Leishmania UDP-sugar pyrophosphorylase: the missing link in galactose salvage?

Identification and functional characterisation of N-acetylglucosamine kinase from Helicoverpa armigera divulge its potential role in growth and development via UDP-GlcNAc salvage pathway

N-acetylglucosamine kinase (NAGK), a major enzyme of sugar-kinase/Hsp70/actin superfamily, catalyses the conversion of N-acetylglucosamine to GlcNAc-6-phosphate, the first step leading to the salvage synthesis of uridine diphosphate N-acetylglucosamine. Here, we present the first report on identification, cloning, recombinant expression and functional characterisation of NAGK from Helicoverpa armigera (HaNAGK). The purified soluble HaNAGK exhibited a molecular mass of ~39 kDa with monomeric conformation. It catalysed the sequential transformation of GlcNAc into UDP-GlcNAc, indicating its role as the initiator of UDP-GlcNAc salvage pathway. HaNAGK exhibited ubiquitous expressions across all the developmental stages and major tissues of H. armigera. The gene was significantly upregulated (80 %; p < 0.01) by the moulting hormone 20-hydroxyecdysone and significantly downregulated (89 %; p < 0.001) by the chitin synthesis inhibitor novaluron, indicating its involvement in ecdysis and chitin metabolism. Furthermore, RNAi of HaNAGK caused poor weight gain, deformed insect bodies, aberrant metamorphosis and pronounced wing abnormalities in >55 % of surviving adults, while recording 7.79 ± 1.52 % and 24.25 ± 7.21 % mortality during larval and pupal stages, respectively. Altogether, the present findings suggest that HaNAGK plays a crucial role in the growth and development of H. armigera and thus, could be considered as a compelling gene of interest while formulating novel pest management strategies.

MC, Concepción JL, Cáceres AJ. Consumption of Galactose by Trypanosoma cruzi Epimastigotes Generates Resistance against Oxidative Stress

In this study, we demonstrate that Trypanosoma cruzi epimastigotes previously grown in LIT medium supplemented with 20 mM galactose and exposed to sub-lethal concentrations of hydrogen peroxide (100 μM) showed two-fold and five-fold viability when compared to epimastigotes grown in LIT medium supplemented with two different glucose concentrations (20 mM and 1.5 mM), respectively. Similar results were obtained when exposing epimastigotes from all treatments to methylene blue 30 μM. Additionally, through differential centrifugation and the selective permeabilization of cellular membranes with digitonin, we found that phosphoglucomutase activity (a key enzyme in galactose metabolism) occurs predominantly within the cytosolic compartment. Furthermore, after partially permeabilizing epimastigotes with digitonin (0.025 mg × mg-1 of protein), intact glycosomes treated with 20 mM galactose released a higher hexose phosphate concentration to the cytosol in the form of glucose-1-phosphate, when compared to intact glycosomes treated with 20 mM glucose, which predominantly released glucose-6-phosphate. These results shine a light on T. cruzi's galactose metabolism and its interplay with mechanisms that enable resistance to oxidative stress.

Study the Mechanism of Antileishmanial Action of Xanthium strumarium Against Amastigotes Stages in Leishmania major: A Metabolomics Approach

Background: Leishmaniasis is among the most important neglected tropical infections, affecting millions of people worldwide. Since 1945, chemotherapy has been the primary treatment for leishmaniasis; however, lengthy and costly treatments associated with various side effects and strains resistant to the conventional therapy have dramatically reduced chemotherapy compounds’ efficacy. Objectives: The antileishmanial activity of the leaf extract of Xanthium strumarium (Asteraceae) was studied. New insights into its mechanism of action toward Leishmania major were provided through a metabolomics-based study. Methods: J774 macrophages were cultured, infected with stationary promastigotes, and treated with different leaf extract concentrations for three days. Antileishmanial activity was assayed by the MTT colorimetric method, and cell metabolites were extracted. 1HNMR spectroscopy was applied, and outliers were analyzed using multivariate statistical analysis. Results: Xanthium strumarium extract (0.15 µg/mL) showed the best activity against L. major amastigotes with the infection rate (IR) and multiplication index (MI) values of 51% and 57%, respectively. The action of X. strumarium extract on amastigotes was comparable with amphotericin B as the positive control (0.015 µg/mL). According to the obtained P-values, pentanoate and coenzyme A biosynthesis, pentose and glucuronate metabolism, valine, leucine and isoleucine biosynthesis, galactose metabolism, amino sugar and nucleotide sugar metabolism were the most important metabolic pathways affected by the plant extract in the amastigote stage of L. major. Conclusions: Our finding demonstrated that X. strumarium leaf extract could be used for discovering and producing novel leishmanicidal medicines. Moreover, the affected metabolic pathways observed in this study could be potential candidates for drug targeting against leishmaniasis.

Elucidating carbohydrate metabolism in Euglena gracilis: Reverse genetics-based evaluation of genes coding for enzymes linked to paramylon accumulation

Euglena gracilis is a eukaryotic single-celled and photosynthetic organism grouped under the kingdom Protista. This phytoflagellate can accumulate the carbon photoassimilate as a linear β-1,3-glucan chain called paramylon. This storage polysaccharide can undergo degradation to provide glucose units to obtain ATP and reducing power both in aerobic and anaerobic growth conditions. Our group has recently characterized an essential enzyme for accumulating the polysaccharide, the UDP-glucose pyrophosphorylase (Biochimie vol 154, 2018, 176-186), which catalyzes the synthesis of UDP-glucose (the substrate for paramylon synthase). Additionally, the identification of nucleotide sequences coding for putative UDP-sugar pyrophosphorylases suggests the occurrence of an alternative source of UDP-glucose. In this study, we demonstrate the active involvement of both pyrophosphorylases in paramylon accumulation. Using techniques of single and combined knockdown of transcripts coding for these proteins, we evidenced a substantial decrease in the polysaccharide synthesis from 39 ± 7 μg/10⁶ cells determined in the control at day 21st of growth. Thus, the paramylon accumulation in Euglena gracilis cells decreased by 60% and 30% after a single knockdown of the expression of genes coding for UDP-glucose pyrophosphorylase and UDP-sugar pyrophosphorylase, respectively. Besides, the combined knockdown of both genes resulted in a ca. 65% reduction in the level of the storage polysaccharide. Our findings indicate the existence of a physiological dependence between paramylon accumulation and the partitioning of sugar nucleotides into other metabolic routes, including the Leloir pathway's functionality in Euglena gracilis.

Biochemical characterization of recombinant UDP-sugar pyrophosphorylase and galactinol synthase from Brachypodium distachyon

Raffinose (Raf) protects plant cells during seed desiccation and under different abiotic stress conditions. The biosynthesis of Raf starts with the production of UDP-galactose by UDP-sugar pyrophosphorylase (USPPase) and continues with the synthesis of galactinol by galactinol synthase (GolSase). Galactinol is then used by Raf synthase to produce Raf. In this work, we report the biochemical characterization of USPPase (BdiUSPPase) and GolSase 1 (BdiGolSase1) from Brachypodium distachyon. The catalytic efficiency of BdiUSPPase was similar with galactose 1-phosphate and glucose 1-phosphate, but 5- to 17-fold lower with other sugar 1-phosphates. The catalytic efficiency of BdiGolSase1 with UDP-galactose was three orders of magnitude higher than with UDP-glucose. A structural model of BdiGolSase1 allowed us to determine the residues putatively involved in the binding of substrates. Among these, we found that Cys²⁶¹ lies within the putative catalytic pocket. BdiGolSase1 was inactivated by oxidation with diamide and H₂O₂. The activity of the diamide-oxidized enzyme was recovered by reduction with dithiothreitol or E. coli thioredoxin, suggesting that BdiGolSase1 is redox-regulated.

Designer biocatalysts for direct incorporation of exogenous galactose into globotriose

Galactose is ubiquitous. The synthesis of galactose-containing oligosaccharides using Leloir galactosyltransferase requires uridine diphosphate (UDP)-galactose as the precursor. Of all UDP-galactose synthesis pathways developed for in vitro synthesis, the salvage pathway represents the simplest route. In this study, for the first time, we designed and constructed an Escherichia coli strain to use salvage pathway for UDP-galactose synthesis, demonstrating effective and direct incorporation of exogenous galactose into globotriose (Gb3). Successful establishment of salvage pathway enabled a complete delineation of carbon and energy source. Consequently, the designed biocatalyst was able to achieve high yield synthesis from galactose (0.95 moles of Gb3/moles galactose consumed) and a high product titer (2 g/L) in shaker flask within 24 hr. Elimination of limitation in acceptor sugar via homologous overexpression of LacY, the transporter for lactose, further improved the synthesis, raising Gb3 titer to 6 g/L in 24 hr and 7.5 g/L in 48 hr. The design principles successfully demonstrated in this study could be broadly applied for synthesis of other galactose-containing oligosaccharides. This study also illustrates a valid strategy to overcome limitation in the transport of acceptor sugar. As lactose is one of the most important basal structures, the significant improvement in synthesis through its enhanced transport could be emulated in numerous other lactose-based oligosaccharides.

Identification of Leishmania major UDP-Sugar Pyrophosphorylase Inhibitors Using Biosensor-Based Small Molecule Fragment Library Screening

Leishmaniasis is a neglected disease that is caused by different species of the protozoan parasite Leishmania, and it currently affects 12 million people worldwide. The antileishmanial therapeutic arsenal remains very limited in number and efficacy, and there is no vaccine for this parasitic disease. One pathway that has been genetically validated as an antileishmanial drug target is the biosynthesis of uridine diphosphate-glucose (UDP-Glc), and its direct derivative UDP-galactose (UDP-Gal). De novo biosynthesis of these two nucleotide sugars is controlled by the specific UDP-glucose pyrophosphorylase (UGP). Leishmania parasites additionally express a UDP-sugar pyrophosphorylase (USP) responsible for monosaccharides salvage that is able to generate both UDP-Gal and UDP-Glc. The inactivation of the two parasite pyrophosphorylases UGP and USP, results in parasite death. The present study reports on the identification of structurally diverse scaffolds for the development of USP inhibitors by fragment library screening. Based on this screening, we selected a small set of commercially available compounds, and identified molecules that inhibit both Leishmania major USP and UGP, with a half-maximal inhibitory concentration in the 100 µM range. The inhibitors were predicted to bind at allosteric regulation sites, which were validated by mutagenesis studies. This study sets the stage for the development of potent USP inhibitors.

UDP-Sugar Producing Pyrophosphorylases: Distinct and Essential Enzymes With Overlapping Substrate Specificities, Providing de novo Precursors for Glycosylation Reactions

Nucleotide sugars are the key precursors for all glycosylation reactions and are required both for oligo- and polysaccharides synthesis and protein and lipid glycosylation. Among all nucleotide sugars, UDP-sugars are the most important precursors for biomass production in nature (e.g., synthesis of cellulose, hemicellulose, and pectins for cell wall production). Several recent studies have already suggested a potential role for UDP-Glc in plant growth and development, and UDP-Glc has also been suggested as a signaling molecule, in addition to its precursor function. In this review, we will cover primary mechanisms of formation of UDP-sugars, by focusing on UDP-sugar metabolizing pyrophosphorylases. The pyrophosphorylases can be divided into three families: UDP-Glc pyrophosphorylase (UGPase), UDP-sugar pyrophosphorylase (USPase), and UDP-N-acetyl glucosamine pyrophosphorylase (UAGPase), which can be distinguished both by their amino acid sequences and by differences in substrate specificity. Substrate specificities of these enzymes are discussed, along with structure-function relationships, based on their crystal structures and homology modeling. Earlier studies with transgenic plants have revealed that each of the pyrophosphorylases is essential for plant survival, and their loss or a decrease in activity results in reproductive impairment. This constitutes a problem when studying exact in vivo roles of the enzymes using classical reverse genetics approaches. Thus, strategies involving the use of specific inhibitors (reverse chemical genetics) are also discussed. Further characterization of the properties/roles of pyrophosphorylases should address fundamental questions dealing with mechanisms and control of carbohydrate synthesis and may allow to identify targets for manipulation of biomass production in plants.

Metabolomics Based Study of the Antileishmanial Activity of Xanthium strumarium Leaf Extract on Promastigotes Phases of Leishmania major by Proton NMR Spectroscopy

BACKGROUND

Xanthium strumarium L. is extensively used as a traditional herb to treat many diseases and is also known as a source of phytochemicals. It has been used traditionally to treat trypanosomiasis, malaria fever, eczema, cancer, ulcer, fever, herpes headache, and skin lesion such as leishmaniasis. In this preliminary study, nuclear magnetic resonance (NMR)-metabolomics approaches was used to evaluate the inhibitory effects and metabolic alterations caused by leaf extract of X. strumarium on the stationary phases of promastigotes in Leishmania major.

METHODS

The promastigotes were cultured in Biochemistry Laboratory at Pasteur Institute of Iran in 2017, stationary phases were obtained from 5 to 6 day-old cultures and treated with different concentrations of the plant's extract. Antileishmanial activity was assayed by MTT method and cell metabolites were extracted. ¹H NMR spectroscopy was applied, and outliers were separated using multivariate statistical analysis.

RESULTS

The most affected metabolic pathways in the experimental groups were limited to amino sugar and nucleotide sugar metabolism, cyanoamino acid metabolism, starch and sucrose metabolism, butanoate metabolism, and galactose metabolism.

CONCLUSION

The ethanolic leaf extract of X. strumarium is a potent growth inhibitor of Leishmania major and can affect vital metabolic pathways of Leishmania promastigotes. The assay provided new perspectives on the development of novel treatment strategies for leishmanial activity derived from natural products.

CRISPR/Cas9 and glycomics tools for Toxoplasma glycobiology

Infection with the protozoan parasite Toxoplasma gondii is a major health risk owing to birth defects, its chronic nature, ability to reactivate to cause blindness and encephalitis, and high prevalence in human populations. Unlike most eukaryotes, Toxoplasma propagates in intracellular parasitophorous vacuoles, but like nearly all other eukaryotes, Toxoplasma glycosylates many cellular proteins and lipids and assembles polysaccharides. Toxoplasma glycans resemble those of other eukaryotes, but species-specific variations have prohibited deeper investigations into their roles in parasite biology and virulence. The Toxoplasma genome encodes a suite of likely glycogenes expected to assemble N-glycans, O-glycans, a C-glycan, GPI-anchors, and polysaccharides, along with their precursors and membrane transporters. To investigate the roles of specific glycans in Toxoplasma, here we coupled genetic and glycomics approaches to map the connections between 67 glycogenes, their enzyme products, the glycans to which they contribute, and cellular functions. We applied a double-CRISPR/Cas9 strategy, in which two guide RNAs promote replacement of a candidate gene with a resistance gene; adapted MS-based glycomics workflows to test for effects on glycan formation; and infected fibroblast monolayers to assess cellular effects. By editing 17 glycogenes, we discovered novel Glc_0-2-Man₆-GlcNAc₂-type N-glycans, a novel HexNAc-GalNAc-mucin-type O-glycan, and Tn-antigen; identified the glycosyltransferases for assembling novel nuclear O-Fuc-type and cell surface Glc-Fuc-type O-glycans; and showed that they are important for in vitro growth. The guide sequences, editing constructs, and mutant strains are freely available to researchers to investigate the roles of glycans in their favorite biological processes.

Facile and Stereo-Selective Synthesis of UDP-α-D-xylose and UDP-β-L-arabinose Using UDP-Sugar Pyrophosphorylase

A novel synthesis of nucleotide sugars was conducted to prepare UDP-α-D-xylose and UDP-β-L-arabinose without utilizing protection strategies or advanced purification techniques. Sugar-1-phosphates of D-xylose and L-arabinose were synthesized from their β-glycosylsulfonylhydrazides and evaluated as substrates for recombinant UDP-sugar pyrophosphorylases from Arabidopsis thaliana or Bifidobacterium infantis to furnish the biologically active nucleotide. The facile, three-step procedure takes advantage of substrate diversity available through chemical synthesis followed by the selectivity of enzyme catalysis. This approach increases the substrate scope of enzymatic preparation and expands access to stereopure nucleotide sugars on preparative scale. Increased production of both sugars has implications for glycoengineering and glycan production using glycosyltransferases.

UDP-glycosyltransferase genes in trypanosomatid genomes have diversified independently to meet the distinct developmental needs of parasite adaptations

BACKGROUND

Trypanosomatid parasites such as Trypanosoma spp. and Leishmania spp. are a major source of infectious disease in humans and domestic animals worldwide. Fundamental to the host-parasite interactions of these potent pathogens are their cell surfaces, which are highly decorated with glycosylated proteins and other macromolecules. Trypanosomatid genomes contain large multi-copy gene families encoding UDP-dependent glycosyltransferases (UGTs), the primary role of which is cell-surface decoration. Here we report a phylogenetic analysis of UGTs from diverse trypanosomatid genomes, the aim of which was to understand the origin and evolution of their diversity.

RESULTS

By combining phylogenetics with analyses of recombination, and selection, we compared UGT repertoire, genomic context and sequence evolution across 19 trypanosomatids. We identified a UGT lineage present in stercorarian trypanosomes and a free-living kinetoplastid Bodo saltans that likely represents the ancestral state of this gene family. The phylogeny of parasite-specific genes shows that UGTs repertoire in Leishmaniinae and salivarian trypanosomes has expanded independently and with distinct evolutionary dynamics. In the former, the ancestral UGT repertoire was organised in a tandem array from which sporadic transpositions to telomeric regions occurred, allowing expansion most likely through telomeric exchange. In the latter, the ancestral UGT repertoire was comprised of seven subtelomeric lineages, two of which have greatly expanded potentially by gene transposition between these dynamic regions of the genome.

CONCLUSIONS

The phylogeny of UGTs confirms that they represent a substantial parasite-specific innovation, which has diversified independently in the distinct trypanosomatid lineages. Nonetheless, developmental regulation has been a strong driver of UGTs diversification in both African trypanosomes and Leishmania.

Identification and characterization of inhibitors of UDP-glucose and UDP-sugar pyrophosphorylases for in vivo studies

UDP-sugars serve as ultimate precursors in hundreds of glycosylation reactions (e.g. for protein and lipid glycosylation, synthesis of sucrose, cell wall polysaccharides, etc.), underlying an important role of UDP-sugar-producing enzymes in cellular metabolism. However, genetic studies on mechanisms of UDP-sugar formation were frequently hampered by reproductive impairment of the resulting mutants, making it difficult to assess an in vivo role of a given enzyme. Here, a chemical library containing 17 500 compounds was separately screened against purified UDP-glucose pyrophosphorylase (UGPase) and UDP-sugar pyrophosphorylase (USPase), both enzymes representing the primary mechanisms of UDP-sugar formation. Several compounds have been identified which, at 50 μm, exerted at least 50% inhibition of the pyrophosphorylase activity. In all cases, both UGPase and USPase activities were inhibited, probably reflecting common structural features of active sites of these enzymes. One of these compounds (cmp #6), a salicylamide derivative, was found as effective inhibitor of Arabidopsis pollen germination and Arabidopsis cell culture growth. Hit optimization on cmp #6 yielded two analogs (cmp #6D and cmp #6D2), which acted as uncompetitive inhibitors against both UGPase and USPase, and were strong inhibitors in the pollen test, with apparent inhibition constants of less than 1 μm. Their effects on pollen germination were relieved by addition of UDP-glucose and UDP-galactose, suggesting that the inhibitors targeted UDP-sugar formation. The results suggest that cmp #6 and its analogs may represent useful tools to study in vivo roles of the pyrophosphorylases, helping to overcome the limitations of genetic approaches.

Characterization of a new UDP-sugar pyrophosphorylase from Hordeum vulgare (barley)

The broad substrate spectrum of UDP-sugar pyrophosphorylases from plant salvage pathways is of high interest for the synthesis of expensive nucleotide sugars by straightforward enzyme cascade reactions in combination with monosaccharide kinases. We here present a new UDP-sugar pyrophosphorylase from Hordeum vulgare with favorable biochemical properties like broad pH and temperature tolerances as well as a broad substrate spectrum and high synthesis stability. Enzyme properties were determined and reaction conditions were optimized by high-through-put multiplexed capillary electrophoresis analysis. In combination with a galactokinase UDP-α-d-galactose (UDP-Gal) was efficiently synthesized with a space-time-yield of 17g/L*h for full conversion of 10mM substrate within 20min by 1.2U of each enzyme.

Substrate Specificity and Inhibitor Sensitivity of Plant UDP-Sugar Producing Pyrophosphorylases

UDP-sugars are essential precursors for glycosylation reactions producing cell wall polysaccharides, sucrose, glycoproteins, glycolipids, etc. Primary mechanisms of UDP sugar formation involve the action of at least three distinct pyrophosphorylases using UTP and sugar-1-P as substrates. Here, substrate specificities of barley and Arabidopsis (two isozymes) UDP-glucose pyrophosphorylases (UGPase), Arabidopsis UDP-sugar pyrophosphorylase (USPase) and Arabidopsis UDP-N-acetyl glucosamine pyrophosphorylase2 (UAGPase2) were investigated using a range of sugar-1-phosphates and nucleoside-triphosphates as substrates. Whereas all the enzymes preferentially used UTP as nucleotide donor, they differed in their specificity for sugar-1-P. UGPases had high activity with D-Glc-1-P, but could also react with Fru-1-P and Fru-2-P (K_m values over 10 mM). Contrary to an earlier report, their activity with Gal-1-P was extremely low. USPase reacted with a range of sugar-1-phosphates, including D-Glc-1-P, D-Gal-1-P, D-GalA-1-P (K_m of 1.3 mM), β-L-Ara-1-P and α-D-Fuc-1-P (K_m of 3.4 mM), but not β-L-Fuc-1-P. In contrast, UAGPase2 reacted only with D-GlcNAc-1-P, D-GalNAc-1-P (K_m of 1 mM) and, to some extent, D-Glc-1-P (K_m of 3.2 mM). Generally, different conformations/substituents at C2, C4, and C5 of the pyranose ring of a sugar were crucial determinants of substrate specificity of a given pyrophosphorylase. Homology models of UDP-sugar binding to UGPase, USPase and UAGPase2 revealed more common amino acids for UDP binding than for sugar binding, reflecting differences in substrate specificity of these proteins. UAGPase2 was inhibited by a salicylate derivative that was earlier shown to affect UGPase and USPase activities, consistent with a common structural architecture of the three pyrophosphorylases. The results are discussed with respect to the role of the pyrophosphorylases in sugar activation for glycosylated end-products.

Multiplexed Capillary Electrophoresis as Analytical Tool for Fast Optimization of Multi-Enzyme Cascade Reactions - Synthesis of Nucleotide Sugars: Dedicated to Prof. Dr. Vladimir Křen on the occasion of his 60th birthday

Nucleotide sugars are considered as bottleneck and expensive substrates for enzymatic glycan synthesis using Leloir-glycosyltransferases. Synthesis from cheap substrates such as monosaccharides is accomplished by multi-enzyme cascade reactions. Optimization of product yields in such enzyme modules is dependent on the interplay of multiple parameters of the individual enzymes and governed by a considerable time effort when convential analytic methods like capillary electrophoresis (CE) or HPLC are applied. We here demonstrate for the first time multiplexed CE (MP-CE) as fast analytical tool for the optimization of nucleotide sugar synthesis with multi-enzyme cascade reactions. We introduce a universal separation method for nucleotides and nucleotide sugars enabling us to analyze the composition of six different enzyme modules in a high-throughput format. Optimization of parameters (T, pH, inhibitors, kinetics, cofactors and enzyme amount) employing MP-CE analysis is demonstrated for enzyme modules for the synthesis of UDP-α-D-glucuronic acid (UDP-GlcA) and UDP-α-D-galactose (UDP-Gal). In this way we achieve high space-time-yields: 1.8 g/L⋆h for UDP-GlcA and 17 g/L⋆h for UDP-Gal. The presented MP-CE methodology has the impact to be used as general analytical tool for fast optimization of multi-enzyme cascade reactions.

Depletion of UDP-Glucose and UDP-Galactose Using a Degron System Leads to Growth Cessation of Leishmania major

Interconversion of UDP-glucose (UDP-Glc) and UDP-galactose (UDP-Gal) by the UDP-Glc 4´-epimerase intimately connects the biosynthesis of these two nucleotide sugars. Their de novo biosynthesis involves transformation of glucose-6-phosphate into glucose-1-phosphate by the phosphoglucomutase and subsequent activation into UDP-Glc by the specific UDP-Glc pyrophosphorylase (UGP). Besides UGP, Leishmania parasites express an uncommon UDP-sugar pyrophosphorylase (USP) able to activate both galactose-1-phosphate and glucose-1-phosphate in vitro. Targeted gene deletion of UGP alone was previously shown to principally affect expression of lipophosphoglycan, resulting in a reduced virulence. Since our attempts to delete both UGP and USP failed, deletion of UGP was combined with conditional destabilisation of USP to control the biosynthesis of UDP-Glc and UDP-Gal. Stabilisation of the enzyme produced by a single USP allele was sufficient to maintain the steady-state pools of these two nucleotide sugars and preserve almost normal glycoinositolphospholipids galactosylation, but at the apparent expense of lipophosphoglycan biosynthesis. However, under destabilising conditions, the absence of both UGP and USP resulted in depletion of UDP-Glc and UDP-Gal and led to growth cessation and cell death, suggesting that either or both of these metabolites is/are essential.

Leishmaniases are a set of tropical and sub-tropical diseases caused by protozoan parasites of the genus Leishmania. They affect about 12 million people and cause a high morbidity. Since treatments against all forms of leishmaniasis are limited in number and efficacy, many efforts are made to identify potential drug targets and develop new therapies. Although considerable progress in genetic manipulation of Leishmania parasites have been made, it remains difficult to study molecules or metabolic pathways essential for parasite viability and growth. In the present work, we used a combination of gene deletion and conditional protein destabilization to demonstrate that biosynthesis of the nucleotide sugar UDP-glucose and its derivative UDP-galactose is essential for parasite growth. Addition of a specific ligand to the culture medium of the engineered parasite protected the targeted enzyme from degradation and enabled cell growth and viability. However, removal of the stabilizing compound led to depletion of UDP-glucose and UDP-galactose, growth arrest and cell death. This work thus opens a new possibility for the study of essential proteins.

Sugar activation and glycosylation in Plasmodium

Glycoconjugates are important mediators of host-pathogen interactions and are usually very abundant in the surface of many protozoan parasites. However, in the particular case of Plasmodium species, previous works show that glycosylphosphatidylinositol anchor modifications, and to an unknown extent, a severely truncated N-glycosylation are the only glycosylation processes taking place in the parasite. Nevertheless, a detailed analysis of the parasite genome and the recent identification of the sugar nucleotide precursors biosynthesized by Plasmodium falciparum support a picture in which several overlooked, albeit not very prominent glycosylations may be occurring during the parasite life cycle. In this work, the authors review recent developments in the characterization of the biosynthesis of glycosylation precursors in the parasite, focusing on the outline of the possible fates of these precursors.

Leishmania major UDP-sugar pyrophosphorylase salvages galactose for glycoconjugate biosynthesis

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Galactose salvage in Leishmania major is mediated by UDP-sugar pyrophosphorylase (USP).

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USP is not rate limiting for glycocalyx biosynthesis under standard growth conditions.

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Salvage by USP contributes to glycoconjugate biosynthesis but is insufficient on its own.

Leishmaniases are a set of tropical and sub-tropical diseases caused by protozoan parasites of the genus Leishmania whose severity ranges from self-healing cutaneous lesions to fatal visceral infections. Leishmania parasites synthesise a wide array of cell surface and secreted glycoconjugates that play important roles in infection. These glycoconjugates are particularly abundant in the promastigote form and known to be essential for establishment of infection in the insect midgut and effective transmission to the mammalian host. Since they are rich in galactose, their biosynthesis requires an ample supply of UDP-galactose. This nucleotide-sugar arises from epimerisation of UDP-glucose but also from an uncharacterised galactose salvage pathway. In this study, we evaluated the role of the newly characterised UDP-sugar pyrophosphorylase (USP) of Leishmania major in UDP-galactose biosynthesis. Upon deletion of the USP encoding gene, L. major lost the ability to synthesise UDP-galactose from galactose-1-phosphate but its ability to convert glucose-1-phosphate into UDP-glucose was fully maintained. Thus USP plays a role in UDP-galactose activation but does not significantly contribute to the de novo synthesis of UDP-glucose. Accordingly, USP was shown to be dispensable for growth and glycoconjugate biosynthesis under standard growth conditions. However, in a mutant seriously impaired in the de novo synthesis of UDP-galactose (due to deficiency of the UDP-glucose pyrophosphorylase) addition of extracellular galactose increased biosynthesis of the cell surface lipophosphoglycan. Thus under restrictive conditions, such as those encountered by Leishmania in its natural habitat, galactose salvage by USP may play a substantial role in biosynthesis of the UDP-galactose pool. We hypothesise that USP recycles galactose from the blood meal within the midgut of the insect for synthesis of the promastigote glycocalyx and thereby contributes to successful vector infection.

A quaternary mechanism enables the complex biological functions of octameric human UDP-glucose pyrophosphorylase, a key enzyme in cell metabolism

In mammals, UDP-glucose pyrophosphorylase (UGP) is the only enzyme capable of activating glucose-1-phosphate (Glc-1-P) to UDP-glucose (UDP-Glc), a metabolite located at the intersection of virtually all metabolic pathways in the mammalian cell. Despite the essential role of its product, the molecular basis of UGP function is poorly understood. Here we report the crystal structure of human UGP in complex with its product UDP-Glc. Beyond providing first insight into the active site architecture, we describe the substrate binding mode and intermolecular interactions in the octameric enzyme that are crucial to its activity. Importantly, the quaternary mechanism identified for human UGP in this study may be common for oligomeric sugar-activating nucleotidyltransferases. Elucidating such mechanisms is essential for understanding nucleotide sugar metabolism and opens the perspective for the development of drugs that specifically inhibit simpler organized nucleotidyltransferases in pathogens.

Metabolic reprogramming during purine stress in the protozoan pathogen Leishmania donovani

The ability of Leishmania to survive in their insect or mammalian host is dependent upon an ability to sense and adapt to changes in the microenvironment. However, little is known about the molecular mechanisms underlying the parasite response to environmental changes, such as nutrient availability. To elucidate nutrient stress response pathways in Leishmania donovani, we have used purine starvation as the paradigm. The salvage of purines from the host milieu is obligatory for parasite replication; nevertheless, purine-starved parasites can persist in culture without supplementary purine for over three months, indicating that the response to purine starvation is robust and engenders parasite survival under conditions of extreme scarcity. To understand metabolic reprogramming during purine starvation we have employed global approaches. Whole proteome comparisons between purine-starved and purine-replete parasites over a 6–48 h span have revealed a temporal and coordinated response to purine starvation. Purine transporters and enzymes involved in acquisition at the cell surface are upregulated within a few hours of purine removal from the media, while other key purine salvage components are upregulated later in the time-course and more modestly. After 48 h, the proteome of purine-starved parasites is extensively remodeled and adaptations to purine stress appear tailored to deal with both purine deprivation and general stress. To probe the molecular mechanisms affecting proteome remodeling in response to purine starvation, comparative RNA-seq analyses, qRT-PCR, and luciferase reporter assays were performed on purine-starved versus purine-replete parasites. While the regulation of a minority of proteins tracked with changes at the mRNA level, for many regulated proteins it appears that proteome remodeling during purine stress occurs primarily via translational and/or post-translational mechanisms.

Leishmania, the cause of a deadly spectrum of diseases in humans, surmounts a number of environmental challenges, including changes in the availability of salvageable nutrients, to successfully colonize its host. Adaptation to environmental stress is clearly of significance in parasite biology, but the underlying mechanisms are not well understood. To simulate the response to periodic nutrient scarcity in vivo, we have induced purine starvation in vitro. Purines are essential for growth and viability, and serve as the major energy currency of cells. Leishmania cannot synthesize purines and must salvage them from the surroundings. Extracellular purine depletion in culture induces a robust survival response in Leishmania, whereby growth arrests, but parasites persist for months. To profile the events that enable endurance of purine starvation, we used shotgun proteomics. Our data suggest that purine starvation induces extensive proteome remodeling, tailored to enhance purine capture and recycling, reduce energy expenditures, and maintain viability of the metabolically active, non-dividing population. Through global and targeted approaches, we reveal that proteome remodeling is multifaceted, and occurs through an array of responses at the mRNA, translational, and post-translational level. Our data provide one of the most inclusive views of adaptation to microenvironmental stress in Leishmania.

Effective sugar nucleotide regeneration for the large-scale enzymatic synthesis of Globo H and SSEA4

We report here the development of chemoenzymatic methods for the large-scale synthesis of cancer-associated antigens globopentaose (Gb5), fucosyl-Gb5 (Globo H), and sialyl-Gb5 (SSEA4) by using overexpressed glycosyltransferases coupled with effective regeneration of sugar nucleotides, including UDP-Gal, UDP-GalNAc, GDP-Fuc, and CMP-Neu5Ac. The enzymes used in the synthesis were first identified from different species through comparative studies and then overexpressed in E. coli and isolated for synthesis. These methods provide multigram quantities of products in high yield with only two or three purification steps and are suitable for the evaluation and development of cancer vaccines and therapeutics.

Biosynthesis of nucleotide sugars by a promiscuous UDP-sugar pyrophosphorylase from Arabidopsis thaliana (AtUSP)

Nucleotide sugars are activated forms of monosaccharides and key intermediates of carbohydrate metabolism in all organisms. The availability of structurally diverse nucleotide sugars is particularly important for the characterization of glycosyltransferases. Given that limited methods are available for preparation of nucleotide sugars, especially their useful non-natural derivatives, we introduced herein an efficient one-step three-enzyme catalytic system for the synthesis of nucleotide sugars from monosaccharides. In this study, a promiscuous UDP-sugar pyrophosphorylase (USP) from Arabidopsis thaliana (AtUSP) was used with a galactokinase from Streptococcus pneumoniae TIGR4 (SpGalK) and an inorganic pyrophosphatase (PPase) to effectively synthesize four UDP-sugars. AtUSP has better tolerance for C4-derivatives of Gal-1-P compared to UDP-glucose pyrophosphorylase from S. pneumoniae TIGR4 (SpGalU). Besides, the nucleotide substrate specificity and kinetic parameters of AtUSP were systematically studied. AtUSP exhibited considerable activity toward UTP, dUTP and dTTP, the yield of which was 87%, 85% and 84%, respectively. These results provide abundant information for better understanding of the relationship between substrate specificity and structural features of AtUSP.

Biosynthesis of GDP-fucose and other sugar nucleotides in the blood stages of Plasmodium falciparum

Carbohydrate structures play important roles in many biological processes, including cell adhesion, cell-cell communication, and host-pathogen interactions. Sugar nucleotides are activated forms of sugars used by the cell as donors for most glycosylation reactions. Using a liquid chromatography-tandem mass spectrometry-based method, we identified and quantified the pools of UDP-glucose, UDP-galactose, UDP-N-acetylglucosamine, GDP-mannose, and GDP-fucose in Plasmodium falciparum intraerythrocytic life stages. We assembled these data with the in silico functional reconstruction of the parasite metabolic pathways obtained from the P. falciparum annotated genome, exposing new active biosynthetic routes crucial for further glycosylation reactions. Fucose is a sugar present in glycoconjugates often associated with recognition and adhesion events. Thus, the GDP-fucose precursor is essential in a wide variety of organisms. P. falciparum presents homologues of GDP-mannose 4,6-dehydratase and GDP-L-fucose synthase enzymes that are active in vitro, indicating that most GDP-fucose is formed by a de novo pathway that involves the bioconversion of GDP-mannose. Homologues for enzymes involved in a fucose salvage pathway are apparently absent in the P. falciparum genome. This is in agreement with in vivo metabolic labeling experiments showing that fucose is not significantly incorporated by the parasite. Fluorescence microscopy of epitope-tagged versions of P. falciparum GDP-mannose 4,6-dehydratase and GDP-L-fucose synthase expressed in transgenic 3D7 parasites shows that these enzymes localize in the cytoplasm of P. falciparum during the intraerythrocytic developmental cycle. Although the function of fucose in the parasite is not known, the presence of GDP-fucose suggests that the metabolite may be used for further fucosylation reactions.

Biochemical and biophysical characterization of the sialyl-/hexosyltransferase synthesizing the meningococcal serogroup W135 heteropolysaccharide capsule

Neisseria meningitidis (Nm) is a leading cause of bacterial meningitis and sepsis. Crucial virulence determinants of pathogenic Nm strains are the polysaccharide capsules that support invasion by hindering complement attack. In NmW-135 and NmY the capsules are built from the repeating units (→ 6)-α-D-Gal-(1 → 4)-α-Neu5Ac-(2 →)n and (→ 6)-α-D-Glc-(1 → 4)-α-Neu5Ac-(2 →)n, respectively. These unusual heteropolymers represent unique examples of a conjugation between sialic acid and hexosyl-sugars in a polymer chain. Moreover, despite the various catalytic strategies needed for sialic acid and hexose transfer, single enzymes (SiaDW-135/Y) have been identified to form these heteropolymers. Here we used SiaDW-135 as a model system to delineate structure-function relationships. In size exclusion chromatography active SiaDW-135 migrated as a monomer. Fold recognition programs suggested two separate glycosyltransferase domains, both containing a GT-B-fold. Based on conserved motifs predicted folds could be classified as a hexosyl- and sialyltransferase. To analyze enzyme properties and interplay of the two identified glycosyltransferase domains, saturation transfer difference NMR and mutational studies were carried out. Simultaneous and independent binding of UDP-Gal and CMP-Sia was seen in the absence of an acceptor as well as when the catalytic cycle was allowed to proceed. Enzyme variants with only one functionality were generated by site-directed mutagenesis and shown to complement each other in trans when combined in an in vitro test system. Together the data strongly suggests that SiaDW-135 has evolved by fusion of two independent ancestral genes encoding sialyl- and galactosyltransferase activity.

Octamerization is essential for enzymatic function of human UDP-glucose pyrophosphorylase

Uridine diphosphate-glucose pyrophosphorylase (UGP) occupies a central position in carbohydrate metabolism in all kingdoms of life, since its product uridine diphosphate-glucose (UDP-glucose) is essential in a number of anabolic and catabolic pathways and is a precursor for other sugar nucleotides. Its significance as a virulence factor in protists and bacteria has given momentum to the search for species-specific inhibitors. These attempts are, however, hampered by high structural conservation of the active site architecture. A feature that discriminates UGPs of different species is the quaternary organization. While UGPs in protists are monomers, di- and tetrameric forms exist in bacteria, and crystal structures obtained for the enzyme from yeast and human identified octameric UGPs. These octamers are formed by contacts between highly conserved amino acids in the C-terminal β-helix. Still under debate is the question whether octamerization is required for the functionality of the human enzyme. Here, we used single amino acid replacements in the C-terminal β-helix to interrogate the impact of highly conserved residues on octamer formation and functional activity of human UGP (hUGP). Replacements were guided by the sequence of Arabidopsis thaliana UGP, known to be active as a monomer. Correlating the data obtained in blue native PAGE, size exclusion chromatography and enzymatic activity testing, we prove that the octamer is the active enzyme form. This new insight into structure-function relationships in hUGP does not only improve the understanding of the catalysis of this important enzyme, but in addition broadens the basis for studies aimed at designing drugs that selectively inhibit UGPs from pathogens.

Substrate kinetics and substrate effects on the quaternary structure of barley UDP-glucose pyrophosphorylase

UDP-Glc pyrophosphorylase (UGPase) is an essential enzyme responsible for production of UDP-Glc, which is used in hundreds of glycosylation reactions involving addition of Glc to a variety of compounds. In this study, barley UGPase was characterized with respect to effects of its substrates on activity and quaternary structure of the protein. Its K(m) values with Glc-1-P and UTP were 0.33 and 0.25 mM, respectively. Besides using Glc-1-P as a substrate, the enzyme had also considerable activity with Gal-1-P; however, the K(m) for Gal-1-P was very high (>10 mM), rendering this reaction unlikely under physiological conditions. UGPase had a relatively broad pH optimum of 6.5-8.5, regardless of the direction of reaction. The enzyme equilibrium constant was 0.4, suggesting slight preference for the Glc-1-P synthesis direction of the reaction. The quaternary structure of the enzyme, studied by Gas-phase Electrophoretic Mobility Macromolecule Analysis (GEMMA), was affected by addition of either single or both substrates in either direction of the reaction, resulting in a shift from UGPase dimers toward monomers, the active form of the enzyme. The substrate-induced changes in quaternary structure of the enzyme may have a regulatory role to assure maximal activity. Kinetics and factors affecting the oligomerization status of UGPase are discussed.

Efficient one-pot multienzyme synthesis of UDP-sugars using a promiscuous UDP-sugar pyrophosphorylase from Bifidobacterium longum (BLUSP)

A promiscuous UDP-sugar pyrophosphorylase (BLUSP) was cloned from Bifidobacterium longum strain ATCC55813 and used efficiently with a Pasteurella multocida inorganic pyrophosphatase (PmPpA) with or without a monosaccharide 1-kinase for one-pot multienzyme synthesis of UDP-galactose, UDP-glucose, UDP-mannose, and their derivatives. Further chemical diversification of a UDP-mannose derivative resulted in the formation of UDP-N-acetylmannosamine.

Characterization of recombinant UDP- and ADP-glucose pyrophosphorylases and glycogen synthase to elucidate glucose-1-phosphate partitioning into oligo- and polysaccharides in Streptomyces coelicolor

Streptomyces coelicolor exhibits a major secondary metabolism, deriving important amounts of glucose to synthesize pigmented antibiotics. Understanding the pathways occurring in the bacterium with respect to synthesis of oligo- and polysaccharides is of relevance to determine a plausible scenario for the partitioning of glucose-1-phosphate into different metabolic fates. We report the molecular cloning of the genes coding for UDP- and ADP-glucose pyrophosphorylases as well as for glycogen synthase from genomic DNA of S. coelicolor A3(2). Each gene was heterologously expressed in Escherichia coli cells to produce and purify to electrophoretic homogeneity the respective enzymes. UDP-glucose pyrophosphorylase (UDP-Glc PPase) was characterized as a dimer exhibiting a relatively high V(max) in catalyzing UDP-glucose synthesis (270 units/mg) and with respect to dTDP-glucose (94 units/mg). ADP-glucose pyrophosphorylase (ADP-Glc PPase) was found to be tetrameric in structure and specific in utilizing ATP as a substrate, reaching similar activities in the directions of ADP-glucose synthesis or pyrophosphorolysis (V(max) of 0.15 and 0.27 units/mg, respectively). Glycogen synthase was arranged as a dimer and exhibited specificity in the use of ADP-glucose to elongate α-1,4-glucan chains in the polysaccharide. ADP-Glc PPase was the only of the three enzymes exhibiting sensitivity to allosteric regulation by different metabolites. Mannose-6-phosphate, phosphoenolpyruvate, fructose-6-phosphate, and glucose-6-phosphate behaved as major activators, whereas NADPH was a main inhibitor of ADP-Glc PPase. The results support a metabolic picture where glycogen synthesis occurs via ADP-glucose in S. coelicolor, with the pathway being strictly regulated in connection with other routes involved with oligo- and polysaccharides, as well as with antibiotic synthesis in the bacterium.

A common structural blueprint for plant UDP-sugar-producing pyrophosphorylases

Plant pyrophosphorylases that are capable of producing UDP-sugars, key precursors for glycosylation reactions, include UDP-glucose pyrophosphorylases (A- and B-type), UDP-sugar pyrophosphorylase and UDP-N-acetylglucosamine pyrophosphorylase. Although not sharing significant homology at the amino acid sequence level, the proteins share a common structural blueprint. Their structures are characterized by the presence of the Rossmann fold in the central (catalytic) domain linked to enzyme-specific N-terminal and C-terminal domains, which may play regulatory functions. Molecular mobility between these domains plays an important role in substrate binding and catalysis. Evolutionary relationships and the role of (de)oligomerization as a regulatory mechanism are discussed.

Biosynthetic origin and mechanism of formation of the aminoribosyl moiety of peptidyl nucleoside antibiotics

Several peptidyl nucleoside antibiotics that inhibit bacterial translocase I involved in peptidoglycan cell wall biosynthesis contain an aminoribosyl moiety, an unusual sugar appendage in natural products. We present here the delineation of the biosynthetic pathway for this moiety upon in vitro characterization of four enzymes (LipM-P) that are functionally assigned as (i) LipO, an L-methionine:uridine-5'-aldehyde aminotransferase; (ii) LipP, a 5'-amino-5'-deoxyuridine phosphorylase; (iii) LipM, a UTP:5-amino-5-deoxy-α-D-ribose-1-phosphate uridylyltransferase; and (iv) LipN, a 5-amino-5-deoxyribosyltransferase. The cumulative results reveal a unique ribosylation pathway that is highlighted by, among other features, uridine-5'-monophosphate as the source of the sugar, a phosphorylase strategy to generate a sugar-1-phosphate, and a primary amine-requiring nucleotidylyltransferase that generates the NDP-sugar donor.

Biosynthesis of galactofuranose in kinetoplastids: novel therapeutic targets for treating leishmaniasis and chagas' disease

Cell surface proteins of parasites play a role in pathogenesis by modulating mammalian cell recognition and cell adhesion during infection. β-Galactofuranose (Galf) is an important component of glycoproteins and glycolipids found on the cell surface of Leishmania spp. and Trypanosoma cruzi. β-Galf-containing glycans have been shown to be important in parasite-cell interaction and protection against oxidative stress. Here, we discuss the role of β-Galf in pathogenesis and recent studies on the Galf-biosynthetic enzymes: UDP-galactose 4′ epimerase (GalE), UDP-galactopyranose mutase (UGM), and UDP-galactofuranosyl transferase (GalfT). The central role in Galf formation, its unique chemical mechanism, and the absence of a homologous enzyme in humans identify UGM as the most attractive drug target in the β-Galf-biosynthetic pathway in protozoan parasites.

Characterization, localization, essentiality, and high-resolution crystal structure of glucosamine 6-phosphate N-acetyltransferase from Trypanosoma brucei

A gene predicted to encode Trypanosoma brucei glucosamine 6-phosphate N-acetyltransferase (TbGNA1; EC 2.3.1.4) was cloned and expressed in Escherichia coli. The recombinant protein was enzymatically active, and its high-resolution crystal structure was obtained at 1.86 Å. Endogenous TbGNA1 protein was localized to the peroxisome-like microbody, the glycosome. A bloodstream-form T. brucei GNA1 conditional null mutant was constructed and shown to be unable to sustain growth in vitro under nonpermissive conditions, demonstrating that there are no metabolic or nutritional routes to UDP-GlcNAc other than via GlcNAc-6-phosphate. Analysis of the protein glycosylation phenotype of the TbGNA1 mutant under nonpermissive conditions revealed that poly-N-acetyllactosamine structures were greatly reduced in the parasite and that the glycosylation profile of the principal parasite surface coat component, the variant surface glycoprotein (VSG), was modified. The significance of results and the potential of TbGNA1 as a novel drug target for African sleeping sickness are discussed.

Structural basis for the broad substrate range of the UDP-sugar pyrophosphorylase from Leishmania major

Nucleotide sugars and the enzymes that are responsible for their synthesis are indispensable for the production of complex carbohydrates and, thus, for elaboration of a protective cellular coat for many organisms such as the protozoan parasite Leishmania. These activated sugars are synthesized de novo or derived from salvaged monosaccharides. In addition to UDP-glucose (UDP-Glc) pyrophosphorylase, which catalyzes the formation of UDP-Glc from substrates UTP and glucose-1-phosphate, Leishmania major and plants express a UDP-sugar pyrophosphorylase (USP) that exhibits broad substrate specificity in vitro. The enzyme, likely involved in monosaccharide salvage, preferentially generates UDP-Glc and UDP-galactose, but it may also activate other hexose- or pentose-1-phosphates such as galacturonic acid-1-phosphate or arabinose-1-phosphate. In order to gain insight into structural features governing the differences in substrate specificity, we determined the crystal structure of the L. major USP in the APO-, UTP-, and UDP-sugar-bound conformations. The overall tripartite structure of USP exhibits a significant structural homology to other nucleotidyldiphosphate-glucose pyrophosphorylases. The obtained USP structures reveal the structural rearrangements occurring during the stepwise binding process of the substrates. Moreover, the different product complexes explain the broad substrate specificity of USP, which is enabled by structural changes in the sugar binding region of the active site.

Identification, subcellular localization, biochemical properties, and high-resolution crystal structure of Trypanosoma brucei UDP-glucose pyrophosphorylase

The protozoan parasite Trypanosoma brucei is the causative agent of the cattle disease Nagana and human African sleeping sickness. Glycoproteins play key roles in the parasite’s survival and infectivity, and the de novo biosyntheses of the sugar nucleotides UDP-galactose (UDP-Gal), UDP-N-acetylglucosamine, and GDP-fucose have been shown to be essential for their growth. The only route to UDP-Gal in T.brucei is through the epimerization of UDP-glucose (UDP-Glc) by UDP-Glc 4′-epimerase. UDP-Glc is also the glucosyl donor for the unfolded glycoprotein glucosyltransferase (UGGT) involved in glycoprotein quality control in the endoplasmic reticulum and is the presumed donor for the synthesis of base J (β-d-glucosylhydroxymethyluracil), a rare deoxynucleotide found in telomere-proximal DNA in the bloodstream form of T.brucei. Considering that UDP-Glc plays such a central role in carbohydrate metabolism, we decided to characterize UDP-Glc biosynthesis in T.brucei. We identified and characterized the parasite UDP-glucose pyrophosphorylase (TbUGP), responsible for the formation of UDP-Glc from glucose-1-phosphate and UTP, and localized the enzyme to the peroxisome-like glycosome organelles of the parasite. Recombinant TbUGP was shown to be enzymatically active and specific for glucose-1-phosphate. The high-resolution crystal structure was also solved, providing a framework for the design of potential inhibitors against the parasite enzyme.

Identification of a novel UDP-sugar pyrophosphorylase with a broad substrate specificity in Trypanosoma cruzi

The diverse types of glycoconjugates synthesized by trypanosomatid parasites are unique compared with the host cells. These glycans are required for the parasite survival, invasion or evasion of the host immune system. Synthesis of those glycoconjugates requires a constant supply of nucleotide-sugars (NDP-sugars), yet little is known about how these NDP-sugars are made and supplied. In the present paper, we report a functional gene from Trypanosoma cruzi that encodes a nucleotidyltransferase, which is capable of transforming different types of sugar 1-phosphates and NTP into NDP-sugars. In the forward reaction, the enzyme catalyses the formation of UDP-glucose, UDP-galactose, UDP-xylose and UDP-glucuronic acid, from their respective monosaccharide 1-phosphates in the presence of UTP. The enzyme could also convert glucose 1-phosphate and TTP into TDP-glucose, albeit at lower efficiency. The enzyme requires bivalent ions (Mg2+ or Mn2+) for its activity and is highly active between pH 6.5 and pH 8.0, and at 30–42 °C. The apparent Km values for the forward reaction were 177 μM (glucose 1-phosphate) and 28.4 μM (UTP) respectively. The identification of this unusual parasite enzyme with such broad substrate specificities suggests an alternative pathway that might play an essential role for nucleotide-sugar biosynthesis and for the regulation of the NDP-sugar pool in the parasite.

Deletion of UDP-glucose pyrophosphorylase reveals a UDP-glucose independent UDP-galactose salvage pathway in Leishmania major

The nucleotide sugar UDP-galactose (UDP-Gal) is essential for the biosynthesis of several abundant glycoconjugates forming the surface glycocalyx of the protozoan parasite Leishmania major. Current data suggest that UDP-Gal could arise de novo by epimerization of UDP-glucose (UDP-Glc) or by a salvage pathway involving phosphorylation of Gal and the action of UDP-glucose:alpha-D-galactose-1-phosphate uridylyltransferase as described by Leloir. Since both pathways require UDP-Glc, inactivation of the UDP-glucose pyrophosphorylase (UGP) catalyzing activation of glucose-1 phosphate to UDP-Glc was expected to deprive parasites of UDP-Gal required for Leishmania glycocalyx formation. Targeted deletion of the gene encoding UGP, however, only partially affected the synthesis of the Gal-rich phosphoglycans. Moreover, no alteration in the abundant Gal-containing glycoinositolphospholipids was found in the deletion mutant. Consistent with these findings, the virulence of the UGP-deficient mutant was only modestly affected. These data suggest that Leishmania elaborates a UDP-Glc independent salvage pathway for UDP-Gal biosynthesis.