Glycosidases and glycosyl transferases in glycoside and oligosaccharide synthesis

Assembly of a three-dimensional array of glycoconjugates by combinatorial biocatalysis in nonaqueous media

Glycoconjugates can be artificially synthesized by combinatorial biocatalysis. An example is given in this paper describing the construction of glycoconjugates array by using glycosidase and lipase in nonaqueous media. This array was started from glucose, with three aryl alcohols as the aglycone moiety of glycosides and five acids or esters as acyl donors for combinatorial acylation of glycosides, affording a three-dimensional array containing about 30 members with diverse structures. The array would be more abundant if more aglycones and acyl donors with other structures were filled in. Indeed, diverse classes of carbohydrates besides glucose can also be employed for generating diverse glycoconjugates due to their different roles in numerous physiological responses. The composition and distribution of the demonstration glycoconjugates array was detected and evaluated by HPLC-MS with electrospray ionization. And also, the distribution of the artificial array can be adjusted by changing the molar ratio of the auxiliary materials.

Glycosynthase activity of hybrid aspen xyloglucan endo-transglycosylase PttXET16-34 nucleophile mutants

Glycosynthases are active-site mutants of glycoside hydrolases that catalyse glycosyl transfer using suitable activated donor substrates without competing product hydrolysis (S. M. Hancock, M. D. Vaughan and S. G. Withers, Curr. Opin. Chem. Biol., 2006, 10, 509-519). Site-directed mutagenesis of the catalytic nucleophile, Glu-85, of a Populus tremula x tremuloides xyloglucan endo-transglycosylase (PttXET16-34, EC 2.4.1.207) into alanine, glycine, and serine yielded enzymes with glycosynthase activity. Product analysis indicated that PttXET16-34 E85A in particular was able to catalyse regio- and stereospecific homo- and hetero-condensations of alpha-xylogluco-oligosaccharyl fluoride donors XXXGalphaF and XLLGalphaF to produce xyloglucans with regular sidechain substitution patterns. This substrate promiscuity contrasts that of the Humicola insolens Cel7B E197A glycosynthase, which was not able to polymerise the di-galactosylated substrate XLLGalphaF. The production of the PttXET16-34 E85A xyloglucosynthase thus expands the repertoire of glycosynthases to include those capable of synthesising structurally homogenenous xyloglucans for applications.

Glycopeptide synthesis through endo-glycosidase-catalyzed oligosaccharide transfer of sugar oxazolines: probing substrate structural requirement

An array of sugar oxazolines was synthesized and tested as donor substrates for the Arthrobacter endo-beta-N-acetylglucosaminidase (Endo-A)-catalyzed glycopeptide synthesis. The experiments revealed that the minimum structure of the donor substrate required for Endo-A catalyzed transglycosylation is a Man beta1-->4-GlcNAc oxazoline moiety. Replacement of the beta-D-Man moiety with beta-D-Glc, beta-D-Gal, and beta-D-GlcNAc monosaccharides resulted in the loss of substrate activity for the disaccharide oxazoline. Despite this, the enzyme could tolerate modifications such as attachment of additional sugar residues or a functional group at the 3- and/or 6-positions of the beta-D-Man moiety, thus allowing a successful transfer of selectively modified oligosaccharides to the peptide acceptor. On the other hand, the enzyme has a great flexibility for the acceptor portion and could take both small and large GlcNAc-peptides as the acceptor. The studies implicate a great potential of the endoglycosidase-catalyzed transglycosylation for constructing both natural and selectively modified glycopeptides.

Synthesis of p-nitrophenyl sulfated disaccharides with β-d-(6-sulfo)-GlcNAc units using β-N-acetylhexosaminidase from Aspergillus oryzae in a transglycosylation reaction

When alpha-D-GlcNAc-OC(6)H(4)NO(2) -p and beta-D-(6-sulfo)-GlcNAc-OC(6)H(4)NO(2)-p (2) were used as substrates, beta-N-acetylhexosaminidase from Aspergillus oryzae transferred the beta-D-(6-sulfo)-GlcNAc(unit from 2 to alpha-D-GlcNAc-OC(6)H(4)NO(2) -p to afford beta-D-(6-sulfo)-GlcNAc-(1-->4)-alpha-D-GlcNAc-OC(6)H(4)NO(2)-p (3) in a yield of 94% based on the amount of donor, 2, added. beta-D-(6-sulfo)-GlcNAc-(1-->4)-alpha-D-Glc-OC(6)H(4)NO(2)-p (4) was obtained with alpha-D-Glc-OC(6)H(4)NO(2) -p as acceptor in a similar manner. With a reaction mixture of 2 and beta-D-GlcNAc-OC(6)H(4)NO(2)-p (1) in a molar ratio of 6:1, the enzyme mediated the transfer of beta-D-GlcNAc from 1 to 2, affording disaccharide beta-D-GlcNAc-(1-->4)-beta-(6-sulfo)-D-GlcNAc-OC(6)H(4)NO(2)-p (5) in a yield of 13% based on the amount of 1 added.

Marine glycosyl hydrolases in the hydrolysis and synthesis of oligosaccharides

The marine ecosystem can be considered a rather unexplored source of biological material (e.g. natural substances with therapeutic activity) and can also be a surprising source of enzymes carrying new and interesting catalytic activities to be applied in biocatalysis. The use of glycosyl hydrolases from marine environments dates back to the end of the 1960s and was mainly focused on the development of sensitive and reliable hydrolytic methods for the analysis of sugar chains. As a result not all the benefits of a particular enzymatic activity have been investigated, especially regarding the transglycosylation potential of these enzymes for the synthesis of glycosidic bonds. In this review, the potential of marine sources will be demonstrated reporting on the few examples found in literature for the synthesis and hydrolysis of biologically relevant oligosaccharides catalyzed by glycosyl hydrolases of marine origin. Particular emphasis is given to the synthesis of glycosidic bonds, which is easy by the use of glycosyl hydrolases. Further aspects considered in this review are applications of these biocatalysts for vegetal waste treatment in recovering useful materials, for structural identification and for preparation of target materials from new purified polysaccharides, for the synthesis or modification of food-related compounds and for glycobiology related studies.

Metabolic engineering of microbes for oligosaccharide and polysaccharide synthesis

Metabolic engineering has recently been embraced as an effective tool for developing whole-cell biocatalysts for oligosaccharide and polysaccharide synthesis. Microbial catalysts now provide a practical means to derive many valuable oligosaccharides, previously inaccessible through other methods, in sufficient quantities to support research and clinical applications. The synthesis process based upon these microbes is scalable as it avoids expensive starting materials. Most impressive is the high product concentrations (up to 188 g/L) achieved through microbe-catalyzed synthesis. The overall cost for selected molecules has been brought to a reasonable range (estimated $ 30–50/g).

Microbial synthesis of oligosaccharides and polysaccharides is a carbon-intensive and energy-intensive process, presenting some unique challenges in metabolic engineering. Unlike nicotinamide cofactors, the required sugar nucleotides are products of multiple interacting pathways, adding significant complexity to the metabolic engineering effort. Besides the challenge of providing the necessary mammalian-originated glycosyltransferases in active form, an adequate uptake of sugar acceptors can be an issue when another sugar is necessary as a carbon and energy source. These challenges are analyzed, and various strategies used to overcome these difficulties are reviewed in this article. Despite the impressive success of the microbial coupling strategy, there is a need to develop a single strain that can achieve at least the same efficiency. Host selection and the manner with which the synthesis interacts with the central metabolism are two important factors in the design of microbial catalysts. Additionally, unlike in vitro enzymatic synthesis, product degradation and byproduct formation are challenges of whole-cell systems that require additional engineering. A systematic approach that accounts for various and often conflicting requirements of the synthesis holds the key to deriving an efficient catalyst.

Metabolic engineering strategies applied to selected polysaccharides (hyaluronan, alginate, and exopolysaccharides for food use) are reviewed in this article to highlight the recent progress in this area and similarity to challenges in oligosaccharide synthesis. Many naturally occurring microbes possess highly efficient mechanisms for polysaccharide synthesis. These mechanisms could potentially be engineered into a microbe for oligosaccharide and polysaccharide synthesis with enhanced efficiency.

Biosynthesis of radiolabeled cellodextrins by the Clostridium thermocellum cellobiose and cellodextrin phosphorylases for measurement of intracellular sugars

The Clostridium thermocellum cellobiose and cellodextrin phosphorylases (glucosyl transferases) in the cell extract were used to synthesize radiolabeled cellodextrins with a degree of polymerization (DP=2-6) from nonradioactive glucose-1-phosphate and radioactive glucose. Chain lengths of synthesized cellodextrin were controlled by the absence or presence of dithiothreitol and by reaction conditions. All cellodextrins have the sole radioactive glucose unit located at the reducing ends. Mixed cellodextrins (G2-G6) were separated efficiently by size-exclusion chromatography or less efficiently by thin-layer chromatography. A new rapid sampling device was developed using disposable syringes containing an ultracold methanol-quenching buffer. It was simple, less costly, and especially convenient for anaerobic fermentation. After an impulse feed of radiolabeled cellobiose, the intracellular sugar levels were measured after a series of operations-sampling, extracting, concentrating, separating, and reading. Results showed that the largest amount of radioactivity was cellobiose with lesser amounts of glucose, cellotriose, and cellotetraose, and an average DP of intracellular cellodextrins was ca. 2.

Substrate flexibility of a 2,6-dideoxyglycosyltransferase

We report the first 2,6-dideoxysugar-O-glycosyltransferase with substrate flexibility at the 2 position, confirm the function of a putative NDP-hexose 2,3-dehydratase in the jadomycin B biosynthetic gene cluster and deduce the substrate flexibility of downstream enzymes in l-digitoxose assembly, enabling reprogramming of biosynthetic gene clusters to modify sugar substituents.

Enzymatic transglycosylation of xylose using a glycosynthase

The application of the hyperactive glycosynthase derived from Agrobacterium sp. beta-glucosidase (AbgE358G-2F6) to the synthesis of xylo-oligosaccharides by using alpha-D-xylopyranosyl fluoride as donor represents the first successful use of glycosynthase technology for xylosyl transfer. Transfer to p-nitrophenyl beta-D-glucopyranoside yields di- and trisaccharide products with beta-(1-->4) linkages in 63% and 35% yields, respectively. By contrast, transfer to p-nitrophenyl beta-D-xylopyranoside yielded the beta-(1-->3) linked disaccharide and beta-D-Xyl-(1-->4)-beta-D-Xyl-(1-->3)-beta-D-Xyl-pNP as major products in 42% and 30% yields, respectively. Transfer of xylose to beta-D-Xyl-(1-->4)-beta-D-Xyl-pNP yielded the beta-(1-->4) linked trisaccharide in 98% yield, thereby indicating that transfers to xylo-disaccharides occur with formation of beta-(1-->4) bonds. Xylosylation of carbamate-protected deoxyxylonojirimycin produced a mixture of di- and tri-'saccharide' products in modest yields.

Purification and characterization of a β-d-mannosidase from the marine anaspidean Aplysia fasciata

A beta-D-mannosidase was purified to homogeneity from visceral mass extract of Aplysia fasciata a mollusc belonging to the order Anaspidea. The purified enzyme is a homodimer with a subunit mass of 130 kDa. Temperature and pH optima of this enzyme were 45 degrees C and 4.5, respectively. Substrate specificity tests revealed that the enzyme exerts only beta-D-mannosidase activity. The K(M) and V(max) values for p-nitrophenyl beta-D-mannopyranoside were determined to be 2.4 mM and 50.3 micromol min(-1)mg(-1), respectively. The catalytic efficiency of this beta-mannosidase (11,519 min(-1)) was significantly higher than those reported for beta-mannosidases from other sources. It was verified that this is an exo-acting glycosyl hydrolase with transglycosidase activity. When the enzyme was incubated in the presence of p-nitrophenyl beta-D-mannopyranoside, self-transfer of the mannosyl group was observed, and a 10-15% yield of a beta-1-4 disaccharide was obtained. When the reaction was performed in the presence of o-nitrophenyl alpha-D-2-deoxy-N-acetyl glucopyranoside in 3:1 molar ratio with respect to the p-nitrophenyl beta-D-mannopyranoside, two regioisomers (85:15, 12% yield) due to the beta-mannosylation of the heteroacceptor in 4 and in 6 positions were formed.

Production of a new sucrose derivative by transglycosylation of recombinant Sulfolobus shibatae β-glycosidase

The gene encoding beta-glycosidase of the hyperthermophilic archaea Sulfolobus shibatae (SSG) was expressed in Escherichia coli. Recombinant SSG (referred to as rSSG hereafter) was efficiently purified, and its transglycosylation activity was tested with lactose as a donor and various sugars as acceptors. When sucrose was used as an acceptor, we found a distinct intermolecular transglycosylation product and confirmed its presence by TLC and high performance anion exchange chromatography (HPAEC). The sucrose transglycosylation product was isolated by paper chromatography, and its chemical structure was determined by 1H and 13C NMR. The sucrose transfer product was determined to be beta-D-galactopyranosyl-(1-->6)-alpha-D-glucopyranosyl-beta-d-fructofuranoside with a galactose molecule linked to sucrose via a beta-(1-->6)-glycosidic bond.

Developing Promiscuous Glycosidases for Glycoside Synthesis: Residues W433 and E432 in Sulfolobus solfataricus β-Glycosidase are Important Glucoside- and Galactoside-Specificity Determinants

Two residues that have been implicated in determining the substrate specificity of the thermophilic beta-glycosidase from the archaeon Sulfolobus solfataricus (SsbetaG), a member of the glycosyl hydrolase family 1, have been mutated by site-directed mutagenesis so as to create more versatile catalysts for carbohydrate chemistry. The wild-type and mutated sequences were expressed in E. coli with a His(7)-tag to allow one-step chromatographic purification. The E432C and W433C mutations removed key interactions with the OH-4 and OH-3 of the sugar substrates, thus reducing the discrimination of glucose, galactose and fucose with respect to other glycosides. This resulted in two glycosidases with greatly broadened substrate specificities. Observed changes include a 24-fold increase in Man:Gal activity and an 18-fold increase in GalA:Gal activity. This promiscuous substrate tolerance was further illustrated by the parallel synthesis of a beta-glycoside library of glucose, galactose, xylose and mannose in one pot at 50 degrees C, in organic solvent. The synthetic potential of the catalysts was further evaluated through alkyl glycoside transglycosylation yields, including the first examples of synthesis of beta-mannosides and beta-xylosides with SsbetaG.

Enzymatic glycosidation of sugar oxazolines having a carboxylate group catalyzed by chitinase

Enzymatic glycosidation using sugar oxazolines 1-3 having a carboxylate group as glycosyl donors and compounds 4-6 as glycosyl acceptors was performed by employing a chitinase from Bacillus sp. as catalyst. All the glycosidations proceeded with full control in stereochemistry at the anomeric carbon of the donor and regio-selectivity of the acceptor. The N,N'-diacetyl-6'-O-carboxymethylchitobiose oxazoline derivative 1 was effectively glycosidated, under catalysis by the enzyme, with methyl N,N'-diacetyl-beta-chitobioside (4), pent-4-enyl N-acetyl-beta-D-glucosaminide (5), and methyl N-acetyl-beta-D-glucosaminide (6), affording in good yields the corresponding oligosaccharide derivatives having 6-O-carboxymethyl group at the nonreducing GlcNAc residue. The N,N'-diacetyl-6-O-carboxymethylchitobiose oxazoline derivative 2 was subjected to catalysis by the enzyme catalysis; however, no glycosidated products were produced through the reactions with 4, 5, and 6. Glycosidation reactions of the beta-d-glucosyluronic-(1-->4)-N-acetyl-D-glucosamine oxazoline derivative 3 proceeded with each of the glycosyl acceptors, giving rise to the corresponding oligosaccharide derivative having a GlcA residue at their nonreducing termini in good yields.

An original chemoenzymatic route for the synthesis of β-d-galactofuranosides using an α-l-arabinofuranosidase

DGalactofuranose is a widespread component of cell wall polysaccharides in bacteria, protozoa and fungi, but is totally absent in mammals. Importantly, galactofuranose is a key constituent of major cell envelope polysaccharides in pathogenic mycobacteria. In this respect, galactofuranose-based glycoconjugates are interesting target molecules for drug design. O-Glycosidases and notably beta-D-galactofuranosidases could be useful tools for the chemoenzymatic synthesis of galactofuranosides, but to date no studies of this type have been reported. Here we report the use of a GH 51 alpha-l-arabinofuranosidase for the synthesis of beta-D-galactofuranosides. We have demonstrated that this enzyme can catalyse both the autocondensation of p-nitrophenyl-beta-D-galactofuranoside and the transgalactofuranosylation of benzyl alpha-D-xylopyranoside, forming p-nitrophenyl beta-D-galactofuranosyl-(1-->2)-beta-D-galactofuranoside and benzyl beta-D-galactofuranosyl-(1-->2)-alpha-D-xylopyranoside, respectively. Both reactions were very regiospecific and the reaction involving benzyl alpha-D-xylopyranoside afforded very high yields (74.8%) of the major product. To our knowledge, this demonstration of chemoenzymatic synthesis of galactofuranosides constitutes the very first use of an O-glycosidase for the synthesis of galactofuranosides.

Enzymatic syntheses and selective hydrolysis of O-β-d-galactopyranosides using a marine mollusc β-galactosidase

The use of crude extract of the hepatopancreas of Aplysia fasciata, a large mollusc belonging to the order Anaspidea containing a beta-galactosidase activity, was reported for the synthesis of different galactosides. Good yields with polar acceptors and the uncommon beta-1-3 selectivity in the transgalactosylation reactions with most of the acceptors were observed. A beta-1-2 selectivity in the hydrolytic conditions was also observed and discussed.

Enzymes in the synthesis of bioactive compounds: the prodigious decades

The growing demand for enantiomerically pure pharmaceuticals has impelled research on enzymes as catalysts for asymmetric synthetic transformations. However, the use of enzymes for this purpose was rather limited until the discovery that enzymes can work in organic solvents. Since the advent of the PCR the number of available enzymes has been growing rapidly and the tailor-made biocatalysts are becoming a reality. Thus, it has been possible the use of enzymes for the synthesis of new innovative medicines such as carbohydrates and their incorporation to modern methods for drug development, such as combinatorial chemistry. Finally, the genomic research is allowing the manipulation of whole genomes opening the door to the combinatorial biosynthesis of compounds. In this review, our intention is to highlight the main landmarks that have led to transfer the chemical efficiency shown by the enzymes in the cell to the synthesis of bioactive molecules in the lab during the last 20 years.

Directed Evolution of a Glycosynthase from Agrobacterium sp. Increases Its Catalytic Activity Dramatically and Expands Its Substrate Repertoire

The Agrobacterium sp. beta-glucosidase (Abg) is a retaining beta-glycosidase and its nucleophile mutants, termed Abg glycosynthases, catalyze the formation of glycosidic bonds using alpha-glycosyl fluorides as donor sugars and various aryl glycosides as acceptor sugars. Two rounds of random mutagenesis were performed on the best glycosynthase to date (AbgE358G), and transformants were screened using an on-plate endocellulase coupled assay. Two highly active mutants were obtained, 1D12 (A19T, E358G) and 2F6 (A19T, E358G, Q248R, M407V) in the first and second rounds, respectively. Relative catalytic efficiencies (kcat/Km) of 1:7:27 were determined for AbgE358G, 1D12, and 2F6, respectively, using alpha-D-galactopyranosyl fluoride and 4-nitrophenyl beta-D-glucopyranoside as substrates. The 2F6 mutant is not only more efficient but also has an expanded repertoire of acceptable substrates. Analysis of a homology model structure of 2F6 indicated that the A19T and M407V mutations do not interact directly with substrates but exert their effects by changing the conformation of the active site. Much of the improvement associated with the A19T mutation seems to be caused by favorable interactions with the equatorial C2-hydroxyl group of the substrate. The alteration of torsional angles of Glu-411, Trp-412, and Trp-404, which are components of the aglycone (+1) subsite, is an expected consequence of the A19T and M407V mutations based on the homology model structure of 2F6.

Anatomy of glycosynthesis: structure and kinetics of the Humicola insolens Cel7B E197A and E197S glycosynthase mutants

The formation of glycoconjugates and oligosaccharides remains one of the most challenging chemical syntheses. Chemo-enzymatic routes using retaining glycosidases have been successfully harnessed but require tight kinetic or thermodynamic control. "Glycosynthases," specifically engineered glycosidases that catalyze the formation of glycosidic bonds from glycosyl donor and acceptor alcohol, are an emerging range of synthetic tools in which catalytic nucleophile mutants are harnessed together with glycosyl fluoride donors to generate powerful and versatile catalysts. Here we present the structural and kinetic dissection of the Humicola insolens Cel7B glycosynthases in which the nucleophile of the wild-type enzyme is mutated to alanine and serine (E197A and E197S). 3-D structures reveal the acceptor and donor subsites and the basis for substrate inhibition. Kinetic analysis shows that the E197S mutant is considerably more active than the corresponding alanine mutant due to a 40-fold increase in k(cat).

P1 Trisaccharide (Galalpha1,4Galbeta1,4GlcNAc) synthesis by enzyme glycosylation reactions using recombinant Escherichia coli

The frequency of Escherichia coli infection has lead to concerns over pathogenic bacteria in our food supply and a demand for therapeutics. Glycolipids on gut cells serve as receptors for the Shiga-like toxin produced by E. coli. Oligosaccharide moiety analogues of these glycolipids can compete with receptors for the toxin, thus acting as antibacterials. An enzymatic synthesis of the P1 trisaccharide (Galalpha1,4Galbeta1,4GlcNAc), one of the oligosaccharide analogues, was assessed in this study. In the proposed synthetic pathway, UDP-glucose was generated from sucrose with an Anabaena sp. sucrose synthase and then converted with an E. coli UDP-glucose 4-epimerase to UDP-galactose. Two molecules of galactose were linked to N-acetylglucosamine subsequently with a Helicobacter pylori beta-l,4-galactosyltransferase and a Neisseria meningitidis alpha-1,4-galactosyltransferase to produce one molecule of P1 trisaccharide. The four enzymes were coexpressed in a single genetically engineered E. coli strain that was then permeabilized and used to catalyze the enzymatic reaction. P1 trisaccharide was accumulated up to 50 mM (5.4 g in a 200-ml reaction volume), with a 67% yield based on the consumption of N-acetylglucosamine. This study provides an efficient approach for the preparative-scale synthesis of P1 trisaccharide with recombinant bacteria.

Prebiotics and synbiotics: towards the next generation

Recent research in the area of prebiotic oligosaccharides and synbiotic combinations with probiotics is leading towards a more targeted development of functional food ingredients. Improved molecular techniques for analysis of the gut microflora, new manufacturing biotechnologies, and increased understanding of the metabolism of oligosaccharides by probiotics are facilitating development. Such developments are leading us to the time when we will be able to rationally develop prebiotics and synbiotics for specific functional properties and health outcomes.

Enzymatic synthesis and characterization of oligosaccharides structurally related to the repeating unit of Pullulan

A trisaccharide (Glcalpha1-4Glcalpha1-6Glc) and a tetrasaccharide (Glcalpha1-4Glcalpha1-4Glcalpha1-6Glc) the structures of which are related to that of repeating unit of pullulan have been obtained, exploiting the transglycolytic activity of Aspergillus niger cyclodextrin glucanotransferase. Both products were obtained in one-pot reaction using as a donor the alpha-cyclodextrin and as an acceptor the disaccharide isomaltose. The regioselectivity of the reaction was 85% for the tetrasaccharide and 80% for the trisaccharide. The yield of reaction resulted to be 42% for the synthesis of trisaccharide and 25% for that of tetrasaccharide. Purification of products was performed by size exclusion chromatography and by semipreparative reverse phase HPLC after reversible derivatization with 2-aminopyridine. Structural characterization was performed by capillary electrophoresis, ion-spray mass spectrometry, and by 13C-NMR spectroscopy. A comparison of these results with those obtained by using alpha-D-glucosidase, which had been effective for the synthesis of the disaccharide isomaltose, is reported.

Chemo-enzymatic synthesis of trimeric sialyl Lewisxpentadecasaccharide

The enzymatic synthesis of trimeric sialyl Lewisxpentadecasaccharide (6), a 15-mer, from a trimannoside precursor required six different glycosyltransferase enzymes and four nucleotide donor sugars. Three N-acetylglucosaminyl residues were transferred from UDP-N-acetylglucosamine to a trimannoside by N-acetylglucosaminyltransferases I, II, and V, respectively. Galactosylation using β(1[Formula: see text]4) galactosyltransferase and UDP-galactose gave three N-acetyl lactosamine units in nonasaccharide 4. Sialylation of 4 with α(2[Formula: see text]3) sialyltransferase and CMP-N-acetylneuraminic acid was followed by fucosylation with α(1[Formula: see text]3) fucosyltransferase and GDP-fucose giving the 15-mer 6 in mg quantities. Compound 4 was also converted to a trimeric Lewisxdodecasaccharide 12-mer with α(1[Formula: see text]3) fucosyltransferase and GDP-fucose and to a trimeric α-2,6-sialyl N-acetyllactosamine dodecasaccharide 12-mer with α(2[Formula: see text]6) sialyltransferase and CMP-N-acetylneuraminic acid. Key words: glycosyltransferases, pentadecasaccharide, sialyl Lewisx.

Combined biosynthetic pathway for de novo production of UDP-galactose: catalysis with multiple enzymes immobilized on agarose beads

Regeneration of sugar nucleotides is a critical step in the biosynthetic pathway for the formation of oligosaccharides. To alleviate the difficulties in the production of sugar nucleotides, we have developed a method to produce uridine diphosphate galactose (UDP-galactose). The combined biosynthetic pathway, which involves seven enzymes, is composed of three parts: i) the main pathway to form UDP-galactose from galactose, with the enzymes galactokinase, galactose-1-phosphate uridyltransferase, UDP-glucose pyrophosphorylase, and inorganic pyrophosphatase, ii) the uridine triphosphate supply pathway catalyzed by uridine monophosphate (UMP) kinase and nucleotide diphosphate kinase, and iii) the adenosine triphosphate (ATP) regeneration pathway catalyzed by polyphosphate kinase with polyphosphate added as an energy resource. All of the enzymes were expressed individually and immobilized through their hexahistidine tags onto nickel agarose beads ("super beads"). The reaction requires a stoichiometric amount of UMP and galactose, and catalytic amounts of ATP and glucose 1-phosphate, all inexpensive starting materials. After continuous circulation of the reaction mixture through the super-bead column for 48 h, 50 % of the UMP was converted into UDP-galactose. The results show that de novo production of UDP-galactose on the super-bead column is more efficient than in solution because of the stability of the immobilized enzymes.

Tailoring the substrate specificity of the beta-glycosidase from the thermophilic archaeon Sulfolobus solfataricus

The substrate specificity of the thermophilic beta-glycosidase (lacS) from the archaeon Sulfolobus solfataricus (SSbetaG), a member of the glycohydrolase family 1, has been analysed at a molecular level using predictions from known protein sequences and structures and through site-directed mutagenesis. Three critical residues were identified and mutated to create catalysts with altered and broadened specificities for use in glycoside synthesis. The wild-type (WT) and mutated sequences were expressed as recombinant fusion proteins in Escherichia coli, with an added His(6)-tag to allow one-step chromatographic purification. Consistent with side-chain orientation towards OH-6, the single Met439-->Cys mutation enhances D-xylosidase specificity 4.7-fold and decreases D-fucosidase activity 2-fold without greatly altering its activity towards other D-glycoside substrates. Glu432-->Cys and Trp433-->Cys mutations directed towards OH-4 and -3, respectively, more dramatically impair glucose (Glc), galactose (Gal), fucose specificity than for other glycosides, resulting in two glycosidases with greatly broadened substrate specificities. These include the first examples of stereospecificity tailoring in glycosidases (e.g. WT-->W433C, k(cat)/K(M) (Gal):k(cat)/K(M) (mannose (Man))=29.4:1-->1.2:1). The robustness and high utility of these broad specificity SSbetaG mutants in parallel synthesis were demonstrated by the formation of libraries of beta-glycosides of Glc, Gal, xylose, Man in one-pot preparations at 50 degrees C in the presence of organic solvents, that could not be performed by SSbetaG-WT.

Rapid screening of the aglycone specificity of glycosidases: applications to enzymatic synthesis of oligosaccharides

BACKGROUND

Retaining glycosidases can catalyse glycosidic bond formation through transglycosylation from a donor sugar to an acceptor bound in the aglycone site. The aglycone specificity of a glycosidase is not easily determined, thereby complicating the choice of the most appropriate glycosidase for use as a catalyst for transglycosylation. We have developed a strategy to rapidly screen the aglycone specificity of a glycosidase and thereby determine which enzymes are best suited to catalyse specific transglycosylation reactions.

RESULTS

The reactivation, or turnover, of a glycosidase trapped as a fluoroglycosyl-enzyme species is accelerated in the presence of a compound that productively binds to the aglycone site. This methodology was used to rapidly screen six glycosidases with 44 potential acceptor sugars. Validation of the screening strategy was demonstrated by the identification of products formed from a transglycosylation reaction with positively screened acceptors for four of the enzymes studied.

CONCLUSIONS

The aglycone specificity of a glycosidase can be rapidly evaluated and requires only an appropriate fluorosugar inactivator, a substrate for assay of activity and a library of compounds for screening.

Heterologous over-expression of alpha-1,6-fucosyltransferase from Rhizobium sp.: application to the synthesis of the trisaccharide beta-D-GlcNAc(1-->4)-[alpha-L-Fuc-(1-->6)]-D-GLcNAc, study of the acceptor specificity and evaluation of polyhydroxylated indolizidines as inhibitors

An efficient heterologous expression system for overproduction of the enzyme alpha-1,6-Fucosyltransferase (alpha-1,6-FucT) from Rhizobium sp. has been developed. The gene codifying for the alpha-1,6-FucT was amplified by PCR using specific primers. After purification, the gene was cloned in the plasmid pKK223-3. The resulting plasmid, pKK1,6FucT, was transformed into the E. coli strain XL1-Blue MRF'. The protein was expressed both as inclusion bodies and in soluble form. Changing the induction time a five-fold increase of enzyme expressed in soluble form was obtained. In this way five units of enzyme alpha-1,6-FucT can be obtained per liter of culture. A crude preparation of the recombinant enzyme was used for the synthesis of the branched trisaccharide alpha-D-GlcNAc-(1-->4)-[alpha-L-Fuc-(1-->6)]-D-GlcNAc (3), from chitobiose (2) and GDP-Fucose (1). After purification, the trisaccharide 3 was obtained in a 84% overall yield. In order to elucidate the structural requirements for the acceptors, the specificity of the enzyme was studied towards mono-, di- and trisaccharides, which are structurally related to chitobiose. The enzyme uses, among others, the disaccharide N-acetyl lactosamine as a good substrate; the monosaccharide GlcNAc is a weak acceptor. Finally, several racemic polyhydroxylated indolizidines have been tested as potential inhibitors of the enzyme. Indolizidine 21 was the best inhibitor with an IC50 of 4.5 x 10(-5) M. Interestingly, this compound turned out to be the best mimic for the structural features of the fucose moiety in the presumed transition state.

Improved oligosaccharide synthesis by protein engineering of beta-glucosidase CelB from hyperthermophilic Pyrococcus furiosus

Enzymatic transglycosylation of lactose into oligosaccharides was studied using wild-type beta-glucosidase (CelB) and active site mutants thereof (M424K, F426Y, M424K/F426Y) and wild-type beta-mannosidase (BmnA) of the hyperthermophilic Pyrococcus furiosus. The effects of the mutations on kinetics, enzyme activity, and substrate specificity were determined. The oligosaccharide synthesis was carried out in aqueous solution at 95 degrees C at different lactose concentrations and pH values. The results showed enhanced synthetic properties of the CelB mutant enzymes. An exchange of one phenylalanine to tyrosine (F426Y) increased the oligosaccharide yield (45%) compared with the wild-type CelB (40%). Incorporation of a positively charged group in the active site (M424K) increased the pH optimum of transglycosylation reaction of CelB. The double mutant, M424K/F426Y, showed much better transglycosylation properties at low (10-20%) lactose concentrations compared to the wild-type. At a lactose concentration of 10%, the oligosaccharide yield for the mutant was 40% compared to 18% for the wild-type. At optimal reaction conditions, a higher ratio of tetrasaccharides to trisaccharides was obtained with the double mutant (0.42, 10% lactose) compared to the wild-type (0.19, 70% lactose). At a lactose concentration as low as 10%, only trisaccharides were synthesized by CelB wild-type. The beta-mannosidase BmnA from P. furiosus showed both beta-glucosidase and beta-galactosidase activity and in the transglycosylation of lactose the maximal oligosaccharide yield of BmnA was 44%. The oligosaccharide yields obtained in this study are high compared to those reported with other transglycosylating beta-glycosidases in oligosaccharide synthesis from lactose.