The glycosomal ATP-dependent phosphofructokinase of Trypanosoma brucei must have evolved from an ancestral pyrophosphate-dependent enzyme

Delineating transitions during the evolution of specialised peroxisomes: Glycosome formation in kinetoplastid and diplonemid protists

One peculiarity of protists belonging to classes Kinetoplastea and Diplonemea within the phylum Euglenozoa is compartmentalisation of most glycolytic enzymes within peroxisomes that are hence called glycosomes. This pathway is not sequestered in peroxisomes of the third Euglenozoan class, Euglenida. Previous analysis of well-studied kinetoplastids, the ‘TriTryps’ parasites Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp., identified within glycosomes other metabolic processes usually not present in peroxisomes. In addition, trypanosomatid peroxins, i.e. proteins involved in biogenesis of these organelles, are divergent from human and yeast orthologues. In recent years, genomes, transcriptomes and proteomes for a variety of euglenozoans have become available. Here, we track the possible evolution of glycosomes by querying these databases, as well as the genome of Naegleria gruberi, a non-euglenozoan, which belongs to the same protist supergroup Discoba. We searched for orthologues of TriTryps proteins involved in glycosomal metabolism and biogenesis. Predicted cellular location(s) of each metabolic enzyme identified was inferred from presence or absence of peroxisomal-targeting signals. Combined with a survey of relevant literature, we refine extensively our previously postulated hypothesis about glycosome evolution. The data agree glycolysis was compartmentalised in a common ancestor of the kinetoplastids and diplonemids, yet additionally indicates most other processes found in glycosomes of extant trypanosomatids, but not in peroxisomes of other eukaryotes were either sequestered in this ancestor or shortly after separation of the two lineages. In contrast, peroxin divergence is evident in all euglenozoans. Following their gain of pathway complexity, subsequent evolution of peroxisome/glycosome function is complex. We hypothesize compartmentalisation in glycosomes of glycolytic enzymes, their cofactors and subsequently other metabolic enzymes provided selective advantage to kinetoplastids and diplonemids during their evolution in changing marine environments. We contend two specific properties derived from the ancestral peroxisomes were key: existence of nonselective pores for small solutes and the possibility of high turnover by pexophagy. Critically, such pores and pexophagy are characterised in extant trypanosomatids. Increasing amenability of free-living kinetoplastids and recently isolated diplonemids to experimental study means our hypothesis and interpretation of bioinformatic data are suited to experimental interrogation.

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.

Kinetic and structural studies of Trypanosoma and Leishmania phosphofructokinases show evolutionary divergence and identify AMP as a switch regulating glycolysis versus gluconeogenesis

Trypanosomatids possess glycosome organelles that contain much of the glycolytic machinery, including phosphofructokinase (PFK). We present kinetic and structural data for PFK from three human pathogenic trypanosomatids, illustrating intriguing differences that may reflect evolutionary adaptations to differing ecological niches. The activity of Leishmania PFK – to a much larger extent than Trypanosoma PFK – is reliant on AMP for activity regulation, with 1 mm AMP increasing the L. infantum PFK (LiPFK) kcat/K_0.5^F6P value by 10‐fold, compared to only a 1.3‐ and 1.4‐fold increase for T. cruzi and T. brucei PFK, respectively. We also show that Leishmania PFK melts at a significantly lower (> 15 °C) temperature than Trypanosoma PFKs and that addition of either AMP or ATP results in a marked stabilization of the protein. Sequence comparisons of Trypanosoma spp. and Leishmania spp. show that divergence of the two genera involved amino acid substitutions that occur in the enzyme’s ‘reaching arms’ and ‘embracing arms’ that determine tetramer stability. The dramatic effects of AMP on Leishmania activity compared with the Trypanosoma PFKs may be explained by differences between the T‐to‐R equilibria for the two families, with the low‐melting Leishmania PFK favouring the flexible inactive T‐state in the absence of AMP. Sequence comparisons along with the enzymatic and structural data presented here also suggest there was a loss of AMP‐dependent regulation in Trypanosoma species rather than gain of this characteristic in Leishmania species and that AMP acts as a key regulator in Leishmania governing the balance between glycolysis and gluconeogenesis.

The kinetic characteristics of human and trypanosomatid phosphofructokinases for the reverse reaction

Eukaryotic ATP-dependent phosphofructokinases (PFKs) are often considered unidirectional enzymes catalysing the transfer of a phospho moiety from ATP to fructose 6-phosphate to produce ADP and fructose 1,6-bisphosphate. The reverse reaction is not generally considered to occur under normal conditions and has never been demonstrated for any eukaryotic ATP-dependent PFKs, though it does occur in inorganic pyrophosphate-dependent PFKs and has been experimentally shown for bacterial ATP-dependent PFKs. The evidence is provided via two orthogonal assays that all three human PFK isoforms can catalyse the reverse reaction in vitro, allowing determination of kinetic properties. Additionally, the reverse reaction was shown possible for PFKs from three clinically important trypanosomatids; these enzymes are contained within glycosomes in vivo. This compartmentalisation may facilitate reversal, given the potential for trypanosomatids to have an altered ATP/ADP ratio in glycosomes compared with the cytosol. The kinetic properties of each trypanosomatid PFK were determined, including the response to natural and artificial modulators of enzyme activity. The possible physiological relevance of the reverse reaction in trypanosomatid and human PFKs is discussed.

Inorganic polyphosphate interacts with nucleolar and glycosomal proteins in trypanosomatids

Inorganic polyphosphate (polyP) is a polymer of three to hundreds of phosphate units bound by high-energy phosphoanhydride bonds and present from bacteria to humans. Most polyP in trypanosomatids is concentrated in acidocalcisomes, acidic calcium stores that possess a number of pumps, exchangers, and channels, and are important for their survival. In this work, using polyP as bait we identified > 25 putative protein targets in cell lysates of both Trypanosoma cruzi and Trypanosoma brucei. Gene ontology analysis of the binding partners found a significant over-representation of nucleolar and glycosomal proteins. Using the polyphosphate-binding domain (PPBD) of Escherichia coli exopolyphosphatase (PPX), we localized long-chain polyP to the nucleoli and glycosomes of trypanosomes. A competitive assay based on the pre-incubation of PPBD with exogenous polyP and subsequent immunofluorescence assay of procyclic forms (PCF) of T. brucei showed polyP concentration-dependent and chain length-dependent decrease in the fluorescence signal. Subcellular fractionation experiments confirmed the presence of polyP in glycosomes of T. brucei PCF. Targeting of yeast PPX to the glycosomes of PCF resulted in polyP hydrolysis, alteration in their glycolytic flux and increase in their susceptibility to oxidative stress.

Recent advances in trypanosomatid research: genome organization, expression, metabolism, taxonomy and evolution

Unicellular flagellates of the family Trypanosomatidae are obligatory parasites of invertebrates, vertebrates and plants. Dixenous species are aetiological agents of a number of diseases in humans, domestic animals and plants. Their monoxenous relatives are restricted to insects. Because of the high biological diversity, adaptability to dramatically different environmental conditions, and omnipresence, these protists have major impact on all biotic communities that still needs to be fully elucidated. In addition, as these organisms represent a highly divergent evolutionary lineage, they are strikingly different from the common 'model system' eukaryotes, such as some mammals, plants or fungi. A number of excellent reviews, published over the past decade, were dedicated to specialized topics from the areas of trypanosomatid molecular and cell biology, biochemistry, host-parasite relationships or other aspects of these fascinating organisms. However, there is a need for a more comprehensive review that summarizing recent advances in the studies of trypanosomatids in the last 30 years, a task, which we tried to accomplish with the current paper.

Farming, slaving and enslavement: histories of endosymbioses during kinetoplastid evolution

Parasitic trypanosomatids diverged from free-living kinetoplastid ancestors several hundred million years ago. These parasites are relatively well known, due in part to several unusual cell biological and molecular traits and in part to the significance of a few - pathogenic Leishmania and Trypanosoma species - as aetiological agents of serious neglected tropical diseases. However, the majority of trypanosomatid biodiversity is represented by osmotrophic monoxenous parasites of insects. In two lineages, novymonads and strigomonads, osmotrophic lifestyles are supported by cytoplasmic endosymbionts, providing hosts with macromolecular precursors and vitamins. Here we discuss the two independent origins of endosymbiosis within trypanosomatids and subsequently different evolutionary trajectories that see entrainment vs tolerance of symbiont cell divisions cycles within those of the host. With the potential to inform on the transition to obligate parasitism in the trypanosomatids, interest in the biology and ecology of free-living, phagotrophic kinetoplastids is beginning to enjoy a renaissance. Thus, we take the opportunity to additionally consider the wider relevance of endosymbiosis during kinetoplastid evolution, including the indulged lifestyle and reductive evolution of basal kinetoplastid Perkinsela.

Glycolysis as the Central Core of Fermentation

The increasing concerns of greenhouse gas emissions have increased the interest in dark fermentation as a means of productions for industrial chemicals, especially from renewable cellulosic biomass. However, the metabolism, including glycolysis, of many candidate organisms for cellulosic biomass conversion through consolidated bioprocessing is still poorly understood and the genomes have only recently been sequenced. Because a variety of industrial chemicals are produced directly from sugar metabolism, the careful understanding of glycolysis from a genomic and biochemical point of view is essential in the development of strategies for increasing product yields and therefore increasing industrial potential. The current review discusses the different pathways available for glycolysis along with unexpected variations from traditional models, especially in the utilization of alternate energy intermediates (GTP, pyrophosphate). This reinforces the need for a careful description of interactions between energy metabolites and glycolysis enzymes for understanding carbon and electron flux regulation.

N-Terminal Presequence-Independent Import of Phosphofructokinase into Hydrogenosomes of Trichomonas vaginalis

Mitochondrial evolution entailed the origin of protein import machinery that allows nuclear-encoded proteins to be targeted to the organelle, as well as the origin of cleavable N-terminal targeting sequences (NTS) that allow efficient sorting and import of matrix proteins. In hydrogenosomes and mitosomes, reduced forms of mitochondria with reduced proteomes, NTS-independent targeting of matrix proteins is known. Here, we studied the cellular localization of two glycolytic enzymes in the anaerobic pathogen Trichomonas vaginalis: PPi-dependent phosphofructokinase (TvPPi-PFK), which is the main glycolytic PFK activity of the protist, and ATP-dependent PFK (TvATP-PFK), the function of which is less clear. TvPPi-PFK was detected predominantly in the cytosol, as expected, while all four TvATP-PFK paralogues were imported into T. vaginalis hydrogenosomes, although none of them possesses an NTS. The heterologous expression of TvATP-PFK in Saccharomyces cerevisiae revealed an intrinsic capability of the protein to be recognized and imported into yeast mitochondria, whereas yeast ATP-PFK resides in the cytosol. TvATP-PFK consists of only a catalytic domain, similarly to "short" bacterial enzymes, while ScATP-PFK includes an N-terminal extension, a catalytic domain, and a C-terminal regulatory domain. Expression of the catalytic domain of ScATP-PFK and short Escherichia coli ATP-PFK in T. vaginalis resulted in their partial delivery to hydrogenosomes. These results indicate that TvATP-PFK and the homologous ATP-PFKs possess internal structural targeting information that is recognized by the hydrogenosomal import machinery. From an evolutionary perspective, the predisposition of ancient ATP-PFK to be recognized and imported into hydrogenosomes might be a relict from the early phases of organelle evolution.

Discovery of PPi-type Phosphoenolpyruvate Carboxykinase Genes in Eukaryotes and Bacteria

Phosphoenolpyruvate carboxykinase (PEPCK) is one of the pivotal enzymes that regulates the carbon flow of the central metabolism by fixing CO2 to phosphoenolpyruvate (PEP) to produce oxaloacetate or vice versa. Whereas ATP- and GTP-type PEPCKs have been well studied, and their protein identities are established, inorganic pyrophosphate (PPi)-type PEPCK (PPi-PEPCK) is poorly characterized. Despite extensive enzymological studies, its protein identity and encoding gene remain unknown. In this study, PPi-PEPCK has been identified for the first time from a eukaryotic human parasite, Entamoeba histolytica, by conventional purification and mass spectrometric identification of the native enzyme, followed by demonstration of its enzymatic activity. A homolog of the amebic PPi-PEPCK from an anaerobic bacterium Propionibacterium freudenreichii subsp. shermanii also exhibited PPi-PEPCK activity. The primary structure of PPi-PEPCK has no similarity to the functional homologs ATP/GTP-PEPCKs and PEP carboxylase, strongly suggesting that PPi-PEPCK arose independently from the other functional homologues and very likely has unique catalytic sites. PPi-PEPCK homologs were found in a variety of bacteria and some eukaryotes but not in archaea. The molecular identification of this long forgotten enzyme shows us the diversity and functional redundancy of enzymes involved in the central metabolism and can help us to understand the central metabolism more deeply.

Glucose metabolism in Trypanosoma cruzi

The causative agent of Chagas disease, Trypanosoma cruzi, metabolizes glucose through two major pathways: glycolysis and the pentose phosphate pathway. Glucose is taken up via one facilitated transporter and its catabolism by the glycolytic pathway leads to the excretion of reduced products, succinate and l-alanine, even in the presence of oxygen; the first six enzymes are located in a peroxisome-like organelle, the glycosome, and the lack of regulatory controls in hexokinase and phosphofructokinase results in the lack of the Pasteur effect. All of the enzymes of the pentose phosphate pathway are present in the four major stages of the parasite's life cycle, and some of them are possible targets for chemotherapy. The gluconeogenic enzymes phosphoenolpyruvate carboxykinase and fructose-1,6-bisphosphatase are present, but there is no reserve polysaccharide.

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.

Molecular and biochemical characterisation of Trypanosoma cruzi phosphofructokinase

The characterisation of the gene encoding Trypanosoma cruzi CL Brener phosphofructokinase (PFK) and the biochemical properties of the expressed enzyme are reported here. In contradiction with previous reports, the PFK genes of CL Brener and YBM strain T. cruzi were found to be similar to their Leishmania mexicana and Trypanosoma brucei homologs in terms of both kinetic properties and size, with open reading frames encoding polypeptides with a deduced molecular mass of 53,483. The predicted amino acid sequence contains the C-terminal glycosome-targeting tripeptide SKL; this localisation was confirmed by immunofluorescence assays. In sequence comparisons with the genes of other eukaryotes, it was found that, despite being an adenosine triphosphate-dependent enzyme, T. cruzi PFK shows significant sequence similarity with inorganic pyrophosphate-dependent PFKs.

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.

The crystal structure of ATP-bound phosphofructokinase from Trypanosoma brucei reveals conformational transitions different from those of other phosphofructokinases

The crystal structure of the ATP-bound form of the tetrameric phosphofructokinase (PFK) from Trypanosoma brucei enables detailed comparisons to be made with the structures of the apoenzyme form of the same enzyme, as well as with those of bacterial ATP-dependent and PP(i)-dependent PFKs. The active site of T. brucei PFK (which is strictly ATP-dependent but belongs to the PP(i)-dependent family by sequence similarities) is a chimera of the two types of PFK. In particular, the active site of T. brucei PFK possesses amino acid residues and structural features characteristic of both types of PFK. Conformational changes upon ATP binding are observed that include the opening of the active site to accommodate the two substrates, MgATP and fructose 6-phosphate, and a dramatic ordering of the C-terminal helices, which act like reaching arms to hold the tetramer together. These conformational transitions are fundamentally different from those of other ATP-dependent PFKs. The substantial differences in structure and mechanism of T. brucei PFK compared with bacterial and mammalian PFKs give optimism for the discovery of species-specific drugs for the treatment of diseases caused by protist parasites of the trypanosomatid family.

Assembly of heterohexameric trypanosome hexokinases reveals that hexokinase 2 is a regulable enzyme

Glycolysis is essential to Trypanosoma brucei, the protozoan parasite that causes African sleeping sickness in humans and nagana in cattle. Hexokinase (HK), the first enzyme in glycolysis, catalyzes the phosphorylation of glucose to form glucose 6-phosphate. T. brucei harbors two HKs that are 98% identical at the amino acid level, T. brucei hexokinase 1 (TbHK1) and TbHK2. Recombinant TbHK1 (rTbHK1) has HK activity, whereas rTbHK2 does not. Unlike other eukaryotic HKs, TbHK1 is not subject to inhibition by ADP and glucose 6-phosphate. However, TbHK1 is inhibited by myristate, a critical fatty acid in T. brucei biology. We report here that rTbHKs, similar to authentic TbHK, form oligomers. Myristate dissociated these assemblies when incubated with either ATP or glucose. Furthermore, oligomer disruption was reversible by removal of myristate. Mixing of rTbHK1 and rTbHK2 monomers followed by reassembly yielded enzyme with an approximately 3-fold increase in specific activity compared with similarly treated rTbHK1 alone. Surprisingly, reassembly of rTbHK2 with an inactive rTbHK1 variant yielded an active HK, revealing for the first time that rTbHK2 is competent for HK activity. Finally, pyrophosphate inhibits active reassembled rTbHK2 oligomers but not oligomeric rTbHK1, suggesting that the two enzymes have distinct regulatory mechanisms.

Design, synthesis and trypanocidal activity of lead compounds based on inhibitors of parasite glycolysis

The glycolytic pathway has been considered a potential drug target against the parasitic protozoan species of Trypanosoma and Leishmania. We report the design and the synthesis of inhibitors targeted against Trypanosoma brucei phosphofructokinase (PFK) and Leishmania mexicana pyruvate kinase (PyK). Stepwise library synthesis and inhibitor design from a rational starting point identified furanose sugar amino amides as a novel class of inhibitors for both enzymes with IC(50) values of 23microM and 26microM against PFK and PyK, respectively. Trypanocidal activity also showed potency in the low micromolar range and confirms these inhibitors as promising candidates for the development towards the design of anti-trypanosomal drugs.

Horizontal gene transfer in trypanosomatids

Trypanosomes harbour a large number of structural and biochemical peculiarities. Kinetoplast DNA, mitochondrial RNA editing, the sequestration of glycolysis inside glycosomes and unique oxidative-stress protection mechanisms (to name but a few) are found only in the members of the order Kinetoplastida. Thus, it is not surprising that they have provoked much speculation about why and how such oddities have evolved in trypanosomes. However, the true reasons for their existence within the eukaryotic world are still far from clear. Here, Fred Opperdoes and Paul Michels argue that the trypanosome-specific evolution of novel processes and organization could only have been made possible by the acquisition of a large number of foreign genes, which entered a trypanosomatid ancestor through lateral gene transfer. Many different organisms must have served as donors. Some of them were viruses, and others were bacteria, such as cyanobacterial endosymbionts and non-phototrophic bacteria.

Age-dependent differential expression and activity of rat liver cytosolic inorganic pyrophosphatase gene

Besides epigenetic factors, the genetic make-up and differential gene expression not only determines aging and disease susceptibility but also the functional activity of cells in an individual. Analysis of a variety of mammalian tissues revealed that the age-associated differentially expressed genes mainly belong to inflammation, stress, and metabolism. Intracellular PPi is a by-product of multiple biosynthetic reactions and its hydrolysis by cytosolic inorganic pyrophosphatase (iPPase) has long been considered as an important homeostatic mechanism favoring biosynthesis. In this paper we report an age-associated increase ( approximately 2-fold) in the expression of rat liver cytosolic iPPase gene by differential display PCR and northern blot analysis. Expression profiling of iPPase by RNA slot blot analysis in several other tissues revealed no significant change with aging. A comparative spectrophotometric and in-gel analysis of iPPase activity in whole cell lysate (WCL) of liver, brain, skeletal muscle, heart, spleen and kidney exhibited that liver of old rats (24 months ) has approximately 2-fold more activity than the adult (4 months) ones and also its activity is highest among the tissues. The specificity of iPPase activity in the spectrophotometric assay and in-gel analysis was confirmed by specific iPPase inhibitors like CaCl(2) and NaF.

Cloning, expression, and functional analysis of rat liver cytosolic inorganic pyrophosphatase gene and characterization of its functional promoter

Inorganic pyrophosphate (PPi) is formed in several metabolic processes and its hydrolysis by the ubiquitously expressed enzyme inorganic pyrophosphatase (iPPase) is essential for the reactions to proceed in the direction of biosynthesis. Recently, we have reported differential expression and activity of cytosolic iPPase in rat liver with aging. In this article we report the cloning of the coding region of rat liver cytosolic iPPase gene in a bacterial expression vector, its expression, purification, and functional analysis by in-gel enzyme assay. SDS-PAGE and Western blot analysis of this expressed protein revealed that its molecular weight (MW) is approximately 33 kDa, while in-gel assay showed that it is functionally active just as the liver cytosolic iPPase. We have determined the genomic organization of this gene by genome blast approach. We have also cloned and characterized its proximal approximate 1 kb functional promoter (-1009 to +82) by transient transfection and luciferase assay of different 5'-deleted iPPase promoter-luciferase constructs and also established its transcription start site by primer extension analysis, along with protein-DNA interaction studies for a few putative transcription factor binding sites.

Purification, microsequencing and cloning of spinach ATP-dependent phosphofructokinase link sequence and function for the plant enzyme

Despite its importance in plant metabolism, no sequences of higher plant ATP-dependent phosphofructokinase (EC 2.7.1.11) are annotated in the databases. We have purified the enzyme from spinach leaves 309-fold to electrophoretic homogeneity. The purified enzyme was a homotetramer of approximately 52 kDa subunits with a specific activity of 600 mU x mg(-1) and a Km value for ATP of 81 microm. The purified enzyme was not activated by phosphate, but slightly inhibited instead, suggesting that it was the chloroplast isoform. The inclusion of adenosine 5'-(beta,gamma-imido)triphosphate was conducive to enzyme activity during the purification protocol. The sequences of eight tryptic peptides from the final protein preparation, which did not utilize pyrophosphate as a phosphoryl donor, were determined and an exactly corresponding cDNA was cloned. The sequence of enzymatically active spinach ATP-dependent phosphofructokinase suggests that a large family of genomics-derived higher plant sequences currently annotated in the databases as putative pyrophosphate-dependent phosphofructokinases according to sequence similarity is misannotated with respect to the cosubstrate.

The first crystal structure of phosphofructokinase from a eukaryote: Trypanosoma brucei

The crystal structure of the ATP-dependent phosphofructokinase (PFK) from Trypanosoma brucei provides the first detailed description of a eukaryotic PFK, and enables comparisons to be made with the crystal structures of bacterial ATP-dependent and PPi-dependent PFKs. The structure reveals that two insertions (the 17-20 and 329-348 loops) that are characteristic of trypanosomatid PFKs, but absent from bacterial and mammalian ATP-dependent PFKs, are located within and adjacent to the active site, and are in positions to play important roles in the enzyme's mechanism. The 90 residue N-terminal extension forms a novel domain that includes an "embracing arm" across the subunit boundary to the symmetry-related subunit in the tetrameric enzyme. Comparisons with the PPi-dependent PFK from Borrelia burgdorferi show that several features thought to be characteristic of PPi-dependent PFKs are present in the trypanosome ATP-dependent PFK. These two enzymes are generally more similar to each other than to the bacterial or mammalian ATP-dependent PFKs. However, there are critical differences at the active site of PPi-dependent PFKs that are sufficient to prevent the binding of ATP. This crystal structure of a eukaryotic PFK has enabled us to propose a detailed model of human muscle PFK that shows active site and other differences that offer opportunities for structure-based drug discovery for the treatment of sleeping sickness and other diseases caused by the trypanosomatid family of protozoan parasites.

Pyruvate phosphate dikinase and pyrophosphate metabolism in the glycosome of Trypanosoma cruzi epimastigotes

Pyruvate phosphate dikinase (PPDK) was recently reported in trypanosomatids, but its metabolic function is not yet known. The present work deals with the cellular localization and the function of the Trypanosoma cruzi enzyme. First, we show by digitonin titration and cell fractionation that the enzyme was essentially present in the glycosome matrix of the epimastigote form. Second, we address the issue of the direction of the reaction inside the glycosome for one part, our bibliographic survey evidenced a quite exergonic DeltaGo' (at least -5.2 kcal/mol at neutral pH and physiologic ionic strength); for another part, no pyrophosphatase (PPase) could be detected in fractions corresponding to the glycosomes; therefore, glycosomal PPDK likely works in the direction of pyruvate production. Third, we address the issue of the origin of the glycosomal pyrophosphate (PPi): several synthetic pathways known to produce PPi are already considered to be glycosomal. This work also indicates the presence of an NADP(+)-dependent beta-oxidation of palmitoyl-CoA in the glycosome. Several pyruvate-consuming activities, in particular alanine dehydrogenase (ADH) and pyruvate carboxylase (PC), were detected in the glycosomal fraction. PPDK appears therefore as a central enzyme in the metabolism of the glycosome of T. cruzi by providing a link between glycolysis, fatty acid oxidation and biosynthetic PPi-producing pathways. Indeed, PPDK seems to replace pyrophosphatase in its classical thermodynamic role of displacing the equilibrium of PPi-producing reactions, as well as in its role of eliminating the toxic PPi.

A novel calcium-dependent soluble inorganic pyrophosphatase from the trypanosomatidLeishmania major

A single-copy gene IPP encoding a putative soluble inorganic pyrophosphatase (LmsPPase, EC 3.6.1.1) was identified in the genome of the parasite protozoan Leishmania major. The full-length coding sequence (ca. 0.8 kb) was obtained from genomic DNA by polymerase chain reaction (PCR) and cloned into an Escherichia coli expression vector, and was overexpressed for functional protein purification and characterization. The recombinant LmsPPase, purified to electrophoretic homogeneity by a two-step chromatography procedure, exhibited a predicted molecular mass of ca. 30 kDa. The enzyme has an absolute requirement for divalent cations, exhibits a pH optimum of 7.5-8.0 and does not hydrolyze polyphosphates or adenosine triphosphate (ATP). LmsPPase differs from previously studied soluble pyrophosphatases with respect to cation selectivity, Ca(2+) being far more effective than Mg(2+). Comparisons to known sPPases show a short N-terminal extension predicted to be a mitochondrial transit peptide, and changes in active-site residues and the neighboring region. Subcellular fractionation of L. major promastigotes suggests a mitochondrial localization. Molecular phylogenetic analysis indicates that LmsPPase is a highly divergent eukaryotic Family I sPPase, perhaps an ancestral class of eukaryotic sPPases functionally adapted to a calcium-rich, probably mitochondrial, environment.

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.

Leishmania donovani phosphofructokinase. Gene characterization, biochemical properties and structure-modeling studies

The characterization of the gene encoding Leishmania donovani phosphofructokinase (PFK) and the biochemical properties of the expressed enzyme are reported. L. donovani has a single PFK gene copy per haploid genome that encodes a polypeptide with a deduced molecular mass of 53 988 and a pI of 9.26. The predicted amino acid sequence contains a C-terminal tripeptide that conforms to an established signal for glycosome targeting. L. donovani PFK showed most sequence similarity to inorganic pyrophosphate (PPi)-dependent PFKs, despite being ATP-dependent. It thereby resembles PFKs from other Kinetoplastida such as Trypanosoma brucei, Trypanoplasma borreli (characterized in this study), and a PFK found in Entamoeba histolytica. It exhibited hyperbolic kinetics with respect to ATP whereas the binding of the other substrate, fructose 6-phosphate, showed slight positive cooperativity. PPi, even at high concentrations, did not have any effect. AMP acted as an activator of PFK, shifting its kinetics for fructose 6-phosphate from slightly sigmoid to hyperbolic, and increasing considerably the affinity for this substrate, whereas GDP did not have any effect. Modelling studies and site-directed mutagenesis were employed to shed light on the structural basis for the AMP effector specificity and on ATP/PPi specificity among PFKs.

The 3.0 A resolution crystal structure of glycosomal pyruvate phosphate dikinase from Trypanosoma brucei

The crystal structure of the glycosomal enzyme pyruvate phosphate dikinase from the African protozoan parasite Trypanosoma brucei has been solved to 3.0 A resolution by molecular replacement. The search model was the 2.3 A resolution structure of the Clostridium symbiosum enzyme. Due to different relative orientations of the domains and sub-domains in the two structures, molecular replacement could be achieved only by positioning these elements (four bodies altogether) sequentially in the asymmetric unit of the P2(1)2(1)2 crystal, which contains one pyruvate phosphate dikinase (PPDK) subunit. The refined model, comprising 898 residues and 188 solvent molecules per subunit, has a crystallographic residual index Rf = 0.245 (cross-validation residual index Rfree = 0.291) and displays satisfactory stereochemistry. Eight regions, comprising a total of 69 amino acid residues at the surface of the molecule, are disordered in this crystal form. The PPDK subunits are arranged around the crystallographic 2-fold axis as a dimer, analogous to that observed in the C. symbiosum enzyme. Comparison of the two structures was carried out by superposition of the models. Although the fold of each domain or sub-domain is similar, the relative orientations of these constitutive elements are different in the two structures. The trypanosome enzyme is more "bent" than the bacterial enzyme, with bending increasing from the center of the molecule (close to the molecular 2-fold axis) towards the periphery where the N-terminal domain is located. As a consequence of this increased bending and of the differences in relative positions of subdomains, the nucleotide-binding cleft in the amino-terminal domain is wider in T. brucei PPDK: the N-terminal fragment of the amino-terminal domain is distant from the catalytic, phospho-transfer competent histidine 482 (ca 10 A away). Our observations suggest that the requirements of domain motion during enzyme catalysis might include widening of the nucleotide-binding cleft to allow access and departure of the AMP or ATP ligand.

Presence of prokaryotic and eukaryotic species in all subgroups of the PP(i)-dependent group II phosphofructokinase protein family

Inorganic pyrophosphate-dependent phosphofructokinase (PP(i)-PFK) of the amitochondriate eukaryote Mastigamoeba balamuthi was sequenced and showed about 60% identity to PP(i)-PFKs from two eubacteria, Propionibacterium freudenreichii and Sinorhizobium meliloti. These gene products represent a newly recognized lineage of PFKs. All four lineages of group II PFKs, as defined by phylogenetic analysis, contained both prokaryotic and eukaryotic species, underlining the complex evolutionary history of this enzyme.

Inorganic pyrophosphate generation and disposition in pathophysiology

Inorganic pyrophosphate (PP(i)) regulates certain intracellular functions and extracellular crystal deposition. PP(i) is produced, degraded, and transported by specialized mechanisms. Moreover, dysregulated cellular PP(i) production, degradation, and transport all have been associated with disease, and PP(i) appears to directly mediate specific disease manifestations. In addition, natural and synthetic analogs of PP(i) are in use or currently under evaluation as prophylactic agents or therapies for disease. This review summarizes recent developments in the understanding of how PP(i) is made and disposed of by cells and assesses the body of evidence for potentially significant physiological functions of intracellular PP(i) in higher organisms. Major topics addressed are recent lines of molecular evidence that directly link decreased and increased extracellular PP(i) levels with diseases in which connective tissue matrix calcification is disordered. To illustrate in depth the effects of disordered PP(i) metabolism, this review weighs the roles in matrix calcification of the transmembrane protein ANK, which regulates intracellular to extracellular movement of PP(i), and the PP(i)-generating phosphodiesterase nucleotide pyrophosphatase family isoenzyme plasma cell membrane glycoprotein-1 (PC-1).

The two phosphofructokinase gene products of Entamoeba histolytica

Two phosphofructokinase genes have been described previously in Entamoeba histolytica. The product of the larger of the two genes codes for a 60-kDa protein that has been described previously as a pyrophosphate (PP(i))-dependent enzyme, and the product of the second, coding for a 48-kDa protein, has been previously reported to be a PP(i)-dependent enzyme with extremely low specific activity. Here it is found that the 48-kDa protein is not a PP(i)-dependent enzyme but a highly active ATP-requiring enzyme (k(cat) = 250 s(-)1) that binds the cosubstrate fructose 6-phosphate (Fru-6-P) with relatively low affinity. This enzyme exists in concentration- and ATP-dependent tetrameric active and dimeric inactive states. Activation is achieved in the presence of nucleoside triphosphates, ADP, and PP(i), but not by AMP, P(i), or the second substrate Fru-6-P. Activation by ATP is facilitated by conditions of molecular crowding. Divalent cations are not required, and no phosphoryl transfer occurs during activation. Kinetics of the activated enzyme show cooperativity with Fru-6-P (Fru-6-P(0.5) = 3.8 mm) and inhibition by high ATP and phosphoenolpyruvate. The enzyme is active without prior activation in extracts of E. histolytica. The level of mRNA, the amount of enzyme protein, and the enzyme activity of the 48-kDa enzyme are about one-tenth that of the 60-kDa enzyme in extracts of E. histolytica trophozoites.

Cloning of a pyruvate phosphate dikinase from Trypanosoma cruzi

We have cloned and characterised a gene that encodes a putative pyruvate phosphate dikinase (PPDK) from Trypanosoma cruzi, an enzyme that catalyses the reversible conversion of phosphoenolpyruvate to pyruvate. PPDK is absent in mammalian cells, but has been found in a wide variety of other organisms, including plants and bacteria. In T. cruzi, two genes (PPDK1 and PPDK2) are present in a tandem array localised on a 1 Mbp chromosome. Northern and Western blot analyses indicates that PPDK is expressed as a 100-kDa protein in epimastigote, amastigote and trypomastigote forms. PPDK1 and PPDK2 encode an identical protein of 100.8 kDa with a C-terminal extension ending with the sequence AKL, a signal for glycosomal import. Both T. cruzi and T. brucei enzymes possess a 23-residue insertion, that is absent in other PPDKs. A three-dimensional alignment with the crystal structure of the enzyme from Clostridium symbiosum predicts that this insertion is located on the surface of the nucleotide-binding domain. Phylogenetic studies indicate that bacterial and protist PPDKs cluster as a separate group from those of plants. The evolutionary implications and possible role of this enzyme in T. cruzi is discussed.

Glycolysis as a target for the design of new anti-trypanosome drugs

Glycolysis is perceived as a promising target for new drugs against parasitic trypanosomatid protozoa because this pathway plays an essential role in their ATP supply. Trypanosomatid glycolysis is unique in that it is compartmentalized, and many of its enzymes display unique structural and kinetic features. Structure- and catalytic mechanism-based approaches are applied to design compounds that inhibit the glycolytic enzymes of the parasites without affecting the corresponding proteins of the human host. For some trypanosomatid enzymes, potent and selective inhibitors have already been developed that affect only the growth of cultured trypanosomatids, and not mammalian cells.

Thermotoga maritima phosphofructokinases: expression and characterization of two unique enzymes

A pyrophosphate-dependent phosphofructokinase (PP(i)-PFK) and an ATP-dependent phosphofructokinase (ATP-PFK) from Thermotoga maritima have been cloned and characterized. The PP(i)-PFK is unique in that the K(m) and V(max) values indicate that polyphosphate is the preferred substrate over pyrophosphate; the enzyme in reality is a polyphosphate-dependent PFK. The ATP-PFK was not significantly affected by common allosteric effectors (e.g., phosphoenolpyruvate) but was strongly inhibited by PP(i) and polyphosphate. The results suggest that the control of the Embden-Meyerhof pathway in this organism is likely to be modulated by pyrophosphate and/or polyphosphate.

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.

Sequencing, cloning, and high-level expression of the pfp gene, encoding a PP(i)-dependent phosphofructokinase from the extremely thermophilic eubacterium Dictyoglomus thermophilum

The sequencing, cloning, and expression of the pfp gene from Dictyoglomus thermophilum, which consists of 1,041 bp and encodes a pyrophosphate-dependent phosphofructokinase, are described. A phylogenetic analysis indicates that the enzyme is closely related to the pyrophosphate-dependent enzyme from Thermoproteus tenax. The recombinant and native enzymes share a high degree of similarity for most properties examined.

Enolase from Trypanosoma brucei, from the amitochondriate protist Mastigamoeba balamuthi, and from the chloroplast and cytosol of Euglena gracilis: pieces in the evolutionary puzzle of the eukaryotic glycolytic pathway

Genomic or cDNA clones for the glycolytic enzyme enolase were isolated from the amitochondriate pelobiont Mastigamoeba balamuthi, from the kinetoplastid Trypanosoma brucei, and from the euglenid Euglena gracilis. Clones for the cytosolic enzyme were found in all three organisms, whereas Euglena was found to also express mRNA for a second isoenzyme that possesses a putative N-terminal plastid-targeting peptide and is probably targeted to the chloroplast. Database searching revealed that Arabidopsis also possesses a second enolase gene that encodes an N-terminal extension and is likely targeted to the chloroplast. A phylogeny of enolase amino acid sequences from 6 archaebacteria, 24 eubacteria, and 32 eukaryotes showed that the Mastigamoeba enolase tended to branch with its homologs from Trypanosoma and from the amitochondriate protist Entamoeba histolytica. The compartment-specific isoenzymes in Euglena arose through a gene duplication independent of that which gave rise to the compartment-specific isoenzymes in Arabidopsis, as evidenced by the finding that the Euglena enolases are more similar to the homolog from the eubacterium Treponema pallidum than they are to homologs from any other organism sampled. In marked contrast to all other glycolytic enzymes studied to date, enolases from all eukaryotes surveyed here (except Euglena) are not markedly more similar to eubacterial than to archaebacterial homologs. An intriguing indel shared by enolase from eukaryotes, from the archaebacterium Methanococcus jannaschii, and from the eubacterium Campylobacter jejuni maps to the surface of the three-dimensional structure of the enzyme and appears to have occurred at the same position in parallel in independent lineages.

Characterization of a vacuolar pyrophosphatase in Trypanosoma brucei and its localization to acidocalcisomes

Inorganic pyrophosphate promoted the acidification of an intracellular compartment in permeabilized procyclic trypomastigotes of Trypanosoma brucei, as measured by acridine orange uptake. The proton gradient generated by pyrophosphate was collapsed by addition of nigericin or NH(4)Cl. Pyrophosphate-driven proton translocation was stimulated by potassium ions and inhibited by KF, by the pyrophosphate analogs imidodiphosphate and aminomethylenediphosphonate (AMDP), and by the thiol reagent p-hydroxymercuribenzoate at concentrations similar to those that inhibit the plant vacuolar H(+)-pyrophosphatase (PPase). The proton translocation activity had a pH optimum around 7.5 and was partially inhibited by 7-chloro-4-nitrobenz-2-oxa-1,3-diazole (10 microM) and unaffected by bafilomycin A(1) (40 nM), concanamycin A (5 nM), sodium o-vanadate (500 microM), oligomycin (1 microM), N-ethylmaleimide (100 microM), and KNO(3). AMDP-sensitive pyrophosphate hydrolysis was detected in both procyclic and bloodstream trypomastigotes. Measurements of acridine orange uptake in permeabilized procyclic trypomastigotes in the presence of different substrates and inhibitors suggested the presence of H(+)-ATPase, H(+)-PPase, and (ADP-dependent) H(+)/Na(+) antiport activity in the same compartment. Separation of bloodstream and procyclic trypomastigote extracts on Percoll gradients yielded fractions that contained H(+)-PPase (both stages) and H(+)/Na(+) exchanger (procyclics) activities but lacked markers for mitochondria, glycosomes, and lysosomes. The organelles in these fractions were identified by electron microscopy and X-ray microanalysis as acidocalcisomes (electron-dense vacuoles). These results provide further evidence for the unique nature of acidocalcisomes in comparison with other, previously described, organelles.

Presence of a plant-like proton-pumping pyrophosphatase in acidocalcisomes of Trypanosoma cruzi

The vacuolar-type proton-translocating pyrophosphatase (V-H+-PPase) is an enzyme previously described in detail only in plants. This paper demonstrates its presence in the trypanosomatid Trypanosoma cruzi. Pyrophosphate promoted organellar acidification in permeabilized amastigotes, epimastigotes, and trypomastigotes of T. cruzi. This activity was stimulated by K+ ions and was inhibited by Na+ ions and pyrophosphate analogs, as is the plant activity. Separation of epimastigote extracts on Percoll gradients yielded a dense fraction that contained H+-PPase activity measured both by proton uptake and phosphate release but lacked markers for mitochondria, lysosomes, glycosomes, cytosol, and plasma membrane. Antiserum raised against specific sequences of the plant V-H+-PPase cross-reacted with a T. cruzi protein, which was also detectable in the dense Percoll fraction. The organelles in this fraction appeared by electron microscopy to consist mainly of acidocalcisomes (acidic calcium storage organelles). This identification was confirmed by x-ray microanalysis. Immunofluorescence and immunoelectron microscopy indicated that the V-H+-PPase was located in the plasma membrane and acidocalcisomes of the three different forms of the parasite. Pyrophosphate was able to drive calcium uptake in permeabilized T. cruzi. This uptake depended upon a proton gradient and was reversed by a specific V-H+-PPase inhibitor. Our results imply that the phylogenetic distribution of V-H+-PPases is much wider than previously perceived but that the enzyme has a unique subcellular location in trypanosomes.

Functional and molecular characterization of a glycosomal PPi-dependent enzyme in trypanosomatids: pyruvate, phosphate dikinase

Trypanosomatids are parasitic protists that have an ATP-dependent glycolysis with no indication of PPi-dependent metabolism. Most of the glycolysis takes place in peroxisome-like organelles, the glycosomes. We characterized in Trypanosoma brucei a single-copy gene encoding a PPi-dependent enzyme, pyruvate, phosphate dikinase (PPDK), which was expressed functionally in Escherichia coli. Specific antibodies detected a 100-kDa protein in procyclic forms but not in mammalian forms of T. brucei, indicating a differential expression. Glycosomal localization of PPDK was determined by immunofluorescence analysis and was confirmed by Western blot analysis on glycosomal fractions by using anti-PPDK antibodies. Expression and localization of recombinant PPDKs in procyclic forms of T. brucei showed that the AKL motif at the C-terminal extremity of PPDK is necessary for glycosomal targeting. PPDK was detected in every trypanosomatid tested-Trypanosoma congolense, Trypanosoma vivax, Trypanosoma cruzi, Phytomonas, Crithidia and Leishmania-with a good correlation between amount of protein and enzymatic activity. The precise role of PPDK in trypanosomatid carbohydrate metabolism remains to be clarified.