The evolution of kinetoplastid glycosomes

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.

The coiled-coil domain of glycosomal membrane-associated Leishmania donovani PEX14: cloning, overexpression, purification and preliminary crystallographic analysis

The glycosomal membrane-associated Leishmania donovani protein PEX14, which plays a crucial role in protein import from the cytosol to the glycosomal matrix, consists of three domains: an N-terminal domain where the signalling molecule binds, a transmembrane domain and an 84-residue coiled-coil domain (CC) that is responsible for oligomerization. CCs are versatile domains that participate in a variety of functions including supramolecular assembly, cellular signalling and transport. Recombinant PEX14 CC was cloned, overexpressed, affinity-purified with in-column thrombin cleavage and further purified by size-exclusion chromatography. Crystals that diffracted to 1.98 Å resolution were obtained from a condition consisting of 1.4 M sodium citrate tribasic dihydrate, 0.1 M HEPES buffer pH 7.5. The crystals belonged to the monoclinic space group C2, with unit-cell parameters a = 143.98, b = 32.62, c = 95.62 Å, β = 94.68°. Structure determination and characterization are in progress.

Using fluorescent proteins to monitor glycosome dynamics in the African trypanosome

Trypanosoma brucei is a kinetoplastid parasite that causes human African trypanosomiasis (HAT), or sleeping sickness, and a wasting disease, nagana, in cattle. The parasite alternates between the bloodstream of the mammalian host and the tsetse fly vector. The composition of many cellular organelles changes in response to these different extracellular conditions. Glycosomes are highly specialized peroxisomes in which many of the enzymes involved in glycolysis are compartmentalized. Glycosome composition changes in a developmental and environmentally regulated manner. Currently, the most common techniques used to study glycosome dynamics are electron and fluorescence microscopy; techniques that are expensive, time and labor intensive, and not easily adapted to high throughput analyses. To overcome these limitations, a fluorescent-glycosome reporter system in which enhanced yellow fluorescent protein (eYFP) is fused to a peroxisome targeting sequence (PTS2), which directs the fusion protein to glycosomes, has been established. Upon import of the PTS2eYFP fusion protein, glycosomes become fluorescent. Organelle degradation and recycling results in the loss of fluorescence that can be measured by flow cytometry. Large numbers of cells (5,000 cells/sec) can be analyzed in real-time without extensive sample preparation such as fixation and mounting. This method offers a rapid way of detecting changes in organelle composition in response to fluctuating environmental conditions.

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.

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.

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.

Biogenesis and function of peroxisomes and glycosomes

Peroxisomes of higher eukaryotes, glycosomes of kinetoplastids, and glyoxysomes of plants are related microbody organelles that perform differing metabolic functions tailored to their cellular environments. The close evolutionary relationship of these organelles is most clearly evidenced by the conservation of proteins involved in matrix protein import and biogenesis. The glycosome can be viewed as an offshoot of the peroxisomal lineage with additional metabolic functions, specifically glycolysis and purine salvage. Within the parasitic protozoa, only kinetoplastids have been conclusively demonstrated to possess glycosomes or indeed any peroxisome-like organelle. The importance of glycosomal pathways and their compartmentation emphasizes the potential of the glycosome and glycosomal proteins as drug targets.

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.

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.

Peroxisomal targeting signal-1 receptor protein PEX5 from Leishmania donovani. Molecular, biochemical, and immunocytochemical characterization

The human pathogens of the Leishmania and Trypanosoma genera compartmentalize glycolytic and other key metabolic pathways in unique subcellular microbodies called glycosomes, organelles related to the peroxisomes of mammals and yeast. The molecular machinery that carries out the specific targeting of glycosomal proteins to the organelle has not been characterized, although the bulk of glycosomal proteins contain the COOH-terminal tripeptide glycosomal peroxisomal targeting signal-1 (PTS-1) similar to the mammalian and fungal peroxisomal targeting signal. To characterize the mechanisms of glycosomal targeting, the gene encoding PEX5, designated LdPEX5, has been isolated from Leishmania donovani. LdPEX5 encodes a 625-amino acid protein with a molecular mass of 69.7 kDa. Like its homologs in yeast and humans, LdPEX5 predicts a protein with seven copies of a tetratricopeptide repeat in its COOH-terminal half proposed to mediate PTS-1 binding and three copies of a WXXX(Y/F) motif in its NH(2) terminus conjectured to be essential for protein translocation into the organelle. LdPEX5 was overexpressed in Escherichia coli and purified to homogeneity for binding experiments and generation of antibodies. Recombinant LdPEX5 bound xanthine phosphoribosyltransferase (XPRT), a PTS-1 containing glycosomal protein with a K(D) of 4.2 nm, but did not bind an XPRT in which the PTS-1 had been deleted. Moreover, binding studies with the COOH-terminal half of the LdPEX5 confirmed that this portion of the PEX5 protein was capable of binding the XPRT PTS-1 with an affinity of 17.3 nm. Confocal microsocopy revealed that LdPEX5 was predominantly in the cytosolic milieu, and genetic analysis implied that LdPEX5 was an essential gene.

Compartmentation protects trypanosomes from the dangerous design of glycolysis

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

Cloning and analysis of the PTS-1 receptor in Trypanosoma brucei

Kinetoplastid organisms, such as the protozoan parasite Trypanosoma brucei, compartmentalise several important metabolic pathways in organelles called glycosomes. Glycosomes are related to peroxisomes of yeast and mammalian cells. A subset of glycosomal matrix proteins is routed to the organelles via the peroxisome-targeting signal type 1 (PTS-1). The PEX5 gene homologue has been cloned from T. brucei coding for a protein of the translocation machinery, the PTS-1 receptor. The gene codes for a polypeptide of 654 amino acids with a calculated molecular mass of 70 kDa. Like its homologue in other organisms T. brucei PTS-1 receptor protein (TbPEX5) is a member of the tetratricopeptide repeat (TPR) protein family and contains several copies of the pentapeptide W-X-X-X-F/Y. Northern and Western blot analysis showed that the protein is expressed at different stages of the life cycle of the parasite. The protein has been overproduced in Escherichia coli and purified using immobilized metal affinity chromatography. The purified protein specifically interacts in vitro with glycosomal phosphoglycerate kinase-C (PGK-C) of T. brucei, a PTS-1 containing protein. The equilibrium dissociation constant (Kd) of PGK-C for purified TbPEX5 is 40 nM. Using biochemical and cytochemical techniques a predominantly cytosolic localization was found for TbPEX5. This is consistent with the idea of receptor cycling between the glycosomes and the cytosol.

What controls glycolysis in bloodstream form Trypanosoma brucei?

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

Purification and properties of phosphoglucose isomerases of Trypanosoma cruzi

Glucosephosphate isomerase (PGI; EC 5.3.1.9) of Trypanosoma cruzi epimastigotes was found in about the same proportion in the glycosome and the cytosol. This subcellular distribution is similar to that of Leishmania mexicana, but contrasts with that of T. brucei bloodstream form, where the enzyme is essentially restricted to the glycosome. Glucosephosphate isomerase was highly purified from a glycosome-enriched fraction and to about 70% purity from the soluble extract. Both enzymes displayed Michaelis-Menten-Henri kinetics. Km values for fructose 6-phosphate were 0.125 +/- 0.07 and 0.80 +/- 0.10 mM for the glycosomal and the cytosolic PGIs, respectively. Erythrose-4-phosphate, 6-phosphogluconate and mannose-6-phosphate were inhibitors for both PGIs. Phosphogluconate and erythrose phosphate showed higher affinity for cytosolic PGI than for glycosomal PGI, by 2.5- and 4-fold respectively. The PGIs differed slightly in their isoelectric point (7.1 +/- 0.15 and 7.5 +/- 0.12) and optimum pH range. Both PGIs also differed in their chromatographic properties (ion-exchange and phenyl Sepharose), indicating a difference in charge and hydrophobicity, with the glycosomal enzyme being more hydrophobic. The molecular mass of both PGIs was 186,000 +/- 9000 Da, which is higher than that of other known PGIs, including those from T. brucei and other trypanosomatids. The molecular mass of the subunit, 63 kDa, is similar to that of PGIs from other sources. It appears that PGIs from T. cruzi are trimeric, in contrast with all other known PGIs which are dimeric.

Molecular analysis of phosphoglycerate kinase in Trypanoplasma borreli and the evolution of this enzyme in kinetoplastida

In the protozoan kinetoplastid organism Trypanoplasma borreli, phosphoglycerate kinase (PGK) activity was found in two different cell compartments: 80% in the cytosol and 20% in peroxisome-like organelles called glycosomes. However, only one functional pgk gene could be detected, in addition to a pseudo-pgk gene. No short-range linkage could be established between these two genes, although they are presumably present on the same chromosome. The intact gene codes for a polypeptide of 411 amino acids, with a C-terminal extension of four residues, -VAKF, a sequence with probably a low targeting efficiency for glycosomes. The calculated net charge and molecular mass of the encoded polypeptide are +13 and 44230Da, respectively. In other Kinetoplastida, different tandemly arranged genes code for distinct PGK isoenzymes in glycosomes and cytosol. By comparison of the pgk gene organization, and a phylogenetic analysis, we have traced a plausible scenario of the evolution of the PGK isoenzymes in these organisms and of the enzymes' intracellular compartmentation.

Elongation and clustering of glycosomes in Trypanosoma brucei overexpressing the glycosomal Pex11p

Kinetoplastid protozoa confine large parts of glycolysis within glycosomes, which are microbodies related to peroxisomes. We cloned the gene encoding the second most abundant integral membrane protein of Trypanosoma brucei glycosomes. The 24 kDa protein is very basic and hydrophobic, with two predicted transmembrane domains. It is targeted to peroxisomes when expressed in mammalian cells and yeast. The protein is a functional homologue of Pex11p from Saccharomyces cerevisiae: pex11Delta mutants, which are defective in peroxisome proliferation, can be complemented by the trypanosome gene. Sequence conservation is significant in the N- and C-terminal domains of all putative Pex11p homologues known, from trypanosomes, yeasts and mammals. Several lines of evidence indicate that these domains are oriented towards the cytosol. TbPex11p can form homodimers, like its yeast counterpart. The TbPEX11 gene is essential in trypanosomes. Inducible overexpression of the protein in T.brucei bloodstream forms causes growth arrest, the globular glycosomes being transformed to clusters of long tubules filling significant proportions of the cytoplasm. Reduced expression results in trypanosomes with fewer, but larger, organelles.

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

Trypanosoma brucei contains an ATP-dependent phosphofructokinase (PFK), located in its glycosomes, which are peroxisome-like organelles sequestering the majority of its glycolytic enzymes. In this paper, we report the cloning and sequencing of the single-copy gene encoding this enzyme. Its amino-acid sequence is more similar to pyrophosphate (PPi)-dependent PFKs than to other ATP-dependent PFKs. A phylogenetic analysis suggests that the enzyme must have been derived from a PPi-dependent ancestral PFK, which changed its phospho-donor specificity during evolution. The enzyme is no longer capable of using PPi as phospho substrate, nor can it catalyze the reverse reaction as PPi-PFKs generally can. Moreover, the presence of a high pyrophosphatase activity in the cell renders it unlikely that PPi can function as free-energy source in present-day trypanosomes. It remains to be determined which mutations were responsible for the change in phospho-substrate specificity of the trypanosomatid PFK. As a result of its particular evolutionary history, the T. brucei PFK shows many structural differences, even at the active site, when compared with other ATP-dependent PFKs. These differences offer great potential for the structure-based design of trypanocidal drugs.

Organization, sequence and stage-specific expression of the phosphoglycerate kinase genes of Leishmania mexicana mexicana

In Leishmania mexicana two genes were detected coding for different isoforms of the glycolytic enzyme phosphoglycerate kinase. This situation contrasts with that observed in other Trypanosomatidae (Trypanosoma brucei, Trypanosoma congolense, Crithidia fasciculata) analyzed previously, which all contain three different genes coding for isoenzymes A, B and C, respectively. All attempts to detect in L. mexicana a type A PGK, or a gene encoding it, proved unsuccesful. We have cloned and characterized the genes PGKB and PGKC. They code for polypeptides of 416 and 478 amino acids with a molecular mass of 45146 and 51318 Da, respectively. The two polypeptides are 99% identical. PGKC is characterized by a 62 residue C-terminal extension with alternating stretches of hydrophobic and charged, mainly positive amino acids. As in other Trypanosomatidae, PGKB is located in the cytosol, PGKC in the glycosomes. However, Leishmania mexicana distinguishes itself from other trypanosomatids by the simultaneous expression of these isoenzymes: approximately 80% of PGK activity is found in the cytosol and 20% in the glycosomes, both in promastigotes and in the amastigote-like form of the parasite.

Characterisation of phosphoglycerate kinase genes in Leishmania major and evidence for the absence of a third closely related gene or isoenzyme

We have characterised the phosphoglycerate kinases (PGKs) in L. major and studied their mRNA and protein expression. Interestingly we have found evidence for only two tandemly linked PGK genes which correspond to the PGK gene B and C homologue in Trypanosoma and Crithidia. The primary structure of the leishmanial PGK genes B and C are virtually identical and differed only by the presence of a 62 amino acid extension at the carboxyl terminal of the PGK gene C homologue which is therefore likely to contain the translocation signal for glycosomal topogenesis. Indeed, the PGK gene C protein was found to be glycosomal (gPGK) while the PGK gene B protein was found to be cytosolic (cPGK). Both PGK genes are expressed in L. major promastigotes with the cPGK transcript expressed at a much higher level (4-5-fold) than the gPGK transcript. Similarly the relative cPGK isoenzyme activity was found to be approximately 4-fold higher than that of the gPGK isoenzyme. Surprisingly in L. major we have found no evidence for the PGK gene A present in all other trypanosomatids studied to date (Trypanosoma brucei, Trypanosoma congolense and Crithidia fasciculata). We therefore consider the possible evolutionary and functional significance of a trypanosomatid with only two PGK isoenzymes.

Cloning, expression, purification and characterization of triosephosphate isomerase from Trypanosoma cruzi

The gene that encodes for triosephosphate isomerase from Trypanosoma cruzi was cloned and sequenced. In T. cruzi, there is only one gene for triosephosphate isomerase. The enzyme has an identity of 72% and 68% with triosephosphate isomerase from Trypanosoma brucei and Leishmania mexicana, respectively. The active site residues are conserved: out of the 32 residues that conform the interface of dimeric triosephosphate isomerase from T. brucei, 29 are conserved in the T. cruzi enzyme. The enzyme was expressed in Escherichia coli and purified to homogeneity. Data from electrophoretic analysis under denaturing techniques and filtration techniques showed that triosephosphate isomerase from T. cruzi is a homodimer. Some of its structural and kinetic features were determined and compared to those of the purified enzymes from T. brucei and L. mexicana. Its circular dichroism spectrum was almost identical to that of triosephosphate isomerase from T. brucei. Its kinetic properties and pH optima were similar to those of T. brucei and L. mexicana, although the latter exhibited a higher Vmax with glyceraldehyde 3-phosphate as substrate. The sensitivity of the three enzymes to the sulfhydryl reagent methylmethane thiosulfonate (MeSO2-SMe) was determined; the sensitivity of the T. cruzi enzyme was about 40 times and 200 times higher than that of the enzymes from T. brucei and L. mexicana, respectively. Triosephosphate isomerase from T. cruzi and L. mexicana have the three cysteine residues that exist in the T. brucei enzyme (positions 14, 39, 126, using the numbering of the T. brucei enzyme); however, they also have an additional residue (position 117). These data suggest that regardless of the high identity of the three trypanosomatid enzymes, there are structural differences in the disposition of their cysteine residues that account for their different sensitivity to the sulfhydryl reagent. The disposition of the cysteine in triosephosphate isomerase from T. cruzi appears to make it unique for inhibition by modification of its cysteine.

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

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

Cloning and characterization of the NAD-linked glycerol-3-phosphate dehydrogenases of Trypanosoma brucei brucei and Leishmania mexicana mexicana and expression of the trypanosome enzyme in Escherichia coli

A polyclonal antiserum raised against the purified glycosomal glycerol-3-phosphate dehydrogenase of Trypanosoma brucei brucei has been used to identify the corresponding cDNA clone in a T.b. brucei expression library. This cDNA was subsequently used to obtain genomic clones containing glycerol-3-phosphate dehydrogenase genes. Two tandemly arranged genes were detected in these clones. Characterization of one of the genes showed that it codes for a polypeptide of 353 amino acids, with a molecular mass of 37,651 Da and a calculated net charge of +8. Using the T.b. brucei gene as a probe, a corresponding glycerol-3-phosphate dehydrogenase gene was also identified in a genomic library of Leishmania mexicana mexicana. The L.m. mexicana gene codes for a polypeptide of 365 amino acids, with a molecular mass of 39,140 Da and a calculated net charge of +8. The amino-acid sequences of both polypeptides are 63% identical and carry a type-1 peroxisomal targeting signal (PTS1) SKM and -SKL at their respective C-termini. Moreover, the L.m. mexicana polypeptide also carries a short N-terminal extension reminiscent of a mitochondrial transit sequence. Subcellular localisation analysis showed that in L.m. mexicana the glycerol-3-phosphate dehydrogenase activity co-fractionated both with mitochondria and with glycosomes. This is not the case in T. brucei, where the enzyme is predominantly glycosomal. The two trypanosomatid sequences resemble their prokaryotic homologues (32-36%) more than their eukaryotic counterparts (25-31%) and carry typical prokaryotic signatures. The possible reason for this prokaryotic nature of a trypanosomatid glycerol-3-phosphate dehydrogenase is discussed.

Regulation and control of compartmentalized glycolysis in bloodstream form Trypanosoma brucei

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

Molecular analysis of glyceraldehyde-3-phosphate dehydrogenase in Trypanoplasma borelli: an evolutionary scenario of subcellular compartmentation in kinetoplastida

In Trypanoplasma borelli, a representative of the Bodonina within the Kinetoplastida, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity was detected in both the cytosol and glycosomes. This situation is similar to that previously found in Trypanosomatidae, belonging to a different Kinetoplastida suborder. In Trypanosomatidae different isoenzymes, only distantly related, are responsible for the activity in the two cell compartments. In contrast, immunoblot analysis indicated that the GAPDH activity in cytosol and glycosomes of T. borelli should be attributed to identical or at least very similar proteins related to the glycosomal GAPDH of Trypanosomatidae. Moreover, only genes related to the glycosomal GAPDH genes of Trypanosomatidae could be detected. All attempts to identify a gene related to the one coding for the trypanosomatid cytosolic GAPDH remained unsuccessful. Two tandemly arranged genes were found which are 95% identical. The two encoded polypeptides differ in 17 residues. Their sequences are 72-77% identical to the glycosomal GAPDH of the other Kinetoplastida and share with them some characteristic features: an excess of positively charged residues, specific insertions, and a small carboxy-terminal extension containing the sequence -AKL. This tripeptide conforms to the consensus signal for targeting of proteins to glycosomes. One of the two gene copies has undergone some mutations at positions coding for highly conserved residues of the active site and the NAD(+)-binding domain of GAPDH. Modeling of the protein's three-dimensional structure suggested that several of the substitutions compensate each other, retaining the functional coenzyme-binding capacity, although this binding may be less tight. The presented analysis of GAPDH in T. borelli gives further support to the assertion that one isoenzyme, the cytosolic one, was acquired by horizontal gene transfer during the evolution of the Kinetoplastida, in the lineage leading to the suborder Trypanosomatina (Trypanosoma, Leishmania), after the divergence from the Bodonina (Trypanoplasma). Furthermore, the data clearly suggest that the original GAPDH of the Kinetoplastida has been compartmentalized during evolution.

Subcellular distribution and characterization of glucosephosphate isomerase in Leishmania mexicana mexicana

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

Structure, function, and biogenesis of glycosomes in kinetoplastida

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