Real-time NMR monitoring of intermediates and labile products of the bifunctional enzyme UDP-apiose/UDP-xylose synthase

Engineered Saccharomyces cerevisiae as a Biosynthetic Platform of Nucleotide Sugars

Glycosylation of biomolecules can greatly alter their physicochemical properties, cellular recognition, subcellular localization, and immunogenicity. Glycosylation reactions rely on the stepwise addition of sugars using nucleotide diphosphate (NDP)-sugars. Making these substrates readily available will greatly accelerate the characterization of new glycosylation reactions, elucidation of their underlying regulation mechanisms, and production of glycosylated molecules. In this work, we engineered Saccharomyces cerevisiae to heterologously express nucleotide sugar synthases to access a wide variety of uridine diphosphate (UDP)-sugars from simple starting materials (i.e., glucose and galactose). Specifically, activated glucose, uridine diphosphate d-glucose (UDP-d-Glc), can be converted to UDP-d-glucuronic acid (UDP-d-GlcA), UDP-d-xylose (UDP-d-Xyl), UDP-d-apiose (UDP-d-Api), UDP-d-fucose (UDP-d-Fuc), UDP-l-rhamnose (UDP-l-Rha), UDP-l-arabinopyranose (UDP-l-Arap), and UDP-l-arabinofuranose (UDP-l-Araf) using the corresponding nucleotide sugar synthases of plant and microbial origins. We also expressed genes encoding the salvage pathway to directly activate free sugars to achieve the biosynthesis of UDP-l-Arap and UDP-l-Araf. We observed strong inhibition of UDP-d-Glc 6-dehydrogenase (UGD) by the downstream product UDP-d-Xyl, which we circumvented using an induction system (Tet-On) to delay the production of UDP-d-Xyl to maintain the upstream UDP-sugar pool. Finally, we performed a time-course study using strains containing the biosynthetic pathways to produce five non-native UDP-sugars to elucidate their time-dependent interconversion and the role of UDP-d-Xyl in regulating UDP-sugar metabolism. These engineered yeast strains are a robust platform to (i) functionally characterize sugar synthases in vivo, (ii) biosynthesize a diverse selection of UDP-sugars, (iii) examine the regulation of intracellular UDP-sugar interconversions, and (iv) produce glycosylated secondary metabolites and proteins.

A mixing microfluidic chip for real-time NMR monitoring of macromolecular reaction

NMR spectroscopy permits real-time monitoring of reactions that involve changes in the spectra of reactants. MICCS (MIcro Channeled Cell for Synthesis monitoring) is a microfluidic chip for such purposes, which is used to rapidly activate reactions by mixing the reactant solutions in the chip inserted into the typical NMR tube. Although it allows monitoring of chemical reactions of small compounds, its simple mixing system dependent on diffusion in the microchannel was not suitable for macromolecules such as proteins with low diffusion rates. Here we developed a new microfluidic chip based on MICCS by incorporating a mixer of split-and-recombination type within the microchannel. We applied it to monitoring of the protein-folding reaction in a stopped-flow mode. A solution of denaturant-unfolded RNase A was injected from a syringe pump into the microchip set inside the NMR magnet and mixed with a buffer for dilution to reach the folding condition. Immediately after dilution, the reaction was initiated and detected by a series of NMR measurements that were synchronized with activation and inactivation of the pump. The process was repeated for accumulation of the data. By analyzing the change of the spectra by factor analysis, a kinetic constant of 0.57 min-1 was obtained.

Deciphering the enzymatic mechanism of sugar ring contraction in UDP-apiose biosynthesis

D-Apiose is a C-branched pentose sugar important for plant cell wall development. Its biosynthesis as UDP-D-apiose involves decarboxylation of the UDP-D-glucuronic acid precursor coupled to pyranosyl-to-furanosyl sugar ring contraction. This unusual multistep reaction is catalyzed within a single active site by UDP-D-apiose/UDP-D-xylose synthase (UAXS). Here, we decipher the UAXS catalytic mechanism based on crystal structures of the enzyme from Arabidopsis thaliana, molecular dynamics simulations expanded by QM/MM calculations, and mutational-mechanistic analyses. Our studies show how UAXS uniquely integrates a classical catalytic cycle of oxidation and reduction by a tightly bound nicotinamide coenzyme with retro-aldol/aldol chemistry for the sugar ring contraction. They further demonstrate that decarboxylation occurs only after the sugar ring opening and identify the thiol group of Cys100 in steering the sugar skeleton rearrangement by proton transfer to and from the C3’. The mechanistic features of UAXS highlight the evolutionary expansion of the basic catalytic apparatus of short-chain dehydrogenases/reductases for functional versatility in sugar biosynthesis.

Aqueous Preparation of Maillard Reaction Intermediate from Glutathione and Xylose and its Volatile Formation During Thermal Treatment

Maillard reaction intermediate (MGX) generated from glutathione and xylose in aqueous medium was prepared via the Maillard reaction performed under a two-stage temperature increase process. The purified MGX was identified by Fourier-transform infrared spectroscopy, mass spectrometry, and nuclear magnetic resonance as N-(1-deoxy-d-xylulos-1-yl)-glutathione (Amadori compound, C₁₅ H₂₅ O₁₀ N₃ S) with five main isomers. The method of Maillard reaction performed under a two-stage temperature increase process was further verified by high-performance liquid chromatography. The optimal reaction time and temperature for the preparation of MGX was determined as 60 min at 90 °C. The yield of MGX was increased from 8.60% to 55.52% through thermal reaction coupled with vacuum dehydration. In addition, rapid and more Maillard-type volatile compounds were formed in MGX during thermal treatment than that in Maillard reaction products or glutathione-xylose mixture. Beside, MGX possessed more pleasing meat-like volatile profile compared with the Amadori compound of glutamic acid-xylose (AAX), cysteine-xylose (ACX), and glycine-xylose (AGX). Therefore, it suggested that the MGX had the potential to achieve a better flavor formation during thermal treatment. PRACTICAL APPLICATION: Maillard reaction intermediates, such as Amadori or Heyns rearrangement products (ARP or HRP), are important flavor precursors, which possess stable physicochemical properties, but tend to degrade into flavor compounds at high temperatures. Maillard reaction intermediate from glutathione and xylose acts as primary flavor enhancers to complete Maillard reaction to produce flavors in the subsequent thermal processing, which can significantly improve and stabilize the flavor quality of the meaty food, and deserves a very broad application prospects. The new developed method will be a significant theoretical basis on research preparation and properties of Maillard reaction intermediates in complex food systems.

Practical preparation of UDP-apiose and its applications for studying apiosyltransferase

UDP-apiose, a donor substrate of apiosyltransferases, is labile because of its intramolecular self-cyclization ability, resulting in the formation of apiofuranosyl-1,2-cyclic phosphate. Therefore, stabilization of UDP-apiose is indispensable for its availability and identifying and characterizing the apiosyltransferases involved in the biosynthesis of apiosylated sugar chains and glycosides. Here, we established a method for stabilizing UDP-apiose using bulky cations as counter ions. Bulky cations such as triethylamine effectively suppressed the degradation of UDP-apiose in solution. The half-life of UDP-apiose was increased to 48.1 ± 2.4 h at pH 6.0 and 25 °C using triethylamine as a counter cation. UDP-apiose coordinated with a counter cation enabled long-term storage under freezing conditions. UDP-apiose was utilized as a donor substrate for apigenin 7-O-β-D-glucoside apiosyltransferase to produce the apiosylated glycoside apiin. This apiosyltransferase assay will be useful for identifying genes encoding apiosyltransferases.

Identification of an apiosyltransferase in the plant pathogen Xanthomonas pisi

The rare branched-chain sugar apiose, once thought to only be present in the plant kingdom, was found in two bacterial species: Geminicoccus roseus and Xanthomonas pisi. Glycans with apiose residues were detected in aqueous methanol-soluble fractions as well as in the insoluble pellet fraction of X. pisi. Genes encoding bacterial uridine diphosphate apiose (UDP-apiose) synthases (bUASs) were characterized in these bacterial species, but the enzyme(s) involved in the incorporation of the apiose into glycans remained unknown. In the X. pisi genome two genes flanking the XpUAS were annotated as hypothetical glycosyltransferase (GT) proteins. The first GT (here on named XpApiT) belongs to GT family 90 and has a Leloir type B fold and a putative lipopolysaccharide-modifying (LPS) domain. The second GT (here on XpXylT) belongs to GT family 2 and has a type A fold. The XpXylT and XpApiT genes were cloned and heterologously expressed in E. coli. Analysis of nucleotide sugar extracts from E. coli expressing XpXylT or XpApiT with UAS showed that recombinant XpApiT utilized UDP-apiose and XpXylT utilized UDP-xylose as substrate. Indirect activity assay (UDP-Glo) revealed that XpApiT is an apiosyltransferase (ApiT) able to specifically use UDP-apiose. Further support for the apiosyltransferase activity was demonstrated by in microbe co-expression of UAS and XpApiT in E. coli showing the utilization of UDP-apiose to generate an apioside detectable in the pellet fraction. This work provides evidence that X. pisi developed the ability to synthesize an apioside of indeterminate function; however, the evolution of the bacterial ApiT remains to be determined. From genetic and evolutionary perspectives, the apiose operon may provide a unique opportunity to examine how genomic changes reflect ecological adaptation during the divergence of a bacterial group.

Correlation of Apiose Levels and Growth Rates in Duckweeds

The carbon assimilated by photosynthesis in plants can be partitioned into starch, soluble sugars, and cell wall polymers. Higher levels of starch accumulation in leaves are usually correlated with a lower growth capacity. Duckweeds are fast-growing aquatic monocot plants that can accumulate high levels of starch. They are an unusual group because their cell wall has very low levels of lignin while accumulating apiogalacturonan, a pectic polysaccharide that could be involved with boron assimilation. In this work, five duckweed species from different genera (Spirodela polyrhiza, Landoltia punctata, Lemna gibba, Wolffiella caudata, and Wolffia borealis) were cultivated under two light intensities (20 and 500 μmoles of photons m⁻² s⁻¹) to evaluate the effects of growth rate on carbohydrate metabolism. A comparative analysis was performed by measuring their relative growth rates (RGR), and their content for starch, as well as soluble and cell wall carbohydrates. We found that the faster-growing species (the Lemnoideae) accumulate lower starch and higher soluble sugars than the slower-growing species within the Wolffioideae. Interestingly, analysis of the cell wall monosaccharides revealed that the slower-growing species displayed lower content of apiose in their walls. Our results indicate that higher accumulation of apiose observed in cell walls of the Lemnoideae species, which likely correlates with a higher proportion of apiogalacturonan, may lead to higher efficiency in the assimilation of boron. This is consistent with the increased RGR observed under conditions with higher apiose in the cell wall, such as higher light intensity. Consistent with their lower growth capacity, the Wolffioideae species we studied shows higher starch accumulation in comparison with the Lemnoideae species. We suggest that apiose levels could be good biomarkers for growth capacity of duckweeds and suggest that boron uptake could be an important factor for growth control in this aquatic plant family.

Changes in the abundance of cell wall apiogalacturonan and xylogalacturonan and conservation of rhamnogalacturonan II structure during the diversification of the Lemnoideae

The diversification of the Lemnoideae was accompanied by a reduction in the abundance of cell wall apiogalacturonan and an increase in xylogalacturonan whereas rhamnogalacturonan II structure and cross-linking are conserved. The subfamily Lemnoideae is comprised of five genera and 38 species of small, fast-growing aquatic monocots. Lemna minor and Spirodela polyrhiza belong to this subfamily and have primary cell walls that contain large amounts of apiogalacturonan and thus are distinct from the primary walls of most other flowering plants. However, the pectins in the cell walls of other members of the Lemnoideae have not been investigated. Here, we show that apiogalacturonan decreased substantially as the Lemnoideae diversified since Wolffiella and Wolffia walls contain between 63 and 88% less apiose than Spirodela, Landoltia, and Lemna walls. In Wolffia, the most derived genus, xylogalacturonan is far more abundant than apiogalacturonan, whereas in Wolffiella pectic polysaccharides have a high arabinose content, which may arise from arabinan sidechains of RG I. The apiose-containing pectin rhamnogalacturonan II (RG-II) exists in Lemnoideae walls as a borate cross-linked dimer and has a glycosyl sequence similar to RG-II from terrestrial plants. Nevertheless, species-dependent variations in the extent of methyl-etherification of RG-II sidechain A and arabinosylation of sidechain B are discernible. Immunocytochemical studies revealed that pectin methyl-esterification is higher in developing daughter frond walls than in mother frond walls, indicating that methyl-esterification is associated with expanding cells. Our data support the notion that a functional cell wall requires conservation of RG-II structure and cross-linking but can accommodate structural changes in other pectins. The Lemnoideae provide a model system to study the mechanisms by which wall structure and composition has changed in closely related plants with similar growth habits.

Synthesis of UDP-apiose in Bacteria: The marine phototroph Geminicoccus roseus and the plant pathogen Xanthomonas pisi

The branched-chain sugar apiose was widely assumed to be synthesized only by plant species. In plants, apiose-containing polysaccharides are found in vascularized plant cell walls as the pectic polymers rhamnogalacturonan II and apiogalacturonan. Apiosylated secondary metabolites are also common in many plant species including ancestral avascular bryophytes and green algae. Apiosyl-residues have not been documented in bacteria. In a screen for new bacterial glycan structures, we detected small amounts of apiose in methanolic extracts of the aerobic phototroph Geminicoccus roseus and the pathogenic soil-dwelling bacteria Xanthomonas pisi. Apiose was also present in the cell pellet of X. pisi. Examination of these bacterial genomes uncovered genes with relatively low protein homology to plant UDP-apiose/UDP-xylose synthase (UAS). Phylogenetic analysis revealed that these bacterial UAS-like homologs belong in a clade distinct to UAS and separated from other nucleotide sugar biosynthetic enzymes. Recombinant expression of three bacterial UAS-like proteins demonstrates that they actively convert UDP-glucuronic acid to UDP-apiose and UDP-xylose. Both UDP-apiose and UDP-xylose were detectable in cell cultures of G. roseus and X. pisi. We could not, however, definitively identify the apiosides made by these bacteria, but the detection of apiosides coupled with the in vivo transcription of bUAS and production of UDP-apiose clearly demonstrate that these microbes have evolved the ability to incorporate apiose into glycans during their lifecycles. While this is the first report to describe enzymes for the formation of activated apiose in bacteria, the advantage of synthesizing apiose-containing glycans in bacteria remains unknown. The characteristics of bUAS and its products are discussed.

Isotope Probing of the UDP-Apiose/UDP-Xylose Synthase Reaction: Evidence of a Mechanism via a Coupled Oxidation and Aldol Cleavage

The C-branched sugar d-apiose (Api) is essential for plant cell-wall development. An enzyme-catalyzed decarboxylation/pyranoside ring-contraction reaction leads from UDP-α-d-glucuronic acid (UDP-GlcA) to the Api precursor UDP-α-d-apiose (UDP-Api). We examined the mechanism of UDP-Api/UDP-α-d-xylose synthase (UAXS) with site-selectively 2 H-labeled and deoxygenated substrates. The analogue UDP-2-deoxy-GlcA, which prevents C-2/C-3 aldol cleavage as the plausible initiating step of pyranoside-to-furanoside conversion, did not give the corresponding Api product. Kinetic isotope effects (KIEs) support an UAXS mechanism in which substrate oxidation by enzyme-NAD+ and retro-aldol sugar ring-opening occur coupled in a single rate-limiting step leading to decarboxylation. Rearrangement and ring-contracting aldol addition in an open-chain intermediate then give the UDP-Api aldehyde, which is intercepted via reduction by enzyme-NADH.

UUAT1 Is a Golgi-Localized UDP-Uronic Acid Transporter That Modulates the Polysaccharide Composition of Arabidopsis Seed Mucilage

UDP-glucuronic acid (UDP-GlcA) is the precursor of many plant cell wall polysaccharides and is required for production of seed mucilage. Following synthesis in the cytosol, it is transported into the lumen of the Golgi apparatus, where it is converted to UDP-galacturonic acid (UDP-GalA), UDP-arabinose, and UDP-xylose. To identify the Golgi-localized UDP-GlcA transporter, we screened Arabidopsis thaliana mutants in genes coding for putative nucleotide sugar transporters for altered seed mucilage, a structure rich in the GalA-containing polysaccharide rhamnogalacturonan I. As a result, we identified UUAT1, which encodes a Golgi-localized protein that transports UDP-GlcA and UDP-GalA in vitro. The seed coat of uuat1 mutants had less GalA, rhamnose, and xylose in the soluble mucilage, and the distal cell walls had decreased arabinan content. Cell walls of other organs and cells had lower arabinose levels in roots and pollen tubes, but no differences were observed in GalA or xylose contents. Furthermore, the GlcA content of glucuronoxylan in the stem was not affected in the mutant. Interestingly, the degree of homogalacturonan methylation increased in uuat1 These results suggest that this UDP-GlcA transporter plays a key role defining the seed mucilage sugar composition and that its absence produces pleiotropic effects in this component of the plant extracellular matrix.

Transcriptome-guided gene isolation and functional characterization of UDP-xylose synthase and UDP-D-apiose/UDP-D-xylose synthase families from Ornithogalum caudatum Ait

The present study first identified the involvement of OcUAXS2 and OcUXS1-3 in anticancer polysaccharides biosynthesis in O. caudatum. UDP-xylose synthase (UXS) and UDP-D-apiose/UDP-D-xylose synthase (UAXS), both capable of converting UDP-D-glucuronic acid to UDP-D-xylose, are believed to transfer xylosyl residue to anticancer polysaccharides biosynthesis in Ornithogalum caudatum Ait. However, the cDNA isolation and functional characterization of genes encoding the two enzymes from O. caudatum has never been documented. Previously, the transcriptome sequencing of O. caudatum was performed in our laboratory. In this study, a total of six and two unigenes encoding UXS and UAXS were first retrieved based on RNA-Seq data. The eight putative genes were then successfully isolated from transcriptome of O. caudatum by reverse transcription polymerase chain reaction (RT-PCR). Phylogenetic analysis revealed the six putative UXS isoforms can be classified into three types, one soluble and two distinct putative membrane-bound. Moreover, the two UAXS isoenzymes were predicted to be soluble forms. Subsequently, these candidate cDNAs were characterized to be bona fide genes by functional expression in Escherichia coli individually. Although UXS and UAXS catalyzed the same reaction, their biochemical properties varied significantly. It is worth noting that a ratio switch of UDP-D-xylose/UDP-D-apiose for UAXS was established, which is assumed to be helpful for its biotechnological application. Furthermore, a series of mutants were generated to test the function of NAD⁺ binding motif GxxGxxG. Most importantly, the present study determined the involvement of OcUAXS2 and OcUXS1-3 in xylose-containing polysaccharides biosynthesis in O. caudatum. These data provide a comprehensive knowledge for UXS and UAXS families in plants.

Functional Characterization of UDP-apiose Synthases from Bryophytes and Green Algae Provides Insight into the Appearance of Apiose-containing Glycans during Plant Evolution

Apiose is a branched monosaccharide that is present in the cell wall pectic polysaccharides rhamnogalacturonan II and apiogalacturonan and in numerous plant secondary metabolites. These apiose-containing glycans are synthesized using UDP-apiose as the donor. UDP-apiose (UDP-Api) together with UDP-xylose is formed from UDP-glucuronic acid (UDP-GlcA) by UDP-Api synthase (UAS). It was hypothesized that the ability to form Api distinguishes vascular plants from the avascular plants and green algae. UAS from several dicotyledonous plants has been characterized; however, it is not known if avascular plants or green algae produce this enzyme. Here we report the identification and functional characterization of UAS homologs from avascular plants (mosses, liverwort, and hornwort), from streptophyte green algae, and from a monocot (duckweed). The recombinant UAS homologs all form UDP-Api from UDP-glucuronic acid albeit in different amounts. Apiose was detected in aqueous methanolic extracts of these plants. Apiose was detected in duckweed cell walls but not in the walls of the avascular plants and algae. Overexpressing duckweed UAS in the moss Physcomitrella patens led to an increase in the amounts of aqueous methanol-acetonitrile-soluble apiose but did not result in discernible amounts of cell wall-associated apiose. Thus, bryophytes and algae likely lack the glycosyltransferase machinery required to synthesize apiose-containing cell wall glycans. Nevertheless, these plants may have the ability to form apiosylated secondary metabolites. Our data are the first to provide evidence that the ability to form apiose existed prior to the appearance of rhamnogalacturonan II and apiogalacturonan and provide new insights into the evolution of apiose-containing glycans.

The inside and outside: topological issues in plant cell wall biosynthesis and the roles of nucleotide sugar transporters

The cell wall is a complex extracellular matrix composed primarily of polysaccharides. Noncellulosic polysaccharides, glycoproteins and proteoglycans are synthesized in the Golgi apparatus by glycosyltransferases (GTs), which use nucleotide sugars as donors to glycosylate nascent glycan and glycoprotein acceptors that are subsequently exported to the extracellular space. Many nucleotide sugars are synthesized in the cytosol, leading to a topological issue because the active sites of most GTs are located in the Golgi lumen. Nucleotide sugar transporters (NSTs) overcome this problem by translocating nucleoside diphosphate sugars from the cytosol into the lumen of the organelle. The structures of the cell wall components synthesized in the Golgi are diverse and complex; therefore, transporter activities are necessary so that the nucleotide sugars can provide substrates for the GTs. In this review, we describe the topology of reactions involved in polysaccharide biosynthesis in the Golgi and focus on the roles of NSTs as well as their impacts on cell wall structure when they are altered.

Characterization of Early Enzymes Involved in TDP-Aminodideoxypentose Biosynthesis en Route to Indolocarbazole AT2433

The characterization of TDP-α-D-glucose dehydrogenase (AtmS8), TDP-α-D-glucuronic acid decarboxylase (AtmS9), and TDP-4-keto-α-D-xylose 2,3-dehydratase (AtmS14), involved in Actinomadura melliaura AT2433 aminodideoxypentose biosynthesis, is reported. This study provides the first biochemical evidence that both deoxypentose and deoxyhexose biosynthetic pathways share common strategies for sugar 2,3-dehydration/reduction and implicates the sugar nucleotide base specificity of AtmS14 as a potential mechanism for sugar nucleotide commitment to secondary metabolism. In addition, a re-evaluation of the AtmS9 homologue involved in calicheamicin aminodeoxypentose biosynthesis (CalS9) reveals that CalS9 catalyzes UDP-4-keto-α-D-xylose as the predominant product, rather than UDP-α-D-xylose as previously reported. Cumulatively, this work provides additional fundamental insights regarding the biosynthesis of novel pentoses attached to complex bacterial secondary metabolites.

Biotechnological advances in UDP-sugar based glycosylation of small molecules

Glycosylation of small molecules like specialized (secondary) metabolites has a profound impact on their solubility, stability or bioactivity, making glycosides attractive compounds as food additives, therapeutics or nutraceuticals. The subsequently growing market demand has fuelled the development of various biotechnological processes, which can be divided in the in vitro (using enzymes) or in vivo (using whole cells) production of glycosides. In this context, uridine glycosyltransferases (UGTs) have emerged as promising catalysts for the regio- and stereoselective glycosylation of various small molecules, hereby using uridine diphosphate (UDP) sugars as activated glycosyldonors. This review gives an extensive overview of the recently developed in vivo production processes using UGTs and discusses the major routes towards UDP-sugar formation. Furthermore, the use of interconverting enzymes and glycorandomization is highlighted for the production of unusual or new-to-nature glycosides. Finally, the technological challenges and future trends in UDP-sugar based glycosylation are critically evaluated and summarized.

Two UDP-glucuronic acid decarboxylases involved in the biosynthesis of a bacterial exopolysaccharide in Paenibacillus elgii

Xylose is described as a component of bacterial exopolysaccharides in only a limited number of bacterial strains. A bacterial strain, Paenibacillus elgii, B69 was shown to be efficient in producing a xylose-containing exopolysaccharide. Sequence analysis was performed to identify the genes encoding the uridine diphosphate (UDP)-glucuronic acid decarboxylase required for the synthesis of UDP-xylose, the precursor of the exopolysaccharide. Two sequences, designated as Peuxs1 and Peuxs2, were found as the candidate genes for such enzymes. The activities of the UDP-glucuronic acid decarboxylases were proven by heterologous expression and real-time nuclear magnetic resonance analysis. The intracellular activity and effect of these genes on the synthesis of exopolysaccharide were further investigated by developing a thymidylate synthase based knockout system. This system was used to substitute the conventional antibiotic resistance gene system in P. elgii, a natural multi-antibiotic resistant strain. Results of intracellular nucleotide sugar analysis showed that the intracellular UDP-xylose and UDP-glucuronic acid levels were affected in Peuxs1 or Peuxs2 knockout strains. The knockout of either Peuxs1 or Peuxs2 reduced the polysaccharide production and changed the monosaccharide ratio. No polysaccharide was found in the Peuxs1/Peuxs2 double knockout strain. Our results show that P. elgii can be efficient in forming UDP-xylose, which is then used for the synthesis of xylose-containing exopolysaccharide.

Real-time protein NMR spectroscopy and investigation of assisted protein folding

BACKGROUND

During protein-folding reactions toward the native structure, short-lived intermediate states can be populated. Such intermediates expose hydrophobic patches and can self-associate leading to non-productive protein misfolding. A major focus of current research is the characterization of short-lived intermediates and how molecular chaperones enable productive folding. Real-time NMR spectroscopy, together with the development of advanced methods, is reviewed here and the potential these methods have to characterize intermediate states as well as interactions with molecular chaperone proteins at single-residue resolution is highlighted.

SCOPE OF REVIEW

Various chaperone interactions can guide the protein-folding reaction and thus are important for protein structure formation, stability, and activity of their substrates. Chaperone-assisted protein folding, characterization of intermediates, and their molecular interactions using real-time NMR spectroscopy will be discussed. Additionally, recent advances in NMR methods employed for characterization of high-energy intermediates will be discussed.

MAJOR CONCLUSIONS

Real-time NMR combines high resolution with kinetic information of protein reactions, which can be employed not only for protein-folding studies and the characterization of folding intermediates but also to investigate the molecular mechanisms of assisted protein folding.

GENERAL SIGNIFICANCE

Real-time NMR spectroscopy remains an effective tool to reveal structural details about the interaction between chaperones and transient intermediates. Methodologically, it provides in-depth understanding of how kinetic intermediates and their thermodynamics contribute to the protein-folding reaction. This review summarizes the most recent advances in this field. This article is part of a Special Issue titled Proline-directed Foldases: Cell Signaling Catalysts and Drug Targets.

Enzymatic Redox Cascade for One-Pot Synthesis of Uridine 5'-Diphosphate Xylose from Uridine 5'-Diphosphate Glucose

Synthetic ways towards uridine 5′-diphosphate (UDP)-xylose are scarce and not well established, although this compound plays an important role in the glycobiology of various organisms and cell types. We show here how UDP-glucose 6-dehydrogenase (hUGDH) and UDP-xylose synthase 1 (hUXS) from Homo sapiens can be used for the efficient production of pure UDP-α-xylose from UDP-glucose. In a mimic of the natural biosynthetic route, UDP-glucose is converted to UDP-glucuronic acid by hUGDH, followed by subsequent formation of UDP-xylose by hUXS. The nicotinamide adenine dinucleotide (NAD⁺) required in the hUGDH reaction is continuously regenerated in a three-step chemo-enzymatic cascade. In the first step, reduced NAD⁺ (NADH) is recycled by xylose reductase from Candida tenuis via reduction of 9,10-phenanthrenequinone (PQ). Radical chemical re-oxidation of this mediator in the second step reduces molecular oxygen to hydrogen peroxide (H₂O₂) that is cleaved by bovine liver catalase in the last step. A comprehensive analysis of the coupled chemo-enzymatic reactions revealed pronounced inhibition of hUGDH by NADH and UDP-xylose as well as an adequate oxygen supply for PQ re-oxidation as major bottlenecks of effective performance of the overall multi-step reaction system. Net oxidation of UDP-glucose to UDP-xylose by hydrogen peroxide (H₂O₂) could thus be achieved when using an in situ oxygen supply through periodic external feed of H₂O₂ during the reaction. Engineering of the interrelated reaction parameters finally enabled production of 19.5 mM (10.5 g l⁻¹) UDP-α-xylose. After two-step chromatographic purification the compound was obtained in high purity (>98%) and good overall yield (46%). The results provide a strong case for application of multi-step redox cascades in the synthesis of nucleotide sugar products.

A general NMR-based strategy for the in situ characterization of sugar-nucleotide-dependent biosynthetic pathways

A simple method for the study of sugar-nucleotide-dependent multienzyme cascades is highlighted where the use of selectively (13)C-labeled sugar nucleotides and inverse (13)C detection NMR offers fast, direct detection and quantification of reactants and products and circumvents the need for chromatographic separation. The utility of the method has been demonstrated by characterizing four previously uncharacterized sugar nucleotide biosynthetic enzymes involved in calicheamicin biosynthesis.

Cooperation of two bifunctional enzymes in the biosynthesis and attachment of deoxysugars of the antitumor antibiotic mithramycin

Two bifunctional enzymes cooperate in the assembly and the positioning of two sugars, D-olivose and D-mycarose, of the anticancer antibiotic mithramycin. MtmC finishes the biosynthesis of both sugar building blocks depending on which MtmGIV activity is supported. MtmGIV transfers these two sugars onto two structurally distinct acceptor substrates. The dual function of these enzymes explains two essential but previously unidentified activities.

Cooperation of two bifunctional enzymes in the biosynthesis and attachment of deoxysugars of the antitumor antibiotic mithramycin

Two bifunctional enzymes cooperate in the assembly and the positioning of two sugars, D-olivose and D-mycarose, of the anticancer antibiotic mithramycin. MtmC finishes the biosynthesis of both sugar building blocks depending on which MtmGIV activity is supported. MtmGIV transfers these two sugars onto two structurally distinct acceptor substrates. The dual function of these enzymes explains two essential but previously unidentified activities.

Analysis of UDP-D-apiose/UDP-D-xylose synthase-catalyzed conversion of UDP-D-apiose phosphonate to UDP-D-xylose phosphonate: implications for a retroaldol-aldol mechanism

UDP-D-apiose/UDP-D-xylose synthase (AXS) catalyzes the conversion of UDP-D-glucuronic acid to UDP-D-apiose and UDP-D-xylose. An acetyl-protected phosphonate analogue of UDP-D-apiose was synthesized and used in an in situ HPLC assay to demonstrate for the first time the ability of AXS to interconvert the two reaction products. Density functional theory calculations provided insight into the energetics of this process and the apparent inability of AXS to catalyze the conversion of UDP-D-xylose to UDP-D-apiose. The data suggest that this observation is unlikely to be due to an unfavorable equilibrium but rather results from substrate inhibition by the most stable chair conformation of UDP-D-xylose. The detection of xylose cyclic phosphonate as the turnover product reveals significant new details about the AXS-catalyzed reaction and supports the proposed retroaldol-aldol mechanism of catalysis.

The Synthesis and Origin of the Pectic Polysaccharide Rhamnogalacturonan II - Insights from Nucleotide Sugar Formation and Diversity

There is compelling evidence showing that the structurally complex pectic polysaccharide rhamnogalacturonan II (RG-II) exists in the primary cell wall as a borate cross-linked dimer and that this dimer is required for the assembly of a functional wall and for normal plant growth and development. The results of several studies have also established that RG-II structure and cross-linking is conserved in vascular plants and that RG-II likely appeared early in the evolution of land plants. Two features that distinguish RG-II from other plant polysaccharides are that RG-II is composed of 13 different glycoses linked to each other by up to 22 different glycosidic linkages and that RG-II is the only polysaccharide known to contain both apiose and aceric acid. Thus, one key event in land plant evolution was the emergence of genes encoding nucleotide sugar biosynthetic enzymes that generate the activated forms of apiose and aceric acid required for RG-II synthesis. Many of the genes involved in the generation of the nucleotide sugars used for RG-II synthesis have been functionally characterized. By contrast, only one glycosyltransferase involved in the assembly of RG-II has been identified. Here we provide an overview of the formation of the activated sugars required for RG-II synthesis and point to the possible cellular and metabolic processes that could be involved in assembling and controlling the formation of a borate cross-linked RG-II molecule. We discuss how nucleotide sugar synthesis is compartmentalized and how this may control the flux of precursors to facilitate and regulate the formation of RG-II.

Evolution of plant nucleotide-sugar interconversion enzymes

Nucleotide-diphospho-sugars (NDP-sugars) are the building blocks of diverse polysaccharides and glycoconjugates in all organisms. In plants, 11 families of NDP-sugar interconversion enzymes (NSEs) have been identified, each of which interconverts one NDP-sugar to another. While the functions of these enzyme families have been characterized in various plants, very little is known about their evolution and origin. Our phylogenetic analyses indicate that all the 11 plant NSE families are distantly related and most of them originated from different progenitor genes, which have already diverged in ancient prokaryotes. For instance, all NSE families are found in the lower land plant mosses and most of them are also found in aquatic algae, implicating that they have already evolved to be capable of synthesizing all the 11 different NDP-sugars. Particularly interesting is that the evolution of RHM (UDP-L-rhamnose synthase) manifests the fusion of genes of three enzymatic activities in early eukaryotes in a rather intriguing manner. The plant NRS/ER (nucleotide-rhamnose synthase/epimerase-reductase), on the other hand, evolved much later from the ancient plant RHMs through losing the N-terminal domain. Based on these findings, an evolutionary model is proposed to explain the origin and evolution of different NSE families. For instance, the UGlcAE (UDP-D-glucuronic acid 4-epimerase) family is suggested to have evolved from some chlamydial bacteria. Our data also show considerably higher sequence diversity among NSE-like genes in modern prokaryotes, consistent with the higher sugar diversity found in prokaryotes. All the NSE families are widely found in plants and algae containing carbohydrate-rich cell walls, while sporadically found in animals, fungi and other eukaryotes, which do not have or have cell walls with distinct compositions. Results of this study were shown to be highly useful for identifying unknown genes for further experimental characterization to determine their functions in the synthesis of diverse glycosylated molecules.

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.

A fluoro analogue of UDP-α-D-glucuronic acid is an inhibitor of UDP-α-D-apiose/UDP-α-D-xylose synthase

UDP-2F-glucuronic acid was synthesized and analyzed as a mechanistic probe to investigate the ring contraction step catalyzed by UDP-d-apiose/UDP-d-xylose synthase (AXS).

UDP-glucuronic acid decarboxylases of Bacteroides fragilis and their prevalence in bacteria

Xylose is rarely described as a component of bacterial glycans. UDP-xylose is the nucleotide-activated form necessary for incorporation of xylose into glycans and is synthesized by the decarboxylation of UDP-glucuronic acid (UDP-GlcA). Enzymes with UDP-GlcA decarboxylase activity include those that lead to the formation of UDP-xylose as the end product (Uxs type) and those synthesizing UDP-xylose as an intermediate (ArnA and RsU4kpxs types). In this report, we identify and confirm the activities of two Uxs-type UDP-GlcA decarboxylases of Bacteroides fragilis, designated BfUxs1 and BfUxs2. Bfuxs1 is located in a conserved region of the B. fragilis genome, whereas Bfuxs2 is in the heterogeneous capsular polysaccharide F (PSF) biosynthesis locus. Deletion of either gene separately does not result in the loss of a detectable phenotype, but deletion of both genes abrogates PSF synthesis, strongly suggesting that they are functional paralogs and that the B. fragilis NCTC 9343 PSF repeat unit contains xylose. UDP-GlcA decarboxylases are often annotated incorrectly as NAD-dependent epimerases/dehydratases; therefore, their prevalence in bacteria is underappreciated. Using available structural and mutational data, we devised a sequence pattern to detect bacterial genes encoding UDP-GlcA decarboxylase activity. We identified 826 predicted UDP-GlcA decarboxylase enzymes in diverse bacterial species, with the ArnA and RsU4kpxs types confined largely to proteobacterial species. These data suggest that xylose, or a monosaccharide requiring a UDP-xylose intermediate, is more prevalent in bacterial glycans than previously appreciated. Genes encoding BfUxs1-like enzymes are highly conserved in Bacteroides species, indicating that these abundant intestinal microbes may synthesize a conserved xylose-containing glycan.

Time-resolved NMR: extracting the topology of complex enzyme networks

The use of nondestructive NMR spectroscopy for enzymatic studies offers unique opportunities to identify nearly all enzymatic byproducts and detect unstable short-lived products or intermediates at the molecular level; however, numerous challenges must be overcome before it can become a widely used tool. The biosynthesis of acetyl-coenzyme A (acetyl-CoA) by acetyl-CoA synthetase is used here as a case study for the development of an analytical NMR-based time-course assay platform. We describe an algorithm to deconvolve superimposed spectra into spectra for individual molecules, and further develop a model to simulate the acetyl-CoA synthetase enzyme reaction network using the data derived from time-course NMR. Simulation shows indirectly that synthesis of acetyl-CoA is mediated via an enzyme-bound intermediate (possibly acetyl-AMP) and is accompanied by a nonproductive loss from an intermediate. The ability to predict enzyme function based on partial knowledge of the enzymatic pathway topology is also discussed.

Plant nucleotide sugar formation, interconversion, and salvage by sugar recycling

Nucleotide sugars are the universal sugar donors for the formation of polysaccharides, glycoproteins, proteoglycans, glycolipids, and glycosylated secondary metabolites. At least 100 genes encode proteins involved in the formation of nucleotide sugars. These nucleotide sugars are formed using the carbohydrate derived from photosynthesis, the sugar generated by hydrolyzing translocated sucrose, the sugars released from storage carbohydrates, the salvage of sugars from glycoproteins and glycolipids, the recycling of sugars released during primary and secondary cell wall restructuring, and the sugar generated during plant-microbe interactions. Here we emphasize the importance of the salvage of sugars released from glycans for the formation of nucleotide sugars. We also outline how recent studies combining biochemical, genetic, molecular and cellular approaches have led to an increased appreciation of the role nucleotide sugars in all aspects of plant growth and development. Nevertheless, our understanding of these pathways at the single cell level is far from complete.

Biosynthesis of a new UDP-sugar, UDP-2-acetamido-2-deoxyxylose, in the human pathogen Bacillus cereus subspecies cytotoxis NVH 391-98

We have identified an operon and characterized the functions of two genes from the severe food-poisoning bacterium, Bacillus cereus subsp. cytotoxis NVH 391-98, that are involved in the synthesis of a unique UDP-sugar, UDP-2-acetamido-2-deoxyxylose (UDP-N-acetyl-xylosamine, UDP-XylNAc). UGlcNAcDH encodes a UDP-N-acetyl-glucosamine 6-dehydrogenase, converting UDP-N-acetylglucosamine (UDP-GlcNAc) to UDP-N-acetyl-glucosaminuronic acid (UDP-GlcNAcA). The second gene in the operon, UXNAcS, encodes a distinct decarboxylase not previously described in the literature, which catalyzes the formation of UDP-XylNAc from UDP-GlcNAcA in the presence of exogenous NAD(+). UXNAcS is specific and cannot utilize UDP-glucuronic acid and UDP-galacturonic acid as substrates. UXNAcS is active as a dimer with catalytic efficiency of 7 mM(-1) s(-1). The activity of UXNAcS is completely abolished by NADH but unaffected by UDP-xylose. A real-time NMR-based assay showed unambiguously the dual enzymatic conversions of UDP-GlcNAc to UDP-GlcNAcA and subsequently to UDP-XylNAc. From the analyses of all publicly available sequenced genomes, it appears that UXNAcS is restricted to pathogenic Bacillus species, including Bacillus anthracis and Bacillus thuringiensis. The identification of UXNAcS provides insight into the formation of UDP-XylNAc. Understanding the metabolic pathways involved in the utilization of this amino-sugar may allow the development of drugs to combat and eradicate the disease.

Identification of a bifunctional UDP-4-keto-pentose/UDP-xylose synthase in the plant pathogenic bacterium Ralstonia solanacearum strain GMI1000, a distinct member of the 4,6-dehydratase and decarboxylase family

The UDP-sugar interconverting enzymes involved in UDP-GlcA metabolism are well described in eukaryotes but less is known in prokaryotes. Here we identify and characterize a gene (RsU4kpxs) from Ralstonia solanacearum str. GMI1000, which encodes a dual function enzyme not previously described. One activity is to decarboxylate UDP-glucuronic acid to UDP-beta-l-threo-pentopyranosyl-4''-ulose in the presence of NAD(+). The second activity converts UDP-beta-l-threo-pentopyranosyl-4''-ulose and NADH to UDP-xylose and NAD(+), albeit at a lower rate. Our data also suggest that following decarboxylation, there is stereospecific protonation at the C5 pro-R position. The identification of the R. solanacearum enzyme enables us to propose that the ancestral enzyme of UDP-xylose synthase and UDP-apiose/UDP-xylose synthase was diverged to two distinct enzymatic activities in early bacteria. This separation gave rise to the current UDP-xylose synthase in animal, fungus, and plant as well as to the plant Uaxs and bacterial ArnA and U4kpxs homologs.

Identification of galacturonic acid-1-phosphate kinase, a new member of the GHMP kinase superfamily in plants, and comparison with galactose-1-phosphate kinase

The process of salvaging sugars released from extracellular matrix, during plant cell growth and development, is not well understood, and many molecular components remain to be identified. Here we identify and functionally characterize a unique Arabidopsis gene encoding an alpha-d-galacturonic acid-1-phosphate kinase (GalAK) and compare it with galactokinase. The GalAK gene appeared to be expressed in all tissues implicating that glycose salvage is a common catabolic pathway. GalAK catalyzes the ATP-dependent conversion of alpha-d-galacturonic acid (d-GalA) to alpha-d-galacturonic acid-1-phosphate (GalA-1-P). This sugar phosphate is then converted to UDP-GalA by a UDP-sugar pyrophosphorylase as determined by a real-time (1)H NMR-based assay. GalAK is a distinct member of the GHMP kinase family that includes galactokinase (G), homoserine kinase (H), mevalonate kinase (M), and phosphomevalonate kinase (P). Although these kinases have conserved motifs for sugar binding, nucleotide binding, and catalysis, they do have subtle difference. For example, GalAK has an additional domain near the sugar-binding motif. Using site-directed mutagenesis we established that mutation in A368S reduces phosphorylation activity by 40%; A41E mutation completely abolishes GalAK activity; Y250F alters sugar specificity and allows phosphorylation of d-glucuronic acid, the 4-epimer of GalA. Unlike many plant genes that undergo duplication, GalAK occurs as a single copy gene in vascular plants. We suggest that GalAK generates GalA-1-P from the salvaged GalA that is released during growth-dependent cell wall restructuring, or from storage tissue. The GalA-1-P itself is then available for use in the formation of UDP-GalA required for glycan synthesis.