific membrane proteins for transport of metabolites. Finally, the disorders of peroxisomal lipid metabolism will be discussed.

Glycolysis in bloodstream form Trypanosoma brucei can be understood in terms of the kinetics of the glycolytic enzymes

In trypanosomes the first part of glycolysis takes place in specialized microbodies, the glycosomes. Most glycolytic enzymes of Trypanosoma brucei have been purified and characterized kinetically. In this paper a mathematical model of glycolysis in the bloodstream form of this organism is developed on the basis of all available kinetic data. The fluxes and the cytosolic metabolite concentrations as predicted by the model were in accordance with available data as measured in non-growing trypanosomes, both under aerobic and under anaerobic conditions. The model also reproduced the inhibition of anaerobic glycolysis by glycerol, although the amount of glycerol needed to inhibit glycolysis completely was lower than experimentally determined. At low extracellular glucose concentrations the intracellular glucose concentration remained very low, and only at 5 mM of extracellular glucose, free glucose started to accumulate intracellularly, in close agreement with experimental observations. This biphasic relation could be related to the large difference between the affinities of the glucose transporter and hexokinase for intracellular glucose. The calculated intraglycosomal metabolite concentrations demonstrated that enzymes that have been shown to be near-equilibrium in the cytosol must work far from equilibrium in the glycosome in order to maintain the high glycolytic flux in the latter.

Metabolic compartmentation in African trypanosomes

Differences between host and parasite energy metabolism are eagerly sought after as potential targets for antiparasite chemotherapy. In Kinetoplastia, the first seven steps of glycolysis are compartmented inside glycosomes, organelles that are related to the peroxisomes of higher eukaryotes. This arrangement is unique in the living world. In this review, Christine Clayton and Paul Michels discuss the implications of this unusual metabolic compartmentation for the regulation of trypanosome energy metabolism, and describe how an adequate supply of energy is maintained in different species and life cycle stages.

The inhibition of pyruvate transport across the plasma membrane of the bloodstream form of Trypanosoma brucei and its metabolic implications

The pyruvate produced by glycolysis in the bloodstream form of the trypanosome is excreted into the host bloodstream by a facilitated diffusion carrier. The sensitivity of pyruvate transport for alpha-cyano-4-hydroxycinnamate and the compound UK5099 [alpha-cyano-beta-(1-phenylindol-3-yl)acrylate], which are known to be selective inhibitors of pyruvate (monocarboxylate) transporters present in mitochondria and the plasma membrane of eukaryotic cells, was examined. The trypanosomal pyruvate carrier was found to be rather insensitive to inhibition by alpha-cyano-4-hydroxycinnamate (Ki = 17 mM) but could be completely blocked by UK5099 (Ki = 49 microM). Inhibition of pyruvate transport resulted in the retention, and concomitant accumulation, of pyruvate within the trypanosomes, causing acidification of the cytosol and osmotic destabilization of the cells. Our results indicate that this physiological state has serious metabolic consequences and ultimately leads to cell death; thereby identifying the pyruvate carrier as a possible target for chemotherapeutic intervention.

Regulation and control of compartmentalized glycolysis in bloodstream form Trypanosoma brucei

Unlike other eukaryotic cells, trypanosomes possess a compartmentalized glycolytic pathway. The conversion of glucose into 3-phosphoglycerate takes place in specialized peroxisomes, called glycosomes. Further conversion of this intermediate into pyruvate occurs in the cytosol. Due to this compartmentation, many regulatory mechanisms operating in other cell types cannot work in trypanosomes. This is reflected by the insensitivity of the glycosomal enzymes to compounds that act as activity regulators in other cell types. Several speculations have been raised about the function of compartmentation of glycolysis in trypanosomes. We calculate that even in a noncompartmentalized trypanosome the flux through glycolysis should not be limited by diffusion. Therefore, the sequestration of glycolytic enzymes in an organelle may not serve to overcome a diffusion limitation. We also search the available data for a possible relation between compartmentation and the distribution of control of the glycolytic flux among the glycolytic enzymes. Under physiological conditions, the rate of glycolytic ATP production in the bloodstream form of the parasite is possibly controlled by the oxygen tension, but not by the glucose concentration. Within the framework of Metabolic Control Analysis, we discuss evidence that glucose transport, although it does not qualify as the sole rate-limiting step, does have a high flux control coefficient. This, however, does not distinguish trypanosomes from other eukaryotic cell types without glycosomes.

Purification and characterization of the native and the recombinant Leishmania mexicana glycosomal glyceraldehyde-3-phosphate dehydrogenase

The gene coding for the glycosomal glyceraldehyde-3-phosphate dehydrogenase from Leishmania mexicana has been cloned into vector pET3A and expressed as a soluble and active protein in Escherichia coli BL21(DE3) in which the endogenous gene has been inactivated by mutation. The recombinant enzyme was purified to near homogeneity by ammonium sulphate precipitation, followed by hydrophobic and cation-exchange chromatography. From a 1-L culture of E. coli cells, 25 mg purified protein was obtained with a specific activity of 125 units/mg. The recombinant protein restores the natural E. coli phenotype when expressed at low level. The enzyme has also been partially purified from glycosomes of cultured L. mexicana promastigotes. The recombinant and the native proteins show identical mobilities on SDS/PAGE, and have the same isoelectric point and similar pH-activity profiles. The kinetics of both enzymes are very similar, the most important aspect being their lower apparent affinity for the cofactor NAD when compared to all other homologous enzymes studied, with the exception of glycosomal glyceraldehyde-3-phosphate dehydrogenase from Trypanosoma brucei.

Subcellular distribution and characterization of glucosephosphate isomerase in Leishmania mexicana mexicana

The glycolytic enzyme glucosephosphate isomerase (PGI) is present in two different cell compartments of Leishmania mexicana promastigotes; more than 90% of the activity was detected in the cytosol, the remainder in glycosomes. This subcellular distribution contrasts with that in Trypanosoma brucei, in which the enzyme activity has been mainly located in the glycosomes. PGI was partially purified from L. mexicana cell extracts. Throughout the purification procedure only one single PGI activity could be detected. The partially purified protein had the same subunit molecular mass (65 kDa) as the previously characterized glycosomal protein of T. brucei. Both proteins were also very similar with respect to their kinetic and antigenic properties. Using the T. brucei glycosomal PGI gene as a hybridization probe, we cloned the corresponding gene of L. mexicana. Only a single PGI locus could be detected in the L. mexicana genome. Characterization of the cloned gene showed that it codes for a polypeptide of 604 amino acids, with a molecular mass of 67,113. The sequences of the Leishmania and Trypanosoma polypeptides are 69% identical. They differ in calculated net charge (-8 versus -2, respectively) and isoelectric point (6.65 versus 7.35). Our data strongly suggest that the PGI activity in the two cell compartments of L. mexicana and T. brucei is not attributable to different isoenzymes. We discuss the possible metabolic function of the highly different enzyme distribution in the two organisms, and the molecular mechanism that could be responsible for it.

Energy metabolism in Leishmania

Alanine plays a key role in the response of promastigotes to osmotic stress and to hypoxia. It is rapidly released in response to hypo-osmolality, is consumed from its large intracellular pool under iso-osmotic conditions even in the presence of glucose, and is synthesized under hyperosmotic conditions even in the absence of glucose. Its rate of oxidation, in the presence or absence of any of ten other amino acids tested, is strongly inhibited by hyperosmolality. Glucose oxidation is also inhibited by hyperosmolality, but to a lesser extent than that of alanine, and is inhibited by alanine, glutamate, and aspartate. Hyperosmolality also inhibits the incorporation of label from [2-14C]acetate into the putative storage carbohydrate, mannan, which occurs via the glyoxylate bypass and the as yet unexplored "mannoneogenic" pathway. The rates of glycolysis and of oxidation of several amino acids decrease with increasing culture age, but the capacity to oxidize fatty acids increases, and in cells from 3-day stationary phase cultures hyperosmolality enhances rather than inhibits alanine oxidation.

The evolution of kinetoplastid glycosomes

The available data on carbohydrate metabolism in Kinetoplastida have been reviewed. Based on the metabolic pattern of different kinetoplastid organisms, on the subcellular distribution of their glycolytic enzymes, and on the structural and regulatory properties of these proteins, we propose that the glycosome developed from an endosymbiont, as a specific manner to control carbohydrate and energy metabolism. It is discussed how the enzymes were subcellularly recompartmentalized during evolution as adaptation to the environment encountered by the organisms.

Structure, function, and biogenesis of glycosomes in kinetoplastida

Glycosomes are intracellular, microbody-like organelles found in all members of the protist order Kinetoplastida examined. Nine enzymes involved in glucose and glycerol metabolism are associated with these organelles. These enzymes are involved in pathways which, in other organisms, are usually located in the cytosol. This paper reviews our current knowledge about the glycosome and its constituent enzymes, with special reference to the organelle of Trypanosoma brucei.

The glycosomes of the Kinetoplastida

Glycosomes are the microbodies of the organisms belonging to the order of the Kinetoplastida, comprising trypanosomes and leishmanias, both pathogens to man. The organelles sequester a number of glycolytic enzymes that are normally located in the cytosol in other eukaryotic organisms, and share some enzymes with peroxisomes and glyoxysomes of other protists, plants and animals. Proteins enter the glycosome by a mechanism of post-translational translocation which involves in some, but not all, cases a C-terminal oligopeptide sequence.

Molecular analysis of the cytosolic and glycosomal glyceraldehyde-3-phosphate dehydrogenase in Leishmania mexicana

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity was detected in two cell compartments of Leishmania mexicana promastigotes. These activities could be attributed to two different isoenzymes, one residing in glycosomes, the other in the cytosol. We have cloned and sequenced the genes for both isoenzymes. The glycosomal enzyme is encoded by two tandemly linked genes of identical sequence and contains features frequently found in glycosomal enzymes: the presence of peptide insertions, a small carboxy-terminal extension with a potential glycosomal targeting signal (-SKM) and an excess of positively charged residues (net charge +7). Only one open reading frame was detected for the cytosolic enzyme. The amino acid sequences of the two proteins are only 55% identical. We discuss some evolutionary aspects of the observed organization of the GAPDH genes in the Trypanosomatidae and the role of the two isoenzymes in the metabolism of these organisms. The possibility to develop GAPDH-specific inhibitors that will be effective against the enzyme of various parasitic members of this family is explored.

Pyruvate transport across the plasma membrane of the bloodstream form of Trypanosoma brucei is mediated by a facilitated diffusion carrier

The characteristics of pyruvate transport across the plasma membrane in the bloodstream form of Trypanosoma brucei were studied using [14C]pyruvate in combination with the silicone-oil centrifugation technique. We present evidence for the existence of a facilitated diffusion carrier in the plasma membrane of T. brucei which specifically mediates the translocation of pyruvate. The uptake of pyruvate followed saturation kinetics (Km 1.96 +/- 0.28 mM; Cmax 36.61 +/- 1.15 nmol pyruvate/30 sec.mg protein), after correction of the data for a nonsaturable diffusion component. The uptake of pyruvate was competitively inhibited by a number of (oxo)monocarboxylic acids, including pyruvate analogs and metabolically related substances, but not by L-lactate. The transport exhibited the phenomenon of transacceleration, indicative for the involvement of a facilitated diffusion carrier. The carrier is highly specific for pyruvate and differs from other known monocarboxylate carriers present in the mitochondrial and/or plasma membrane of other eukaryotic cells in that it does not transport L-lactate.

Specificity of glucose transport in Trypanosoma brucei. Effective inhibition by phloretin and cytochalasin B

Glucose transport in the bloodstream form of the protozoan parasite Trypanosoma brucei was characterized by enzymatically measuring the D-glucose uptake. Uptake kinetics showed a concentration-dependent saturable process, typical for a carrier-mediated transport system, with an apparent Km = 0.49 +/- 0.14 mM and Vmax = 252 +/- 43 nmol.min-1.mg cell protein-1 (equal to 2.25 x 10(8) trypanosomes). The specificity of glucose transport was investigated by inhibitor studies. Glucose uptake was shown to be sodium independent; neither the Na+/K(+)-ATPase inhibitor ouabain (1 mM) nor the ionophor monensin (1 microM) inhibited uptake. Transport was also unaffected by the H(+)-ATPase inhibitor N,N'-dicyclohexylcarbodiimide (DCCD; 20 microM) and the uncoupler carbonylcyanide-4-(trifluoromethoxy)phenylhydrazone (FCCP; 1 microM). However, highly significant inhibition was obtained with both phloretin (82% at 0.13 mM; Ki = 64 microM) and cytochalasin B (77% at 0.3 mM; Ki = 0.44 mM), and partial inhibition with phlorizin (14% at 0.5 mM; Ki = 3.0 mM). In each case, inhibition was noncompetitive, partially reversible (45%) for phloretin and completely reversible for cytochalasin B and phlorizin. Measurement of the temperature-dependent glucose uptake between 25 degrees C and 37 degrees C resulted in a temperature quotient of Q10 = 1.97 +/- 0.02 and an activation energy of Ea = 52.12 +/- 1.00 kJ/mol for glucose uptake. We conclude that glucose uptake in T. brucei bloodstream forms occurs via a facilitated diffusion system, clearly distinguished from the human erythrocyte-type glucose transporter with about a 10-fold higher affinity for glucose and about a 1000-fold decreased sensitivity to the inhibitor cytochalasin B.

Factors influencing the latency of the peroxisomal enzyme dihydroxyacetone-phosphate acyltransferase (DHAP-AT) in permeabilized human skin fibroblasts

In selectively permeabilized fibroblasts suspended in a medium mimicking the composition of the cytosol the peroxisomal enzyme dihydroxyacetone-phosphate acyltransferase (DHAP-AT) was found to exhibit about 80% latency (Wolvetang, E.J., Tager, J.M. and Wanders, R.J.A. (1990) Biochem. Biophys. Res. Commun. 1035, 6-11). We investigated which components of the cytosol mimicking medium are important for latency of DHAP-AT and unmasking of latent DHAP-AT activity by ATP. We show that the latency of DHAP-AT is critically dependent upon the presence of reduced glutathione in the medium and that the in vivo prevailing GSH/GSSG ratio is sufficient to maintain DHAP-AT latency. Although thiol-groups in the peroxisomal membrane seem to be essential for the integrity of peroxisomes in selectively permeabilized fibroblasts no latency of DHAP-AT is observed in buffered sucrose media or in cell homogenates, irrespective of the presence of GSH in the medium used. We suggest that during homogenization irreversible damage is inflicted upon the peroxisomal membrane and/or that more factors than at present investigated are involved in maintaining peroxisomal integrity. Furthermore, we demonstrate that cations play a role in the stimulatory effect of ATP on latent DHAP-AT activity while a proton gradient is not directly involved in the stimulatory effect of ATP on latent DHAP-AT activity.

Characterization of pyruvate kinase of Trypanosoma brucei and its role in the regulation of carbohydrate metabolism

Pyruvate kinase from Trypanosoma brucei is a labile enzyme, losing its activity within several hours. In mixtures containing 50 mM triethanolamine buffer, pH 7.2, 25% glycerol and 0.5 mM inorganic phosphate the enzyme remained active and could be purified to homogeneity with a specific activity of 417 units mg-1 and a yield of 65%. The enzyme has an activation energy of 31.9 kJ mol-1. Magnesium and potassium ions are essential for activity. Cobalt or manganese ions replace Mg2+ but this leads to a decrease in maximal velocity. Potassium ions can be substituted by ammonium ions, while sodium ions behave as a competitive inhibitor with respect to both K+ and NH4+. All metal ions studied displayed sigmoidal kinetics. The enzyme is activated, with decreasing efficiency by fructose 2-phosphorothioate 6-phosphate, fructose 2,6-bisphosphate, fructose 1,6-bisphosphate and glucose 1,6-bisphosphate. They all display hyperbolic kinetics. Glycerate 2,3-bisphosphate, glyceraldehyde 3-phosphate, CoASAc, oxalate, AMP, ADP, and ATP inhibit the enzyme. At substrate saturation PK was activated by Pi up to a concentration of 0.8 mM. At higher Pi concentrations the enzyme is inhibited. The enzyme is unaffected by most amino acids, only phenylalanine stimulates and tyrosine inhibits.

Differences in glucose transport between blood stream and procyclic forms of Trypanosoma brucei rhodesiense

In African trypanosomes the requirements for glucose and its metabolism vary in different stages of the life cycle. Here we present evidence that cultured procyclic trypanosomes of Trypanosoma brucei rhodesiense uptake glucose against a concentration gradient in a time and dose-dependent manner. Moreover, glucose transport is completely inhibited by the sulphydryl inhibitor N-ethylmaleimide, suggesting the presence of a protein moiety as the carrier molecule. Comparison of glucose uptake in bloodstream and procyclic trypanosomes point to the possibility that different transporters may function in the 2 developmental stages. Glucose uptake by bloodstream trypanosomes requires Na+ ions and is inhibited by phlorizin, an inhibitor of Na(+)-dependent glucose transporters in mammalian cells. Conversely, procyclic trypanosomes transport glucose in a Na(+)-dependent manner, and transport is not affected by phlorizin. Finally, the putative procyclic glucose transporter has a higher affinity for glucose (apparent Km 23 microM) than the bloodstream carrier (apparent Km 237 microM).

The cytosolic and glycosomal glyceraldehyde-3-phosphate dehydrogenase from Trypanosoma brucei. Kinetic properties and comparison with homologous enzymes

The protozoan haemoflagellate Trypanosoma brucei has two NAD-dependent glyceraldehyde-3-phosphate dehydrogenase isoenzymes, each with a different localization within the cell. One isoenzyme is found in the cytosol, as in other eukaryotes, while the other is found in the glycosome, a microbody-like organelle that fulfils an essential role in glycolysis. The kinetic properties of the purified glycosomal and cytosolic isoenzymes were compared with homologous enzymes from other organisms. Both trypanosome enzymes had pH/activity profiles similar to that of other glyceraldehyde-3-phosphate dehydrogenases, with optimal activity around pH 8.5-9. Only the yeast enzyme showed its maximal activity at a lower pH. The glycosomal enzyme was more sensitive to changes in ionic strength below 0.1 M, while the cytosolic enzyme resembled more the enzymes from rabbit muscle, human erythrocytes and yeast. The affinity for NAD of the glycosomal enzyme was 5-10-fold lower than that of the cytosolic, as well as the other enzymes. A similar, but less pronounced, difference was found for its affinity for NADH. These differences are explained by a number of amino acid substitutions in the NAD-binding domain of the glycosomal isoenzyme. In addition, the effects of suramin, gossypol, agaricic acid and pentalenolactone on the trypanosome enzymes were studied. The trypanocidal drug suramin inhibited both enzymes, but in a different manner. Inhibition of the cytosolic enzyme was competitive with NAD, while in the case of the glycosomal isoenzyme, with NAD as substrate, the drug had an effect both on Km and Vmax. The most potent inhibitor was pentalenolactone, which at micromolar concentrations inhibited the glycosomal enzyme and the enzymes from yeast and Bacillus stearothermophilus in a reversible manner, while the rabbit muscle enzyme was irreversibly inhibited.