Metabolic aspects of glycosomes in trypanosomatidae - new data and views

Structural insights into the recognition of peroxisomal targeting signal 1 by Trypanosoma brucei peroxin 5

Glycosomes are peroxisome-like organelles essential for trypanosomatid parasites. Glycosome biogenesis is mediated by proteins called "peroxins," which are considered to be promising drug targets in pathogenic Trypanosomatidae. The first step during protein translocation across the glycosomal membrane of peroxisomal targeting signal 1 (PTS1)-harboring proteins is signal recognition by the cytosolic receptor peroxin 5 (PEX5). The C-terminal PTS1 motifs interact with the PTS1 binding domain (P1BD) of PEX5, which is made up of seven tetratricopeptide repeats. Obtaining diffraction-quality crystals of the P1BD of Trypanosoma brucei PEX5 (TbPEX5) required surface entropy reduction mutagenesis. Each of the seven tetratricopeptide repeats appears to have a residue in the alpha(L) conformation in the loop connecting helices A and B. Five crystal structures of the P1BD of TbPEX5 were determined, each in complex with a hepta- or decapeptide corresponding to a natural or nonnatural PTS1 sequence. The PTS1 peptides are bound between the two subdomains of the P1BD. These structures indicate precise recognition of the C-terminal Leu of the PTS1 motif and important interactions between the PTS1 peptide main chain and up to five invariant Asn side chains of PEX5. The TbPEX5 structures reported here reveal a unique hydrophobic pocket in the subdomain interface that might be explored to obtain compounds that prevent relative motions of the subdomains and interfere selectively with PTS1 motif binding or release in trypanosomatids, and would therefore disrupt glycosome biogenesis and prevent parasite growth.

Residues in an ATP binding domain influence sugar binding in a trypanosome hexokinase

Trypanosoma brucei harbors two hexokinases (TbHK1 and TbHK2) that are 98% identical at the amino acid level. We previously found that recombinant TbHK1 (rTbHK1) has hexokinase activity, while rTbHK2 has not, a finding attributed to differences in the C-termini of the proteins. Sequence analysis suggests that the C-termini of TbHKs are part of a newly identified conserved motif found in other eukaryotic hexokinases. Here, we have explored the role of tail residues in the differences in catalytic activity between TbHK1 and TbHK2. Our studies reveal that tail residues D454, F462, M466, and N469 are essential for HK activity while both I458 and V468 are required for catalysis and substrate specificity. To activate rTbHK2, all of the residues important for activity in rTbHK1 (D454, V458, F462, M466, V468, and N469) were required. These results indicate that the overall structure of the C-terminal tail influences the HK activity of rTbHK1.

Control and regulation of gene expression: quantitative analysis of the expression of phosphoglycerate kinase in bloodstream form Trypanosoma brucei

Isoenzymes of phosphoglycerate kinase in Trypanosoma brucei are differentially expressed in its two main life stages. This study addresses how the organism manages to make sufficient amounts of the isoenzyme with the correct localization, which processes (transcription, splicing, and RNA degradation) control the levels of mRNAs, and how the organism regulates the switch in isoform expression. For this, we combined new quantitative measurements of phosphoglycerate kinase mRNA abundance, RNA precursor stability, trans splicing, and ribosome loading with published data and made a kinetic computer model. For the analysis of regulation we extended regulation analysis. Although phosphoglycerate kinase mRNAs are present at surprisingly low concentrations (e.g. 12 molecules per cell), its protein is highly abundant. Substantial control of mRNA and protein levels was exerted by both mRNA synthesis and degradation, whereas splicing and precursor degradation had little control on mRNA and protein concentrations. Yet regulation of mRNA levels does not occur by transcription, but by adjusting mRNA degradation. The contribution of splicing to regulation is negligible, as for all cases where splicing is faster than RNA precursor degradation.

Molecular and biochemical characterization of novel glucokinases from Trypanosoma cruzi and Leishmania spp

Glucokinase genes, found in the genome databases of Trypanosoma cruzi and Leishmania major, were cloned and sequenced. Their expression in Escherichia coli resulted in the synthesis of soluble and active enzymes, TcGlcK and LmjGlcK, with a molecular mass of 43 kDa and 46 kDa, respectively. The enzymes were purified, and values of their kinetic parameters determined. The K(m) values for glucose were 1.0 mM for TcGlcK and 3.3 mM for LmjGlcK. For ATP, the K(m) values were 0.36 mM (TcGlcK) and 0.35 mM (LmjGlcK). A lower K(m) value for glucose (2.55 mM) was found when the (His)(6)-tag was removed from the recombinant LmjGlcK, whereas the TcGlcK retained the same value. The V(max)'s of the T. cruzi and L. major GlcKs were 36.3 and 30.9 U/mg of protein, respectively. No inhibition was exerted by glucose-6-phosphate. Similarly, no inhibition by inorganic pyrophosphate was found in contrast to previous observations made for the T. cruzi and L. mexicana hexokinases. Both trypanosomatid enzymes were only able to phosphorylate glucose indicating that they are true glucokinases. Gel-filtration chromatography showed that the GlcK of both trypanosomatids may occur as a monomer or dimer, dependent on the protein concentration. Both GlcK sequences have a type-1 peroxisome-targeting signal. Indeed, they were shown to be present inside glycosomes using three different methods. These glucokinases present highest, albeit still a moderate 24% sequence identity with their counterpart from Trichomonas vaginalis, which has been classified into group A of the hexokinase family. This group comprises mainly eubacterial and cyanobacterial glucokinases. Indeed, multiple sequence comparisons, as well as kinetic properties, strongly support the notion that these trypanosomatid enzymes belong to group A of the hexokinases, in which they, according to a phylogenetic analysis, form a separate cluster.

Antiproliferative effect of dihydroxyacetone on Trypanosoma brucei bloodstream forms: cell cycle progression, subcellular alterations, and cell death

We evaluated the effects of dihydroxyacetone (DHA) on Trypanosoma brucei bloodstream forms. DHA is considered an energy source for many different cell types. T. brucei takes up DHA readily due to the presence of aquaglyceroporins. However, the parasite is unable to use it as a carbon source because of the absence of DHA kinase (DHAK). We could not find a homolog of the relevant gene in the genomic database of T. brucei and have been unable to detect DHAK activity in cell lysates of the parasite, and the parasite died quickly if DHA was the sole energy source in the medium. In addition, during trypanosome cultivation, DHA induced growth inhibition with a 50% inhibitory concentration of about 1 mM, a concentration that is completely innocuous to mammals. DHA caused cell cycle arrest in the G(2)/M phase of up to 70% at a concentration of 2 mM. Also, DHA-treated parasites showed profound ultrastructural alterations, including an increase of vesicular structures within the cytosol and the presence of multivesicular bodies, myelin-like structures, and autophagy-like vacuoles, as well as a marked disorder of the characteristic mitochondrion structure. Based on the toxicity of DHA for trypanosomes compared with mammals, we consider DHA a starting point for a rational design of new trypanocidal drugs.

Leishmania mexicana: Molecular cloning and characterization of enolase

The gene of Leishmania mexicana enolase was cloned and overexpressed in Escherichia coli as an active enzyme; the protein was biochemically analyzed. This enolase shares with enolases from other trypanosomatids the presence of three atypical residues, each with a reactive side group, near the active site, already described for the enzyme from Trypanosoma brucei. The natural enzyme was purified, using a three-step procedure, from a cytosolic fraction of L. mexicana promastigotes. The kinetic properties of the purified recombinant enzyme were similar to those of the natural enzyme. Both the recombinant and natural enzyme were inhibited by inorganic pyrophosphate. Subcellular localization analysis after differential centrifugation showed that the enzyme activity is only associated with the cytosolic fraction. However, an apparently inactive form of enolase was detected by Western blots in the microsomal fraction. Digitonin treatment of parasites and immunofluorescence studies with permeabilized and non-permeabilized parasites showed that enolase is also associated with membranes and it was found at the external face of the plasma membrane.

Occurrence of multiple, independent gene fusion events for the fifth and sixth enzymes of pyrimidine biosynthesis in different eukaryotic groups

The genes encoding orotate phosphoribosyltransferase (OPRT) and orotidine-5'-monophosphate decarboxylase (OMPDC), the fifth and sixth enzymes in the de novo pyrimidine biosynthetic pathway, are fused as OPRT-OMPDC in most eukaryotic groups. On the other hand, the inversely linked OMPDC-OPRT fusion is present in trypanosomatids, belonging to kinetoplastids together with bodonids in a supergroup, Euglenozoa. Here, we show the presence of OMPDC-OPRT in the bodonid, Bodo caudatus, while OPRT-OMPDC in Euglena gracilis, another euglenozoan species belonging to euglenoids. These results suggest that the OMPDC-OPRT fusion event occurred in a common ancestor of kinetoplastids. Genome sequence database searches further revealed the presence of OMPDC-OPRT in stramenopiles and cyanobacteria. Phylogenetic reconstruction of OPRT and OMPDC rejected statistically the monophyly of the OPRT domains of stramenopile and kinetoplastid OMPDC-OPRT, demonstrating that these gene fusions do not share a common evolutionary origin, despite the identical gene order. Thus, the OMPDC-OPRT fusion is likely to have emerged independently in these eukaryotic groups. Phylogenetic analyses also suggested that cyanobacterial OMPDC-OPRT arose via lateral transfer. We conclude that gene fusion events occur more frequently than previously thought and that lateral gene transfer has made a marked contribution to establishment of the rearranged structure of OPRT and OMPDC genes in eukaryotes.

Characterization and developmentally regulated localization of the mitochondrial carrier protein homologue MCP6 from Trypanosoma brucei

Proteins of the mitochondrial carrier family (MCF) are located mainly in the inner mitochondrial membrane and mediate the transport of a large range of metabolic intermediates. The genome of Trypanosoma brucei harbors 29 genes encoding different MCF proteins. We describe here the characterization of MCP6, a novel T. brucei MCF protein. Sequence comparison and phylogenetic reconstruction revealed that MCP6 is closely related to different mitochondrial ADP/ATP and calcium-dependent solute carriers, including the ATP-Mg/Pi carrier of Homo sapiens. However, MCP6 lacks essential amino acids and sequence motifs conserved in these metabolite transporters, and functional reconstitution and transport assays with E. coli suggested that this protein indeed does not function as an ADP/ATP or ATP-Mg/Pi carrier. The subcellular localization of MCP6 is developmentally regulated: in bloodstream-form trypanosomes, the protein is predominantly glycosomal, whereas in the procyclic form, it is found mainly in the mitochondria. Depletion of MCP6 in procyclic trypanosomes resulted in growth inhibition, an increased cell size, aberrant numbers of nuclei and kinetoplasts, and abnormal kinetoplast morphology, suggesting that depletion of MCP6 inhibits division of the kinetoplast.

Characterization of the role of the receptors PEX5 and PEX7 in the import of proteins into glycosomes of Trypanosoma brucei

Peroxins 5 and 7 are receptors for protein import into the peroxisomal matrix. We studied the involvement of these peroxins in the biogenesis of glycosomes in the protozoan parasite Trypanosoma brucei. Glycosomes are peroxisome-like organelles in which a major part of the glycolytic pathway is sequestered. We here report the characterization of the T. brucei homologue of PEX7 and provide several data strongly suggesting that it can bind to PEX5. Depletion of PEX5 or PEX7 by RNA interference had a severe effect on the growth of both the bloodstream-form of the parasite, that relies entirely on glycolysis for its ATP supply, and the procyclic form representative of the parasite living in the tsetse-fly midgut and in which also other metabolic pathways play a prominent role. The role of the two receptors in import of glycosomal matrix proteins with different types of peroxisome/glycosome-targeting signals (PTS) was analyzed by immunofluorescence and subcellular fractionation studies. Knocking down the expression of either receptor gene resulted, in procyclic cells, in the mislocalization of proteins with both a type 1 or 2 targeting motif (PTS1, PTS2) located at the C- and N-termini, respectively, and proteins with a sequence-internal signal (I-PTS) to the cytosol. Electron microscopy confirmed the apparent integrity of glycosomes in these procyclic cells. In bloodstream-form trypanosomes, PEX7 depletion seemed to affect only the subcellular distribution of PTS2-proteins. Western blot analysis suggested that, in both life-cycle stages of the trypanosome, the levels of both receptors are controlled in a coordinated fashion, by a mechanism that remains to be determined. The observation that both PEX5 and PEX7 are essential for the viability of the parasite indicates that the respective branches of the glycosome-import pathway in which each receptor acts might be interesting drug targets.

The ins and outs of peroxisomes: Co-ordination of membrane transport and peroxisomal metabolism

Peroxisomes perform a range of metabolic functions which require the movement of substrates, co-substrates, cofactors and metabolites across the peroxisomal membrane. In this review, we discuss the evidence for and against specific transport systems involved in peroxisomal metabolism and how these operate to co-ordinate biochemical reactions within the peroxisome with those in other compartments of the cell.

Purification and characterization of hexokinase from Leishmania mexicana

Hexokinase from Leishmania mexicana was purified to homogeneity from a glycosome-enriched fraction obtained after a differential centrifugation of promastigote form. The kinetic properties of the pure enzyme were determined and the Km values for glucose (Km = 66 microM) and ATP (Km = 303 muM) were comparable to those from hexokinase of Trypanosoma cruzi. L. mexicana hexokinase was able to use fructose (Km = 142 microM), which reflects the condition found in the insect host. In contrast with hexokinases from other trypanosomatids, the enzyme exhibited a moderate sensitivity to inhibition by glucose 6-phosphate. This inhibition was competitive with respect to both ATP and glucose, indicating that an allosteric site for glucose 6-phosphate does not exist in this enzyme. The enzyme was also inhibited by inorganic pyrophosphate, the inhibition being higher than that observed for T. cruzi enzyme. As expected, the enzyme was localized, by immunofluorescence analysis, in glycosomes and is present in both promastigotes and true amastigotes obtained from hamster lesion. Hexokinase specific activity increased with the aging of promastigote culture, and this increment was related to glucose consumption. However, the level of the hexokinase protein remains constant as determined by Western blotting. Several hypotheses are discussed to explain this result.

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.

Comparative proteomics of glycosomes from bloodstream form and procyclic culture form Trypanosoma brucei brucei

Peroxisomes are present in nearly every eukaryotic cell and compartmentalize a wide range of important metabolic processes. Glycosomes of Kinetoplastid parasites are peroxisome-like organelles, characterized by the presence of the glycolytic pathway. The two replicating stages of Trypanosoma brucei brucei, the mammalian bloodstream form (BSF) and the insect (procyclic) form (PCF), undergo considerable adaptations in metabolism when switching between the two different hosts. These adaptations involve also substantial changes in the proteome of the glycosome. Comparative (non-quantitative) analysis of BSF and PCF glycosomes by nano LC-ESI-Q-TOF-MS resulted in the validation of known functional aspects of glycosomes and the identification of novel glycosomal constituents.

Fumarate is an essential intermediary metabolite produced by the procyclic Trypanosoma brucei

The procyclic stage of Trypanosoma brucei, a parasitic protist responsible for sleeping sickness in humans, converts most of the consumed glucose into excreted succinate, by succinic fermentation. Succinate is produced by the glycosomal and mitochondrial NADH-dependent fumarate reductases, which are not essential for parasite viability. To further explore the role of the succinic fermentation pathways, we studied the trypanosome fumarases, the enzymes providing fumarate to fumarate reductases. The T. brucei genome contains two class I fumarase genes encoding cytosolic (FHc) and mitochondrial (FHm) enzymes, which account for total cellular fumarase activity as shown by RNA interference. The growth arrest of a double RNA interference mutant cell line showing no fumarase activity indicates that fumarases are essential for the parasite. Interestingly, addition of fumarate to the medium rescues the growth phenotype, indicating that fumarate is an essential intermediary metabolite of the insect stage trypanosomes. We propose that trypanosomes use fumarate as an essential electron acceptor, as exemplified by the fumarate dependence previously reported for an enzyme of the essential de novo pyrimidine synthesis (Takashima, E., Inaoka, D. K., Osanai, A., Nara, T., Odaka, M., Aoki, T., Inaka, K., Harada, S., and Kita, K. (2002) Mol. Biochem. Parasitol. 122, 189-200).

The mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase of Trypanosomatidae and the glycosomal redox balance of insect stages of Trypanosoma brucei and Leishmania spp

The genes for the mitochondrial FAD-dependent glycerol-3-phosphate dehydrogenase were identified in Trypanosoma brucei and Leishmania major genomes. We have expressed the L. major gene in Saccharomyces cerevisiae and confirmed the subcellular localization and activity of the produced enzyme. Using cultured T. brucei procyclic and Leishmania mexicana promastigote cells with a permeabilized plasma membrane and containing intact glycosomes, it was shown that dihydroxyacetone phosphate is converted into pyruvate, and stimulates oxygen consumption, indicating that all components of the glycerol 3-phosphate/dihydoxyacetone phosphate shuttle between glycosomes and mitochondrion are present in these insect stages of both organisms. A computer model has been prepared for the energy and carbohydrate metabolism of these cells. It was used in an elementary mode analysis to get insight into the metabolic role of the shuttle in these insect-stage parasites. Our analysis suggests that the shuttle fulfils important roles for these organisms, albeit different from its well-known function in the T. brucei bloodstream form. It allows (1) a high yield of further metabolizable glycolytic products by decreasing the need to produce a secreted end product of glycosomal metabolism, succinate; (2) the consumption of glycerol and glycerol 3-phosphate derived from lipids; and (3) to keep the redox balance of the glycosome finely tuned due to a highly flexible and redundant system.

Niche metabolism in parasitic protozoa

Complete or partial genome sequences have recently become available for several medically and evolutionarily important parasitic protozoa. Through the application of bioinformatics complete metabolic repertoires for these parasites can be predicted. For experimentally intractable parasites insight provided by metabolic maps generated in silico has been startling. At its more extreme end, such bioinformatics reckoning facilitated the discovery in some parasites of mitochondria remodelled beyond previous recognition, and the identification of a non-photosynthetic chloroplast relic in malarial parasites. However, for experimentally tractable parasites, mapping of the general metabolic terrain is only a first step in understanding how the parasite modulates its streamlined, yet still often puzzlingly complex, metabolism in order to complete life cycles within host, vector, or environment. This review provides a comparative overview and discussion of metabolic strategies used by several different parasitic protozoa in order to subvert and survive host defences, and illustrates how genomic data contribute to the elucidation of parasite metabolism.

Functional characterisation of the iron superoxide dismutase gene repertoire in Trypanosoma brucei

Superoxide dismutases (SOD) are a family of antioxidant enzymes that function by removing superoxide anions from the cellular environment. Here, we show that the African trypanosome, Trypanosoma brucei, expresses four SOD isoforms, three of which we have validated biochemically as iron dependent, a feature normally associated with prokaryotic SODs. Localisation studies reveal that two of the enzymes are found predominantly in a parasite-specific organelle, the glycosome (TbSODB1 and TbSODB2), while the other two are targeted to the mitochondrion (TbSODA and TbSODC). Functional analysis of the SOD repertoire in bloodstream form parasites was performed using an inducible RNA interference (RNAi) approach. Down-regulation of the glycosomal SOD transcripts corresponded with a significant reduction in the corresponding proteins and a dramatic level of cell death within the population. The importance of one of the mitochondrial enzymes (TbSODA) only became apparent when parasites were exposed to the superoxide-generating agent paraquat following induction of RNAi. These experiments therefore identify essential components of the superoxide metabolising arm of the T. brucei oxidative defence system and validate these enzymes as parasite-specific targets for drug design.

Peroxisomal ABC transporters

Peroxisomes perform a range of different functions, dependent upon organism, tissue type, developmental stage or environmental conditions, many of which are connected with lipid metabolism. This review summarises recent research on ATP binding cassette (ABC) transporters of the peroxisomal membrane (ABC subfamily D) and their roles in plants, fungi and animals. Analysis of mutants has revealed that peroxisomal ABC transporters play key roles in specific metabolic and developmental functions in different organisms. A common function is import of substrates for beta-oxidation but much remains to be determined concerning transport substrates and mechanisms which appear to differ significantly between phyla.

Chemotherapeutic efficacy of ascofuranone in Trypanosoma vivax-infected mice without glycerol

Ascofuranone, an antibiotic isolated from Ascochyta visiae, showed trypanocidal activity in Trypanosoma vivax-infected mice. A single dose of 50 mg/kg ascofuranone effectively cured the mice without the help of glycerol. Repeated administrations of this drug further enhanced its chemotherapeutic effect. After two, three, and four consecutive days treatment, the doses needed to cure the infection decreased to 25, 12, and 6 mg/kg, so that the total doses administered were 50, 36 and 24 mg/kg, respectively. Ascofuranone (50 mg/kg) also had a prophylactic effect against T. vivax infection within the first two days after administration. This prophylactic activity diminished to 80% by day 3 and completely disappeared four days after administration. Of particular interest in this study was that ascofuranone had trypanocidal activity in T. vivax-infected mice in the absence of glycerol, whereas co-administration of glycerol or repeated administrations of this drug are needed for Trypanosoma brucei brucei infection. Our present results strongly suggest that ascofuranone is also an effective tool in chemotherapy against African trypanosomiasis in domestic animals.

The malate dehydrogenase isoforms from Trypanosoma brucei: subcellular localization and differential expression in bloodstream and procyclic forms

Trypanosoma brucei procyclic forms possess three different malate dehydrogenase isozymes that could be separated by hydrophobic interaction chromatography and were recognized as the mitochondrial, glycosomal and cytosolic malate dehydrogenase isozymes. The latter is the only malate dehydrogenase expressed in the bloodstream forms, thus confirming that the expression of malate dehydrogenase isozymes is regulated during the T. brucei life cycle. To achieve further biochemical characterization, the genes encoding mitochondrial and glycosomal malate dehydrogenase were cloned on the basis of previously reported nucleotide sequences and the recombinant enzymes were functionally expressed in Escherichia coli cultures. Mitochondrial malate dehydrogenase showed to be more active than glycosomal malate dehydrogenase in the reduction of oxaloacetate; nearly 80% of the total activity in procyclic crude extracts corresponds to the former isozyme which also catalyzes, although less efficiently, the reduction of p-hydroxyphenyl-pyruvate. The rabbit antisera raised against each of the recombinant isozymes showed that the three malate dehydrogenases do not cross-react immunologically. Immunofluorescence experiments using these antisera confirmed the glycosomal and mitochondrial localization of glycosomal and mitochondrial malate dehydrogenase, as well as a cytosolic localization for the third malate dehydrogenase isozyme. These results clearly distinguish Trypanosoma brucei from Trypanosoma cruzi, since in the latter parasite a cytosolic malate dehydrogenase is not present and mitochondrial malate dehydrogenase specifically reduces oxaloacetate.

6-Phosphofructo-2-kinase and fructose-2,6-bisphosphatase in Trypanosomatidae. Molecular characterization, database searches, modelling studies and evolutionary analysis

Fructose 2,6-bisphosphate is a potent allosteric activator of trypanosomatid pyruvate kinase and thus represents an important regulator of energy metabolism in these protozoan parasites. A 6-phosphofructo-2-kinase, responsible for the synthesis of this regulator, was highly purified from the bloodstream form of Trypanosoma brucei and kinetically characterized. By searching trypanosomatid genome databases, four genes encoding proteins homologous to the mammalian bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) were found for both T. brucei and the related parasite Leishmania major and four pairs in Trypanosoma cruzi. These genes were predicted to each encode a protein in which, at most, only a single domain would be active. Two of the T. brucei proteins showed most conservation in the PFK-2 domain, although one of them was predicted to be inactive due to substitution of residues responsible for ligating the catalytically essential divalent metal cation; the two other proteins were most conserved in the FBPase-2 domain. The two PFK-2-like proteins were expressed in Escherichia coli. Indeed, the first displayed PFK-2 activity with similar kinetic properties to that of the enzyme purified from T. brucei, whereas no activity was found for the second. Interestingly, several of the predicted trypanosomatid PFK-2/FBPase-2 proteins have long N-terminal extensions. The N-terminal domains of the two polypeptides with most similarity to mammalian PFK-2s contain a series of tandem repeat ankyrin motifs. In other proteins such motifs are known to mediate protein-protein interactions. Phylogenetic analysis suggests that the four different PFK-2/FBPase-2 isoenzymes found in Trypanosoma and Leishmania evolved from a single ancestral bifunctional enzyme within the trypanosomatid lineage. A possible explanation for the evolution of multiple monofunctional enzymes and for the presence of the ankyrin-motif repeats in the PFK-2 isoenzymes is presented.

Peroxisomal matrix protein import: the transient pore model

Peroxisomes import folded, even oligomeric, proteins, which distinguishes the peroxisomal translocation machinery from the well-characterized translocons of other organelles. How proteins are transported across the peroxisomal membrane is unclear. Here, we propose a mechanistic model that conceptually divides the import process into three consecutive steps: the formation of a translocation pore by the import receptor, the ubiquitylation of the import receptors, and pore disassembly/ receptor recycling.

Experimental and in silico analyses of glycolytic flux control in bloodstream form Trypanosoma brucei

A mathematical model of glycolysis in bloodstream form Trypanosoma brucei was developed previously on the basis of all available enzyme kinetic data (Bakker, B. M., Michels, P. A. M., Opperdoes, F. R., and Westerhoff, H. V. (1997) J. Biol. Chem. 272, 3207-3215). The model predicted correctly the fluxes and cellular metabolite concentrations as measured in non-growing trypanosomes and the major contribution to the flux control exerted by the plasma membrane glucose transporter. Surprisingly, a large overcapacity was predicted for hexokinase (HXK), phosphofructokinase (PFK), and pyruvate kinase (PYK). Here, we present our further analysis of the control of glycolytic flux in bloodstream form T. brucei. First, the model was optimized and extended with recent information about the kinetics of enzymes and their activities as measured in lysates of in vitro cultured growing trypanosomes. Second, the concentrations of five glycolytic enzymes (HXK, PFK, phosphoglycerate mutase, enolase, and PYK) in trypanosomes were changed by RNA interference. The effects of the knockdown of these enzymes on the growth, activities, and levels of various enzymes and glycolytic flux were studied and compared with model predictions. Data thus obtained support the conclusion from the in silico analysis that HXK, PFK, and PYK are in excess, albeit less than predicted. Interestingly, depletion of PFK and enolase had an effect on the activity (but not, or to a lesser extent, expression) of some other glycolytic enzymes. Enzymes located both in the glycosomes (the peroxisome-like organelles harboring the first seven enzymes of the glycolytic pathway of trypanosomes) and in the cytosol were affected. These data suggest the existence of novel regulatory mechanisms operating in trypanosome glycolysis.

Energy generation in insect stages of Trypanosoma brucei: metabolism in flux

The generation of energy in African trypanosomes is a subject of undoubted importance. In bloodstream-form organisms, substrate-level phosphorylation of glucose is sufficient to provide the energy needs of the parasite. The situation in procyclic-form trypanosomes is more complex. For many years, it was accepted that glucose metabolism followed a conventional scheme involving glycolysis, the tricarboxylic acid cycle and ATP-producing oxidative phosphorylation linked to the electron-transport chain. However, progress in sequencing the Trypanosoma brucei genome and the development of gene-knockout and RNA interference technology has provided novel insight. Coupling these new technologies with classical approaches, including NMR and mass spectrometry to analyse glycolytic intermediates and end products, has yielded several surprises. In this article, we summarize how these recent data have helped to change the view of metabolism in procyclic-form T. brucei.

The suppression of galactose metabolism in procylic form Trypanosoma brucei causes cessation of cell growth and alters procyclin glycoprotein structure and copy number

Galactose metabolism is essential in bloodstream form Trypanosoma brucei and is initiated by the enzyme UDP-Glc 4'-epimerase. Here, we show that the parasite epimerase is a homodimer that can interconvert UDP-Glc and UDP-Gal but not UDP-GlcNAc and UDP-GalNAc. The epimerase was localized to the glycosomes by immunofluorescence microscopy and subcellular fractionation, suggesting a novel compartmentalization of galactose metabolism in this organism. The epimerase is encoded by the TbGALE gene and procyclic form T. brucei single-allele knockouts, and conditional (tetracycline-inducible) null mutants were constructed. Under non-permissive conditions, conditional null mutant cultures ceased growth after 8 days and resumed growth after 15 days. The resumption of growth coincided with constitutive re-expression epimerase mRNA. These data show that galactose metabolism is essential for cell growth in procyclic form T. brucei. The epimerase is required for glycoprotein galactosylation. The major procyclic form glycoproteins, the procyclins., were analyzed in TbGALE single-allele knockouts and in the conditional null mutant after removal of tetracycline. The procyclins contain glycosylphosphatidylinositol membrane anchors with large poly-N-acetyl-lactosamine side chains. The single allele knockouts exhibited 30% reduction in procyclin galactose content. This example of haploid insufficiency suggests that epimerase levels are close to limiting in this life cycle stage. Similar analyses of the conditional null mutant 9 days after the removal of tetracycline showed that the procyclins were virtually galactose-free and greatly reduced in size. The parasites compensated, ultimately unsuccessfully, by expressing 10-fold more procyclin. The implications of these data with respect to the relative roles of procyclin polypeptide and carbohydrate are discussed.

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 mitochondrial NADH-dependent fumarate reductase involved in the production of succinate excreted by procyclic Trypanosoma brucei

Trypanosoma brucei is a parasitic protist responsible for sleeping sickness in humans. The procyclic stage of T. brucei expresses a soluble NADH-dependent fumarate reductase (FRDg) in the peroxisome-like organelles called glycosomes. This enzyme is responsible for the production of about 70% of the excreted succinate, the major end product of glucose metabolism in this form of the parasite. Here we functionally characterize a new gene encoding FRD (FRDm1) expressed in the procyclic stage. FRDm1 is a mitochondrial protein, as evidenced by immunolocalization, fractionation of digitonin-permeabilized cells, and expression of EGFP-tagged FRDm1 in the parasite. RNA interference was used to deplete FRDm1, FRDg, or both together. The analysis of the resulting mutant cell lines showed that FRDm1 is responsible for 30% of the cellular NADH-FRD activity, which solves a long standing debate regarding the existence of a mitochondrial FRD in trypanosomatids. FRDg and FRDm1 together account for the total NADH-FRD activity in procyclics, because no activity was measured in the double mutant lacking expression of both proteins. Analysis of the end products of 13C-enriched glucose excreted by these mutant cell lines showed that FRDm1 contributes to the production of between 14 and 44% of the succinate excreted by the wild type cells. In addition, depletion of one or both FRD enzymes results in up to 2-fold reduction of the rate of glucose consumption. We propose that FRDm1 is involved in the maintenance of the redox balance in the mitochondrion, as proposed for the ancestral soluble FRD presumably present in primitive anaerobic cells.

The effect of over-expression of the alternative oxidase in the procyclic forms of Trypanosoma brucei

Trypanosome alternative oxidase (TAO) is the cyanide-resistant but SHAM-sensitive terminal oxidase of the mitochondrial electron transport chain in African trypanosomes. The bloodstream forms of Trypanosoma brucei lack cytochromes and respire exclusively via TAO. On the other hand, the insect, or procyclic form possesses a fully developed cytochrome system, and down regulates TAO several folds by reducing the stability of the TAO transcript. We expressed an ectopic copy of TAO in the procyclic form from a tetracycline regulated stable expression vector, in which the TAO 3'-UTR was replaced by T. brucei aldolase 3'-UTR. The TAO transcript produced from the ectopic copy was stably accumulated in the procyclic form. Upon induction with doxycycline, TAO protein level was gradually increased about five-fold within 72 h. TAO over-expression did not show any effect on the growth of the parasite. The rate of respiration and the SHAM-sensitive respiratory pathway capacity was increased about two- and five-fold, respectively, and the cytochrome-mediated respiratory pathway capacity was reduced two- to three-folds within 5 days after induction of TAO. Doxycycline induced TAO+ cells preferentially utilized CN-resistant, SHAM-sensitive pathway of respiration, whereas, in the control cells 70-80% of total respiration was via the CN-sensitive pathway. Moreover, we have found that increased expression of TAO caused about two-fold down regulation of cytochrome oxidase subunit IV, and cytochrome c1 protein level and also caused a four-fold up-regulation of the expression of the surface coat protein, GPEET procyclin in the procyclic form. This suggests that the expression of two terminal oxidases and the coat protein is linked in T. brucei.

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.

Proline metabolism in procyclic Trypanosoma brucei is down-regulated in the presence of glucose

Proline metabolism has been studied in procyclic form Trypanosoma brucei. These parasites consume six times more proline from the medium when glucose is in limiting supply than when this carbohydrate is present as an abundant energy source. The sensitivity of procyclic T. brucei to oligomycin increases by three orders of magnitude when the parasites are obliged to catabolize proline in medium depleted in glucose. This indicates that oxidative phosphorylation is far more important to energy metabolism in this latter case than when glucose is available and the energy needs of the parasite can be fulfilled by substrate level phosphorylation alone. A gene encoding proline dehydrogenase, the first enzyme of the proline catabolic pathway, was cloned. RNA interference studies revealed the loss of this activity to be conditionally lethal. Proline dehydrogenase defective parasites grew as wild-type when glucose was available, but, unlike wild-type cells, they failed to proliferate using proline. In parasites grown in the presence of glucose, proline dehydrogenase activity was markedly lower than when glucose was absent from the medium. Proline uptake too was shown to be diminished when glucose was abundant in the growth medium. Wild-type cells were sensitive to 2-deoxy-D-glucose if grown using proline as the principal carbon source, but not in glucose-rich medium, indicating that this non-catabolizable glucose analogue might also stimulate repression of proline utilization. These results indicate that the ability of trypanosomes to use proline as an energy source can be regulated depending upon the availability of glucose.

Cloning, Heterologous Expression, and Characterization of Three Aquaglyceroporins from Trypanosoma brucei

Trypanosoma brucei, causative for African sleeping sickness, relies exclusively on glycolysis for ATP production. Under anaerobic conditions, glucose is converted to equimolar amounts of glycerol and pyruvate, which are both secreted from the parasite. As we have shown previously, glycerol transport in T. brucei occurs via specific membrane proteins (Wille, U., Schade, B., and Duszenko, M. (1998) Eur. J. Biochem. 256, 245-250). Here, we describe cloning and biochemical characterization of the three trypanosomal aquaglyceroporins (AQP; TbAQP1-3), which show a 40-45% identity to mammalian AQP3 and -9. AQPs belong to the major intrinsic protein family and represent channels for small non-ionic molecules. Both TbAQP1 and TbAQP3 contain two highly conserved NPA motifs within the pore-forming region, whereas TbAQP2 contains NSA and NPS motifs instead, which are only occasionally found in AQPs. For functional characterization, all three proteins were heterologously expressed in yeast and Xenopus oocytes. In the yeast fps1Delta mutant, TbAQPs suppressed hypoosmosensitivity and rendered cells to a hyper-osmosensitive phenotype, as expected for unregulated glycerol channels. Under iso- and hyperosmotic conditions, these cells constitutively released glycerol, consistent with a glycerol efflux function of TbAQP proteins. TbAQP expression in Xenopus oocytes increased permeability for water, glycerol and, interestingly, dihydroxyacetone. Except for urea, TbAQPs were virtually impermeable for other polyols; only TbAQP3 transported erythritol and ribitol. Thus, TbAQPs represent mainly water/glycerol/dihydroxyacetone channels involved in osmoregulation and glycerol metabolism in T. brucei. This function and especially the so far not investigated transport of dihydroxyacetone may be pivotal for the survival of the parasite survival under non-aerobic or osmotic stress conditions.

The crystal structure of glucose-6-phosphate isomerase from Leishmania mexicana reveals novel active site features

Glucose-6-phosphate isomerase catalyzes the reversible aldose-ketose isomerization of D-glucose-6-phosphate to D-fructose-6-phosphate in glycolysis and gluconeogenesis, and in the recycling of hexose-6-phosphate in the pentose phosphate pathway. The unicellular protozoans, Trypanosoma brucei, T. cruzi and Leishmania spp., of the order Kinetoplastida are important human parasites responsible for African sleeping sickness, Chagas' disease and leishmaniases, respectively. In these parasites, glycolysis is an important (and in some cases the only) metabolic pathway for ATP supply. The first seven of the 10 enzymes that participate in glycolysis, as well as an important fraction of the enzymes of the pentose phosphate pathway, are compartmentalized in peroxisome-like organelles called glycosomes. The dependence of the parasites on glycolysis, the importance of the pentose phosphate pathway in defense against oxidative stress, and the unique compartmentalization of these pathways, point to the enzymes contained in the glycosome as potential targets for drug design. The present report describes the first crystallographic structure of a parasite (Leishmania mexicana) glucose-6-phosphate isomerase. A comparison of the atomic structure of L. mexicana, human and other mammalian PGIs, which highlights unique features of the parasite's enzyme, is presented.

Interaction of substituted hexose analogues with the Trypanosoma brucei hexose transporter

Glucose metabolism is essential for survival of bloodstream form Trypanosoma brucei subspecies which cause human African trypanosomiasis (sleeping sickness). Hexose analogues may represent good compounds to inhibit glucose metabolism in these cells. Delivery of such compounds to the parasite is a major consideration in drug development. A series of D-glucose and D-fructose analogues were developed to explore the limits of the structure-activity relationship of the THT1 hexose transporter of bloodstream form African trypanosomes, a portal that might be exploited for drug uptake. D-glucose analogues with substituents at the C2 and C6 position continued to interact with the exofacial hexose binding site of the transporter. There was a limit to the size at C6 which still permitted recognition, although compounds carrying large groups at position C2 were still recognised. However, radiolabelled N-acetyl-D-[1-14C] glucosamine was not internalised by trypanosomes, in spite of the ability of this compound to inhibit glucose uptake, indicating that there is a limit to the size of C2 substituent that allows translocation. Addition of an alkylating group (bromoacetyl) at position C2 in the D-glucose series and at position 6 in the D-fructose set, created two analogues which interact with the transporter and kill trypanosomes in vitro. This indicates that inhibition of the transporter may be a good means of killing trypanosomes.

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.

The trypanosomiases

The trypanosomiases consist of a group of important animal and human diseases caused by parasitic protozoa of the genus Trypanosoma. In sub-Saharan Africa, the final decade of the 20th century witnessed an alarming resurgence in sleeping sickness (human African trypanosomiasis). In South and Central America, Chagas' disease (American trypanosomiasis) remains one of the most prevalent infectious diseases. Arthropod vectors transmit African and American trypanosomiases, and disease containment through insect control programmes is an achievable goal. Chemotherapy is available for both diseases, but existing drugs are far from ideal. The trypanosomes are some of the earliest diverging members of the Eukaryotae and share several biochemical peculiarities that have stimulated research into new drug targets. However, differences in the ways in which trypanosome species interact with their hosts have frustrated efforts to design drugs effective against both species. Growth in recognition of these neglected diseases might result in progress towards control through increased funding for drug development and vector elimination.

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.

Depletion of GIM5 causes cellular fragility, a decreased glycosome number, and reduced levels of ether-linked phospholipids in trypanosomes

Microbody division in mammalian cells, trypanosomes, and yeast depends on the PEX11 microbody membrane proteins. The function of PEX11 is not understood, and the suggestion that it affects microbody (peroxisome) numbers in mammals and yeast, because it plays a role in beta-oxidation of fatty acids, is controversial. PEX11 and two PEX11-related proteins, GIM5A and GIM5B, are the predominant membrane proteins of the microbodies (glycosomes) of Trypanosoma brucei. The compartmentation of glycosomal enzymes is essential in trypanosomes. Deletion of the GIM5A gene from the form of the parasite that lives in the mammalian blood has no effect on trypanosome growth, but depletion of GIM5B on a gim5a null background causes death. We show here that procyclic trypanosomes, adapted for life in the Tsetse fly vector, survive without GIM5A and with very low levels of GIM5B. The depleted cells have fewer glycosomes than usual and are osmotically fragile, which is a novel observation for a microbody defect. Thus trypanosomes require both GIM5B and PEX11 for the maintenance of normal glycosome numbers. Procyclic cells lacking GIM5A, like mouse cells partially defective in PEX11, have fewer ether-linked phospholipids, even when GIM5B levels are not reduced. Metabolite measurements on GIM5A/B-depleted bloodstream form trypanosomes suggested a change in the flux through the glycolytic pathway. We conclude that PEX11 family proteins play important roles in determining microbody membrane structure, with secondary effects on a subset of microbody metabolic pathways.

Pentose phosphate metabolism in Leishmania mexicana

The metabolism of pentose phosphates was studied in Leishmania mexicana promastigotes. Each of the enzymes of the classical pentose phosphate pathway (PPP) has been identified and specific activities measured. Functioning of the PPP was demonstrated in non-growing cells by measuring the evolution of 14CO2 from [1-14C]D-glucose and [6-14C]D-glucose under normal conditions and also under selective stimulation of the PPP by exposure to methylene blue. The proportion of glucose which passes through the PPP increases in the latter condition, thus suggesting a protective role against oxidant stress. The incorporation into nucleic acids of ribose 5-phosphate provided via either glucose or free ribose was also determined. Results indicate that the PPP enables glucose to serve as a source of ribose 5-phosphate in nucleotide biosynthesis. Moreover, free ribose is incorporated efficiently, implying the presence of a ribose uptake system and also of ribokinase. Ribose was shown to be accumulated by a carrier mediated process in L. mexicana promastigotes and ribokinase activity was also measured in these cells.

RNA interference identifies two hydroperoxide metabolizing enzymes that are essential to the bloodstream form of the african trypanosome

Detoxification of hydroperoxides in trypanosomes is mediated by a series of linked redox pathways that are dependent on the parasite-specific thiol trypanothione for reducing equivalents. These pathways are characterized by differences in subcellular location, electron transport molecules, and substrate specificity. To determine the functional significance of the enzymes involved, we have used a tetracycline-inducible RNA interference system to down-regulate expression of each of the corresponding transcripts in bloodstream form Trypanosoma brucei. We have identified two peroxidases, a cytosolic peroxiredoxin (TbCPX) and a member of the non-selenium glutathione-dependent peroxidase family (TbGPXI), that appear to be essential for the viability of this clinically relevant stage of the parasite life cycle. The addition of tetracycline to the cultures resulted in a major reduction in mRNA levels and enzyme activity, a dramatic fall in growth rate, and significant cell death. Furthermore, within 20 h of adding tetracycline, cells in which the cytosolic peroxiredoxin transcript was targeted were found to be 16-fold more susceptible to killing by exogenous hydrogen peroxide. We also observed that knockdown of the tryparedoxin TbT-PNI, a thioredoxin-like protein that facilitates electron transport to both TbCPX and TbGPXI, resulted in a reduction in growth rate. These experiments therefore identify redox pathways that are essential for oxidative defense in T. brucei and validate the corresponding peroxidases as targets for drug design.

Characterization of Trypanosoma brucei PEX14 and its role in the import of glycosomal matrix proteins

It has been shown previously in various organisms that the peroxin PEX14 is a component of a docking complex at the peroxisomal membrane, where it is involved in the import of matrix proteins into the organelle after their synthesis in the cytosol and recognition by a receptor. Here we present a characterization of the Trypanosoma brucei homologue of PEX14. It is shown that the protein is associated with glycosomes, the peroxisome-like organelles of trypanosomatids in which most glycolytic enzymes are compartmentalized. The N-terminal part of the protein binds specifically to TbPEX5, the cytosolic receptor for glycosomal matrix proteins with a peroxisome-targeting signal type 1 (PTS-1). TbPEX14 mRNA depletion by RNA interference results, in both bloodstream-form and procyclic, insect-stage T. brucei, in mislocalization of glycosomal proteins to the cytosol. The mislocalization was observed for different classes of matrix proteins: proteins with a C-terminal PTS-1, a N-terminal PTS-2 and a polypeptide internal I-PTS. The RNA interference experiments also showed that TbPEX14 is essential for the survival of bloodstream-form and procyclic trypanosomes. These data indicate the protein's great potential as a target for selective trypanocidal drugs.

Metabolic analysis in drug design

Biotechnology is often presented as if progress in the past two decades represented a major success, but the reality is quite different. For example, ten major classes of antibiotics were discovered between 1935 and 1963, but after 1963 there has been just one, the oxazolidones. To illustrate the possibilities of doing better by taking account of the real behaviour of metabolic systems, we can examine how one might modify the activity of an enzyme in the cell (for example by genetic manipulation, or by the action of an inhibitor, etc.) to satisfy a technological aim. For example, if the objective is to eliminate a pest, one might suppose that the effect of an inhibitor could be to depress an essential flux to a level insufficient for life, or to raise the concentration of an intermediate to a toxic level. The former may seem the more obvious, but the latter is easier to achieve in practice, and there are some excellent examples of industrial products that work in that way, such as the herbicide 'Roundup' and antimalarials of the quinine class. A study of glycolysis in the parasite Trypanosoma brucei (which causes African sleeping sickness) indicates that for this approach to work the selected target enzyme must have a substrate with a concentration that is not limited by stoichiometric constraints. That is not necessarily easy to find in a complicated system, and typically needs the metabolic network to be analysed in the computer.

Genetic characterization of glucose transporter function in Leishmania mexicana

Both insect and mammalian life cycle stages of Leishmania mexicana take up glucose and express all three isoforms encoded by the LmGT glucose transporter gene family. To evaluate glucose transporter function in intact parasites, a null mutant line has been created by targeted disruption of the LmGT locus that encompasses the LmGT1, LmGT2, and LmGT3 genes. This deltalmgt null mutant exhibited no detectable glucose transport activity. The growth rate of the deltalmgt knockout in the promastigote stage was reduced to a rate comparable with that of WT cells grown in the absence of glucose. deltalmgt cells also exhibited dramatically reduced infectivity to macrophages, demonstrating that expression of LmGT isoforms is essential for viability of amastigotes. Furthermore, WT L. mexicana were not able to grow as axenic culture form amastigotes if glucose was withdrawn from the medium, implying that glucose is an essential nutrient in this life cycle stage. Expression of either LmGT2 or LmGT3, but not of LmGT1, in deltalmgt null mutants significantly restored growth as promastigotes, but only LmGT3 expression substantially rescued amastigote growth in macrophages. Subcellular localization of the three isoforms was investigated in deltalmgt cells expressing individual LmGT isoforms. Using anti-LmGT antiserum and GFP-tagged LmGT fusion proteins, LmGT2 and LmGT3 were localized to the cell body, whereas LmGT1 was localized specifically to the flagellum. These results establish that each glucose transporter isoform has distinct biological functions in the parasite.

The role of glutathione peroxidases in trypanosomatids

Reactive oxygen species are the unwanted by-products of aerobic metabolism. To protect cells against their potentially lethal effects a series of pathways have evolved that are collectively called the oxidative defence system. In most eukaryotes, catalases and selenium-dependent glutathione peroxidases form the front line of defence against hydroperoxide-mediated damage. However, these activities are lacking in members of the Trypanosomatidae family of protozoan parasites. Instead these organisms contain several enzyme-mediated pathways for removal of hydroperoxides that are centred upon the unusual thiol trypanothione. Here we discuss the biochemical properties of one group of these enzymes, the non-selenium glutathione-dependent peroxidases, and outline the roles that they play in protecting the parasite against hydroperoxides associated with biological membranes.

Cell fractionation of parasitic protozoa: a review

Cell fractionation, a methodological strategy for obtaining purified organelle preparations, has been applied successfully to parasitic protozoa by a number of investigators. Here we present and discuss the work of several groups that have obtained highly purified subcellular fractions from trypanosomatids, Apicomplexa and trichomonads, and whose work have added substantially to our knowledge of the cell biology of these parasites.

Plant-like traits associated with metabolism of Trypanosoma parasites

Trypanosomatid parasites cause serious diseases among humans, livestock, and plants. They belong to the order of the Kinetoplastida and form, together with the Euglenida, the phylum Euglenozoa. Euglenoid algae possess plastids capable of photosynthesis, but plastids are unknown in trypanosomatids. Here we present molecular evidence that trypanosomatids possessed a plastid at some point in their evolutionary history. Extant trypanosomatid parasites, such as Trypanosoma and Leishmania, contain several "plant-like" genes encoding homologs of proteins found in either chloroplasts or the cytosol of plants and algae. The data suggest that kinetoplastids and euglenoids acquired plastids by endosymbiosis before their divergence and that the former lineage subsequently lost the organelle but retained numerous genes. Several of the proteins encoded by these genes are now, in the parasites, found inside highly specialized peroxisomes, called glycosomes, absent from all other eukaryotes, including euglenoids.

Phospholipids in parasitic protozoa

Parasitic protozoa are surrounded by membrane structures that have a different lipid and protein composition relative to membranes of the host. The parasite membranes are essential structurally and also for parasite specific processes, like host cell invasion, nutrient acquisition or protection against the host immune system. Furthermore, intracellular parasites can modulate membranes of their host, and trafficking of membrane components occurs between host membranes and those of the intracellular parasite. Phospholipids are major membrane components and, although many parasites scavenge these phospholipids from their host, most parasites also synthesise phospholipids de novo, or modify a large part of the scavenged phospholipids. It was recently shown that some parasites like Plasmodium have unique phospholipid metabolic pathways. This review will focus on new developments in research on phospholipid metabolism of parasitic protozoa in relation to parasite-specific membrane structures and function, as well as on several targets for interference with the parasite phospholipid metabolism with a view to developing new anti-parasitic drugs.

Molecular and biochemical characterization of hexokinase from Trypanosoma cruzi

The Trypanosoma cruzi hexokinase gene has been cloned, sequenced, and expressed as an active enzyme in Escherichia coli. Sequence analysis revealed 67% identity with its counterpart in Trypanosoma brucei but low similarity with all other available hexokinase sequences including those of human. It contains an N-terminal peroxisome-targeting signal (PTS-2) and has a calculated basic isoelectric point (pI = 9.67), a feature often associated with glycosomal proteins. The polypeptide has a predicted mass of approximately 50 kDa similar to that of many non-vertebrate hexokinases and the vertebrate hexokinase isoenzyme IV. The natural enzyme was purified to homogeneity from T. cruzi epimastigotes and appeared to exist in several aggregation states, an apparent tetramer being the predominant form. Its kinetic properties were compared with those of the purified recombinant protein. Higher K(m) values for glucose and ATP were found for the (His)(6)-tag-containing recombinant hexokinase. However, removal of the tag produced an enzyme displaying similar values as the natural enzyme (K(m) for glucose = 43 and 60 microM for the natural and the recombinant protein, respectively). None of these enzymes presented activity with fructose. As reported previously for hexokinases from several trypanosomatids, no inhibition was exerted by glucose 6-phosphate (G6-P). In contrast, a mixed-type inhibition was observed with inorganic pyrophosphate (PPi, K(i) = 0.5mM).