Efficient preparation of natural and synthetic galactosides with a recombinant beta-1,4-galactosyltransferase-/UDP-4'-gal epimerase fusion protein

A promiscuous glycosyltransferase generates poly-β-1,4-glucan derivatives that facilitate mass spectrometry-based detection of cellulolytic enzymes

Promiscuous activity of a glycosyltransferase was exploited to polymerise glucose from UDP-glucose via the generation of β-1,4-glycosidic linkages. The biocatalyst was incorporated into biocatalytic cascades and chemo-enzymatic strategies to synthesise cello-oligosaccharides with tailored functionalities on a scale suitable for employment in mass spectrometry-based assays. The resulting glycan structures enabled reporting of the activity and selectivity of celluloltic enzymes.

A Sulfonamide Sialoside Analogue for Targeting Siglec-8 and -F on Immune Cells

The Siglec family of cell surface receptors have emerged as attractive targets for cell-directed therapies due to their restricted expression on immune cells, endocytic properties, and ability to modulate receptor signaling. Human Siglec-8, for instance, has been identified as a therapeutic target for the treatment of eosinophil and mast cell disorders. A promising strategy to target Siglecs involves the use of liposomal nanoparticles with a multivalent display of Siglec ligands. A key challenge for this approach is the identification of a high affinity ligand for the target Siglec. Here, we report the development of a ligand of Siglec-8 and its closest murine functional orthologue Siglec-F that is capable of targeting liposomes to cells expressing Siglec-8 or -F. A glycan microarray library of synthetic 9-N-sulfonyl sialoside analogues was screened to identify potential lead compounds. The best ligand, 9-N-(2-naphthyl-sulfonyl)-Neu5Acα2-3-[6-O-sulfo]-Galβ1-4GlcNAc (6'-O-sulfo ^NSANeu5Ac) combined the lead 2-naphthyl sulfonyl C-9 substituent with the preferred sulfated scaffold. The ligand 6'-O-sulfo ^NSANeu5Ac was conjugated to lipids for display on liposomes to evaluate targeted delivery to cells. Targeted liposomes showed strong in vitro binding/uptake and selectivity to cells expressing Siglec-8 or -F and, when administered to mice, exhibit in vivo targeting to Siglec-F⁺ eosinophils.

One-Pot Multienzyme Cofactors Recycling (OPME-CR) System for Lactose and Non-natural Saccharide Conjugated Polyphenol Production

A one-pot multienzyme cofactors recycling (OPME-CR) system was designed for the synthesis of UDP-α-d-galactose, which was combined with LgtB, a β-(1,4) galactosyltransferase from Neisseria meningitidis, to modify various polyphenol glycosides. This system recycles one mole of ADP and one mole of UDP to regenerate one mole of UDP-α-d-galactose by consuming two moles of acetylphosphate and one mole of d-galactose in each cycle. The ATP additionally used to generate UDP from UMP was also recycled at the beginning of the reaction. The engineered cofactors recycling system with LgtB efficiently added a d-galactose unit to a variety of sugar units such as d-glucose, rutinose, and 2-deoxy-d-glucose. The temperature, pH, incubation time, and divalent metal ions for the OPME-CR system were optimized. The maximum number of UDP-α-d-galactose regeneration cycles (RC_max) was 18.24 by fed batch reaction. The engineered system generated natural and non-natural polyphenol saccharides efficiently and cost-effectively.

Enzyme and microbial technology for synthesis of bioactive oligosaccharides: an update

Oligosaccharides, in either free or bound forms, play crucial roles in a wide range of biological processes. Increasing appreciation of their roles in cellular communication, interaction, pathogenesis, and prebiotic functions has stimulated tremendous interests in their synthesis. Pure and structurally defined oligosaccharides are essential for fundamental studies. On the other hand, for those with near term medical and nutraceutical applications, their large-scale synthesis is necessary. Unfortunately, oligosaccharides are notoriously difficult in their synthesis, and their enormous diverse structures leave a vast gap between what have been synthesized in laboratory and those present in various biological systems. While enzymes and microbes are nature's catalysts for oligosaccharides, their effective use is not without challenges. Using examples of galactose-containing oligosaccharides, this review analyzes the pros and cons of these two forms of biocatalysts and provides an updated view on the status of biocatalysis in this important field. Over the past few years, a large number of novel galactosidases were discovered and/or engineered for improved synthesis via transglycosylation. The use of salvage pathway for regeneration of uridine diphosphate (UDP)-galactose has made the use of Leloir glycosyltransferases simpler and more efficient. The recent success of large-scale synthesis of 2' fucosyllactose heralded the power of whole-cell biocatalysis as a scalable technology. While it still lags behind enzyme catalysis in terms of the number of oligosaccharides synthesized, an acceleration in the use of this form of biocatalyst is expected as rapid advances in synthetic biology have made the engineering of whole cell biocatalysts less arduous and less time consuming.

Converting Pasteurella multocidaα2-3-sialyltransferase 1 (PmST1) to a regioselective α2-6-sialyltransferase by saturation mutagenesis and regioselective screening

A microtiter plate-based screening assay capable of determining the activity and regioselectivity of sialyltransferases was developed. This assay was used to screen two single-site saturation libraries of Pasteurella multocidaα2-3-sialyltransferase 1 (PmST1) for α2-6-sialyltransferase activity and total sialyltransferase activity. PmST1 double mutant P34H/M144L was found to be the most effective α2-6-sialyltransferase and displayed 50% reduced donor hydrolysis and 50-fold reduced sialidase activity compared to the wild-type PmST1. It retained the donor substrate promiscuity of the wild-type enzyme and was used in an efficient one-pot multienzyme (OPME) system to selectively catalyze the sialylation of the terminal galactose residue in a multigalactose-containing tetrasaccharide lacto-N-neotetraoside.

Immunization with Outer Membrane Vesicles Displaying Designer Glycotopes Yields Class-Switched, Glycan-Specific Antibodies

The development of antibodies against specific glycan epitopes poses a significant challenge due to difficulties obtaining desired glycans at sufficient quantity and purity, and the fact that glycans are usually weakly immunogenic. To address this challenge, we leveraged the potent immunostimulatory activity of bacterial outer membrane vesicles (OMVs) to deliver designer glycan epitopes to the immune system. This approach involved heterologous expression of two clinically important glycans, namely polysialic acid (PSA) and Thomsen-Friedenreich antigen (T antigen) in hypervesiculating strains of non-pathogenic Escherichia coli. The resulting glycOMVs displayed structural mimics of PSA or T antigen on their surfaces, and induced high titers of glycan-specific IgG antibodies following immunization in mice. In the case of PSA glycOMVs, serum antibodies potently killed Neisseria meningitidis serogroup B (MenB), whose outer capsule is PSA, in a serum bactericidal assay. These findings demonstrate the potential of glycOMVs for inducing class-switched, humoral immune responses against glycan antigens.

Donor substrate promiscuity of bacterial β1-3-N-acetylglucosaminyltransferases and acceptor substrate flexibility of β1-4-galactosyltransferases

β1-3-N-Acetylglucosaminyltransferases (β3GlcNAcTs) and β1-4-galactosyltransferases (β4GalTs) have been broadly used in enzymatic synthesis of N-acetyllactosamine (LacNAc)-containing oligosaccharides and glycoconjugates including poly-LacNAc, and lacto-N-neotetraose (LNnT) found in the milk of human and other mammals. In order to explore oligosaccharides and derivatives that can be synthesized by the combination of β3GlcNAcTs and β4GalTs, donor substrate specificity studies of two bacterial β3GlcNAcTs from Helicobacter pylori (Hpβ3GlcNAcT) and Neisseria meningitidis (NmLgtA), respectively, using a library of 39 sugar nucleotides were carried out. The two β3GlcNAcTs have complementary donor substrate promiscuity and 13 different trisaccharides were produced. They were used to investigate the acceptor substrate specificities of three β4GalTs from Neisseria meningitidis (NmLgtB), Helicobacter pylori (Hpβ4GalT), and bovine (Bβ4GalT), respectively. Ten of the 13 trisaccharides were shown to be tolerable acceptors for at least one of these β4GalTs. The application of NmLgtA in one-pot multienzyme (OPME) synthesis of two trisaccharides including GalNAcβ1-3Galβ1-4GlcβProN3 and Galβ1-3Galβ1-4Glc was demonstrated. The study provides important information for using these glycosyltransferases as powerful catalysts in enzymatic and chemoenzymatic syntheses of oligosaccharides and derivatives which can be useful probes and reagents.

Chemoenzymatic synthesis of α-dystroglycan core M1 O-mannose glycans

The diversity-oriented chemoenzymatic synthesis of α-dystroglycan (α-DG) core M1 O-mannose glycans has been achieved via a three-step sequential one-pot multienzyme (OPME) glycosylation of a chemically prepared disaccharyl serine intermediate. The high flexibility and efficiency of this chemoenzymatic strategy was demonstrated for the synthesis of three more complex core M1 O-mannose glycans for the first time along with three previously reported core M1 structures.

Complete switch from α-2,3- to α-2,6-regioselectivity in Pasteurella dagmatis β-d-galactoside sialyltransferase by active-site redesign

Incorporation of Pro7His and Met117Ala substitutions resulted in a completely regioselective and highly efficient α-2,6-sialyltransferase.

Sequential one-pot multienzyme (OPME) synthesis of lacto-N-neotetraose and its sialyl and fucosyl derivatives

A highly efficient sequential one-pot multienzyme (OPME) approach for the synthesis of lacto-N-neotetraose (LNnT) and its derivatives at preparative scale was reported.

Synthesis of biologically active N- and O-linked glycans with multisialylated poly-N-acetyllactosamine extensions using P. damsela α2-6 sialyltransferase

Sialosides on N- and O-linked glycoproteins play a fundamental role in many biological processes, and synthetic glycan probes have proven to be valuable tools for elucidating these functions. Though sialic acids are typically found α2-3- or α2-6-linked to a terminal nonreducing end galactose, poly-LacNAc extended core-3 O-linked glycans isolated from rat salivary glands and human colonic mucins have been reported to contain multiple internal Neu5Acα2-6Gal epitopes. Here, we have developed an efficient approach for the synthesis of a library of N- and O-linked glycans with multisialylated poly-LacNAc extensions, including naturally occurring multisialylated core-3 O-linked glycans. We have found that a recombinant α2-6 sialyltransferase from Photobacterium damsela (Pd2,6ST) exhibits unique regioselectivity and is able to sialylate internal galactose residues in poly-LacNAc extended glycans which was confirmed by MS/MS analysis. Using a glycan microarray displaying this library, we found that Neu5Acα2-6Gal specific influenza virus hemagglutinins, siglecs, and plant lectins are largely unaffected by adjacent internal sialylation, and in several cases the internal sialic acids are recognized as ligands. Polyclonal IgY antibodies specific for internal sialoside epitopes were elicited in inoculated chickens.

Recognition of sialylated poly-N-acetyllactosamine chains on N- and O-linked glycans by human and avian influenza A virus hemagglutinins

Human influenza viruses are proposed to recognize sialic acids (pink diamonds) on glycans extended with poly-LacNAc chains (LacNAc=(yellow circle+blue square)). N- and O-linked glycans were extended with different poly-LacNAc chains with α2-3- and α2-6-linked sialic acids recognized by human and avian influenza viruses, respectively. The specificity of recombinant hemagglutinins (receptors in green) was investigated by using glycan microarray technology.

Structural characterization of the hemagglutinin receptor specificity from the 2009 H1N1 influenza pandemic

Influenza virus hemagglutinin (HA) is the viral envelope protein that mediates viral attachment to host cells and elicits membrane fusion. The HA receptor-binding specificity is a key determinant for the host range and transmissibility of influenza viruses. In human pandemics of the 20th century, the HA normally has acquired specificity for human-like receptors before widespread infection. Crystal structures of the H1 HA from the 2009 human pandemic (A/California/04/2009 [CA04]) in complex with human and avian receptor analogs reveal conserved recognition of the terminal sialic acid of the glycan ligands. However, favorable interactions beyond the sialic acid are found only for α2-6-linked glycans and are mediated by Asp190 and Asp225, which hydrogen bond with Gal-2 and GlcNAc-3. For α2-3-linked glycan receptors, no specific interactions beyond the terminal sialic acid are observed. Our structural and glycan microarray analyses, in the context of other high-resolution HA structures with α2-6- and α2-3-linked glycans, now elucidate the structural basis of receptor-binding specificity for H1 HAs in human and avian viruses and provide a structural explanation for the preference for α2-6 siaylated glycan receptors for the 2009 pandemic swine flu virus.

Preparation of oligosaccharides by homogenous enzymatic synthesis and solid phase extraction

This communication describes a method for enzymatic preparation of bioactive glycans, which integrated the high-efficiency of homogenous phase enzymatic reaction and fast separation of solid phase extraction.

Engineering of glycosylation in yeast and other fungi: current state and perspectives

With the increasing demand for recombinant proteins and glycoproteins, research on hosts for producing these proteins is focusing increasingly on more cost-effective expression systems. Yeasts and other fungi are promising alternatives because they provide easy and cheap systems that can perform eukaryotic post-translational modifications. Unfortunately, yeasts and other fungi modify their glycoproteins with heterogeneous high-mannose glycan structures, which is often detrimental to a therapeutic protein's pharmacokinetic behavior and can reduce the efficiency of downstream processing. This problem can be solved by engineering the glycosylation pathways to produce homogeneous and, if so desired, human-like glycan structures. In this review, we provide an overview of the most significant recently reported approaches for engineering the glycosylation pathways in yeasts and fungi.

Chemo-enzymatic synthesis of poly-N-acetyllactosamine (poly-LacNAc) structures and their characterization for CGL2-galectin-mediated binding of ECM glycoproteins to biomaterial surfaces

Poly-N-acetyllactosamine (poly-LacNAc) structures have been identified as important ligands for galectin-mediated cell adhesion to extra-cellular matrix (ECM) proteins. We here present the biofunctionalization of surfaces with poly-LacNAc structures and subsequent binding of ECM glycoproteins. First, we synthesized beta-GlcNAc glycosides carrying a linker for controlled coupling onto chemically functionalized surfaces. Then we produced poly-LacNAc structures with defined lengths using human beta1,4-galactosyltransferase-1 and beta1,3-N-acetylglucosaminyltransferase from Helicobacter pylori. These compounds were also used for kinetic characterization of glycosyltransferases and lectin binding assays. A mixture of poly-LacNAc-structures covalently coupled to functionalized microtiter plates were identified for best binding to our model galectin His(6)CGL2. We further demonstrate for the first time that these poly-LacNAc surfaces are suitable for further galectin-mediated binding of the ECM glycoproteins laminin and fibronectin. This new technology should facilitate cell adhesion to biofunctionalized surfaces by imitating the natural ECM microenvironment.

Bifunctional CD22 ligands use multimeric immunoglobulins as protein scaffolds in assembly of immune complexes on B cells

CD22 is a B cell-specific sialic acid-binding immunoglobulin-like lectin (Siglec) whose function as a regulator of B cell signaling is modulated by its interaction with glycan ligands bearing the sequence NeuAc alpha2-6Gal. To date, only highly multivalent polymeric ligands (n = 450) have achieved sufficient avidity to bind to CD22 on native B cells. Here we demonstrate that a synthetic bifunctional molecule comprising a ligand of CD22 linked to an antigen (nitrophenol; NP) can use a monoclonal anti-NP IgM as a decavalent protein scaffold to efficiently drive assembly of IgM-CD22 complexes on the surface of native B cells. Surprisingly, anti-NP antibodies of lower valency, IgA (n = 4) and IgG (n = 2), were also found to drive complex formation, though with lower avidity. Ligands bearing alternate linkers of variable length and structure were constructed to establish the importance of a minimal length requirement, and versatility in the structural requirement. We show that the ligand drives assembly of IgM complexes exclusively on the surface of B cells and not other classes of white blood cells that do not express CD22, which lends itself to the possibility of targeting B cells in certain hematopoietic malignancies.

On-virus construction of polyvalent glycan ligands for cell-surface receptors

Glycans arrayed on the exterior of virus particles were used as substrates for glycosyltransferase reactions to build di- and trisaccharides from the virus surface. The resulting particles exhibited tight and specific associations with cognate receptors on beads and cells, in one example defeating in cis cell-surface interactions in a manner characteristic of polyvalent binding. Combined with the ability of viruses to provide structurally well-defined attachment points, the methodology provides a convenient and powerful way to prepare complex carbohydrate ligands for clustered receptors.

Glycan microarrays for screening sialyltransferase specificities

Here we demonstrate that glycan microarrays can be used for high-throughput acceptor specificity screening of various recombinant sialyltransferases. Cytidine-5'-monophospho-N-acetylneuraminic acid (CMP-Neu5Ac) was biotinylated at position 9 of N-acetylneuraminic acid (Neu5Ac) by chemoenzymatic synthesis generating CMP-9Biot-Neu5Ac. The activated sugar nucleotide was used as donor substrate for various mammalian sialyltranferases which transferred biotinylated sialic acids simultaneously onto glycan acceptors immobilized onto a microarray glass slide. Biotinylated glycans detected with fluorescein-streptavidin conjugate to generate a specificity profile for each enzyme both confirming previously known specificities and reveal additional specificity information. Human alpha2,6sialyltransferase-I (hST6Gal-I) also sialylates chitobiose structures (GlcNAcbeta1-4GlcNAc)(n) including N-glycans, rat alpha2,3sialyltransferase (rST3Gal-III) tolerates fucosylated acceptors such as Lewis(a), human alpha2,3sialyltransferase-IV (hST3Gal-IV) broadly sialylates oligosaccharides of types 1-4 and porcine alpha2,3sialyltransferase-I (pST3Gal-I) sialylates ganglio-oligosaccharides and core 2 O-glycans in our array system. Several of these sialyltransferases perform a substitution reaction and exchange a sialylated acceptor with a biotinylated sialic acid but are restricted to the most specific acceptor substrates. Thus, this method allows for a rapid generation of enzyme specificity information and can be used towards synthesis of new carbohydrate compounds and expand the glycan array compound library.

Synthesis of N-acetyllactosamine-containing oligosaccharides, galectin ligands

The following spacered oligosaccharides were synthesized: GlcNAcbetal-3Galbetal-4GlcNAcbeta-sp, GlcNAcbetal-6Galbeta1-4GlcNAcbeta -sp, GlcNAcbeta -3(GlcNAcbeta1-6)Galbeta-4GllcNAcbeta-sp, Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAcbeta-sp, Galbeta1-4GlcNAcbetal-6Galbetal-4GlcNAcbeta-sp, Galbeta1-4GlcNAcbeta -3(Galbeta1-4GlcNAcbeta 1-6)Galbeta1-4GlcNAcbeta-sp, GlcNAcbeta1-3(Galbeta1-4GlcNAcbetal-6)Galbeta 1-4GlcNAcbeta-sp, and Galbeta1-4GlcNAcbetal-3(GlcNAcbetal-6)Galbetal-4GlcNAcbeta-sp (sp = O(CH2)2NH2). They represent N-acetyllactosamines substituted with N-acetylgly-cosamine or N-acetyllalctosamine residue at 03, O6, or at both positions of galactose. Glycosylation was achieved by coupling with N-trichloroethoxycarbonyl-protected glucosamine bromide in the presence of silver triflate.

Construction and Structural Characterization of Versatile Lactosaminoglycan-Related Compound Library for the Synthesis of Complex Glycopeptides and Glycosphingolipids

We have established a facile and efficient protocol for the preparative-scale synthesis of various compound libraries related to lactosaminoglycans: cell surface oligosaccharides composed of N-acetyllactosamine as a repeating disaccharide unit, based on chemical and enzymatic approaches. Substrate specificity and feasibility of a bacterial glycosyltransferase, Neisseria meningitidis beta1,3-N-acetylglucosaminyltransferase (LgtA), were investigated in order to synthesize various key intermediates suited for the construction of mammalian O-glycopeptides and glycosphingolipids containing poly-N-acetyllactosamine structures. Recombinant LgtA exhibited the highest glycosyltransferase activity with strongly basic conditions (pH = 10, glycine-NaOH buffer) and a broad range of optimal temperatures from 20 to 30 degrees C. Interestingly, it was found that LgtA discriminates L-serine and L-threonine and functions both as a core-1 beta1,3-N-acetylglucosaminyltransferase and core-2 beta1,3-N-acetylglucosaminyltransferase toward Fmoc-Ser derivatives, while LgtA showed only core-2 beta1,3-N-acetylglucosaminyltransferase activity in the presence of Fmoc-Thr derivatives. Combined use of LgtA with human beta1,4-galactosyltransferase allowed for controlled sugar extension reactions from synthetic sugar amino acids and gave synthetic lactosaminoglycans, such as a decasaccharide derivative, Galbeta(1 --> 4)GlcNAcbeta(1 --> 3)Galbeta(1 --> 4)GlcNAcbeta(1 --> 3)Galbeta(1 --> 4)GlcNAcbeta(1 --> 3)Galbeta(1 --> 4)GlcNAcbeta(1 --> 6)[Galbeta(1 --> 3)]GalNAcalpha1 --> Fmoc-Ser-OH (6), and a dodecasaccharide derivative, Galbeta(1 --> 4)GlcNAcbeta(1 --> 3)Galbeta(1 --> 4)GlcNAcbeta(1 --> 3)Galbeta(1 --> 4)GlcNAcbeta(1 --> 6)[Galbeta(1 --> 4)GlcNAcbeta(1 --> 3)Galbeta(1 --> 4)GlcNAcbeta(1 --> 3)Galbeta(1 --> 3)]GalNAcalpha1 --> Fmoc-Ser-OH (9). A partially protected pentasaccharide intermediate, GlcNAcbeta(1 --> 3)Galbeta(1 --> 4)GlcNAcbeta(1 --> 6)[Galbeta(1 --> 3)]GalNAcalpha1 --> Fmoc-Thr-OH (11), was applied for the microwave-assisted solid-phase synthesis of a MUC1-related glycopeptide 19 (MW = 2610.1). The findings suggest that this sugar extension strategy can be employed for the modification of lactosyl ceramide mimetic polymers to afford convenient precursors for the synthesis of various glycosphingolipids.

Outer membrane mutation effects on UDP-glucose permeability and whole-cell catalysis rate

In whole-cell biocatalysis, cell envelopes represent a formidable barrier for substrates to permeate. The present research addresses this critical issue by investigating the effects of outer membrane mutation on uridine diphosphate (UDP)-glucose-utilizing enzymes in whole-cell systems. Owing to the severe limitation in substrate permeability, the wild-type Escherichia coli cells only exhibited as low as 4% of available enzyme activities. The reduction of the barriers of the outer membrane permeability (by mutations in its structure) led to a striking acceleration (up to 14-fold) of the reaction rate in cells expressing UDP-glucose dehydrogenase. Mutations in the lipopolysaccharide synthesis pathway or Braun's lipoprotein are both effective. The acceleration was dependent upon the substrate concentrations as well as the enzyme expression level. In addition, the mutation has been demonstrated to be much more effective than the freeze-thaw permeabilizing method. An application of outer membrane mutants was illustrated with the synthesis of a disaccharide (N-acetyllactosamine) from UDP-glucose. Both reaction rate and product yield were enhanced significantly (more than twofold) in the lipoprotein mutant, demonstrating the importance of the outer membrane permeability barrier and the advantages of using outer membrane mutants in synthesis. This research and the results outlined in this paper point to a valid strategy in addressing permeability issues in whole-cell biocatalysis. It also highlights a need for an assessment of substrate permeability in biocatalysis research and development.

Large-scale chemoenzymatic synthesis of blood group and tumor-associated poly-N-acetyllactosamine antigens

Poly-N-acetyllactosamines (pLNs) are common terminal sugars of many N- and O-linked glycan structures present in glycoproteins and glycolipids. Utilizing various glycosyltransferases, we developed new and efficient chemoenzymatic methods for the synthesis of pLNs in gram-scale. Specifically, the use of sialyltransferases and fucosyltransferases enabled us to synthesize and purify 24 blood group and tumor-associated pLN derivatives with alpha-(2-->3)- and alpha-(2-->6)-linked sialic acid, as well as with alpha-(1-->2)- and alpha-(1-->3)-linked fucose. All synthesized derivatives were linked to a short 2-azidoethyl spacer for further modification.

Metabolic engineering of Agrobacterium sp. for UDP-galactose regeneration and oligosaccharide synthesis

Curdlan-producing Agrobacterium sp. is unique in possessing a highly efficient UDP-glucose regeneration system. A broad-host-range expression strategy was successfully developed to exploit the unique metabolic capability for UDP-galactose regeneration during oligosaccharide synthesis. The engineered Agrobacterium cells functioned as a UDP-galactose regeneration system, allowing galactose-containing disaccharides to be synthesized from glucose or other simple sugars. Unexpectedly, a lag period of 24h preceded the active synthesis, which could be eliminated with rifampicin. An intracellular nucleotide profiling revealed that the UMP level was elevated by 3.8 fold in the presence of rifampicin, suggesting that rifampicin simulated a nitrogen-limitation condition that triggered the metabolic change. Product selectivity was improved nearly 40-fold by using high acceptor concentration and restricting glucose supply. N-acetyllactosamine concentration near 20 mM (7.5 g/l) was obtained, demonstrating the effectiveness of the engineered strain in UDP-galactose regeneration. This organism could be engineered to regenerate other UDP-sugar nucleotides using the same strategy as illustrated here.

Chemoenzymatic synthesis of glycan libraries

The expanding interest for carbohydrates and glycoconjugates in cell communication has led to an increased demand of these structures for biological studies. Complicated chemical strategies in glycan synthesis are now more frequently replaced by regio- and stereo-specific enzymes. The exploration of microbial resources and improved production of mammalian enzymes have established glycosyltransferases as an efficient complementary tool for glycan synthesis. In this chapter, we demonstrate the feasibility of preparative enzymatic synthesis of different categories of glycans, such as blood group and tumor-associated poly-N-acetyllactosamines antigens, ganglio-oligosaccharides, N- and O-glycans. The enzymatic approach has generated over 100 novel oligosaccharides in amounts allowing milligram to gram distribution to many researchers in the field. Our diverse library has also formed the foundation for the successful developments of both the noncovalent enzyme-linked immunosorbent assay glycan array and the covalent printed glycan microarray.

Synthesis and molecular recognition of carbohydrate-centered multivalent glycoclusters by a plant lectin RCA120

Water soluble and lectin-recognizable carbohydrate-centered glycoclusters were prepared efficiently by the Huisgen 1,3-cycloaddition reaction of methyl-2,3,4,6-tetra-O-propargyl beta-D-galactopyranoside with 2-azidoethyl glycosides of lactose and N-acetyllactosamine. Their binding by a plant lectin RCA120 was examined by capillary affinity electrophoresis using fluorescence-labeled asialoglycans from human alpha1-acid glycoprotein. The glycoclusters showed 400-fold stronger inhibitory effect than free lactose, manifesting strong multivalency effect.

Chemoenzymatic synthesis of 2-azidoethyl-ganglio-oligosaccharides GD3, GT3, GM2, GD2, GT2, GM1, and GD1a

We have synthesized several ganglio-oligosaccharide structures using glycosyltransferases from Campylobacter jejuni. The enzymes, alpha-(2-->3/8)-sialyltransferase (Cst-II), beta-(1-->4)-N-acetylgalactosaminyltransferase (CgtA), and beta-(1-->3)-galactosyltransferase (CgtB), were produced in large-scale fermentation from Escherichia coli and further characterized based on their acceptor specificities. 2-Azidoethyl-glycosides corresponding to the oligosaccharides of GD3 (alpha-D-Neup5Ac-(2-->8)-alpha-D-Neup5Ac-(2-->3)-beta-D-Galp-(1-->4)-beta-D-Glcp-), GT3 (alpha-D-Neup5Ac-(2-->8)-alpha-D-Neup5Ac-(2-->8)-alpha-D-Neup5Ac-(2-->3)-beta-D-Galp-(1-->4)-beta-D-Glcp-), GM2 (beta-D-GalpNAc-(1-->4)-[alpha-D-Neup5Ac-(2-->3)]-beta-D-Galp-(1-->4)-beta-D-Glcp-), GD2 (beta-D-GalpNAc-(1-->4)-[alpha-D-Neup5Ac-(2-->8)-alpha-D-Neup5Ac-(2-->3)]-beta-D-Galp-(1-->4)-beta-D-Glcp-), GT2 (beta-D-GalpNAc-(1-->4)-[alpha-D-Neup5Ac-(2-->8)-alpha-D-Neup5Ac-(2-->8)-alpha-D-Neup5Ac-(2-->3)]-beta-D-Galp-(1-->4)-beta-D-Glcp-), and GM1 (beta-D-Galp-(1-->3)-beta-D-GalpNAc-(1-->4)-[alpha-D-Neup5Ac-(2-->3)]-beta-D-Galp-(1-->4)-beta-D-Glcp-) were synthesized in high yields (gram-scale). In addition, a mammalian alpha-(2-->3)-sialyltransferase (ST3Gal I) was used to sialylate GM1 and generate GD1a (alpha-D-Neup5Ac-(2-->3)-beta-D-Galp-(1-->3)-beta-D-GalpNAc-(1-->4)-[alpha-D-Neup5Ac-(2-->3)]-beta-D-Galp-(1-->4)-beta-D-Glcp-) oligosaccharide. We also cloned and expressed a rat UDP-N-acetylglucosamine-4'epimerase (GalNAcE) in E. coli AD202 cells for cost saving in situ conversion of less expensive UDP-GlcNAc to UDP-GalNAc.

A pH-sensitive assay for galactosyltransferase

We report here a new pH-indicator-based assay for galactosyltransferase. The method is simple and fast, requires no specialized equipment, labeled substrate, or other expensive materials, and is thus expected to have broad applications including automated high-throughput screening. The method is based upon the detection of absorbance change of a pH indicator, phenol red, in response to proton release that accompanies the galactosyltransferase-catalyzed galactose transfer. The assay was used to compare three galactosyltransferases in our collection. As demonstrated here, subtle differences in substrate specificity were readily discerned with this sensitive method. All three enzymes accept both N-acetylglucosamine and glucose as acceptor but the relative activity varies with the origin of the enzyme. The method was demonstrated to be useful in the initial characterization of recombinant galactosyltransferase from crude cell extract. Optimal metal cofactor Mn(2+) concentration and temperature were determined with the method. Overall, the method offers a great improvement over current methods in reducing time and material consumption. It is the first pH-sensitive method for galactosyltransferase. The principles of using pH indicator in galactosyltransferase assay should be applicable to other glycosyltransferase enzymes.

Potent glycan inhibitors of myelin-associated glycoprotein enhance axon outgrowth in vitro

Myelin-associated glycoprotein (MAG, Siglec-4) is one of several endogenous axon regeneration inhibitors that limit recovery from central nervous system injury and disease. Molecules that block such inhibitors may enhance axon regeneration and functional recovery. MAG, a member of the Siglec family of sialic acid-binding lectins, binds to sialoglycoconjugates on axons and particularly to gangliosides GD1a and GT1b, which may mediate some of the inhibitory effects of MAG. In a prior study, we identified potent monovalent sialoside inhibitors of MAG using a novel screening platform. In the current study, the most potent of these were tested for their ability to reverse MAG-mediated inhibition of axon outgrowth from rat cerebellar granule neurons in vitro. Monovalent sialoglycans enhanced axon regeneration in proportion to their MAG binding affinities. The most potent glycoside was disialyl T antigen (NeuAcalpha2-3Galbeta1-3[NeuAcalpha2-6]GalNAc-R), followed by 3-sialyl T antigen (NeuAcalpha2-3Galbeta1-3GalNAc-R), structures expressed on O-linked glycoproteins as well as on gangliosides. Prior studies indicated that blocking gangliosides reversed MAG inhibition. In the current study, blocking O-linked glycoprotein sialylation with benzyl-alpha-GalNAc had no effect. The ability to reverse MAG inhibition with monovalent glycosides encourages further exploration of glycans and glycan mimetics as blockers of MAG-mediated axon outgrowth inhibition.

High-throughput identification of fucosyltransferase inhibitors using carbohydrate microarrays

A noncovalent carbohydrate microarray was used to screen for possible inhibitors to fucosyltransferases, which are critical to the synthesis of inflammation mediators like sialyl Lewis x (SLe(x)). Inhibition was followed by observation of the transferred fucose on the carbohydrate array with the lectin Tetragonolobus purpureas. Of these compounds, four inhibitors with nanomolar Ki(s) were discovered, with three of the top five inhibitors exhibiting a common architecture.

Dimeric galectin-1 binds with high affinity to alpha2,3-sialylated and non-sialylated terminal N-acetyllactosamine units on surface-bound extended glycans

Galectin-1 is a member of the galectin family of glycan-binding proteins and occurs as an approximately 29.5-kDa noncovalent homodimer (dGal-1) that is widely expressed in many tissues. Here, we report that human recombinant dGal-1 bound preferentially and with high affinity (apparent K(d) approximately 2-4 microM) to immobilized extended glycans containing terminal N-acetyllactosamine (LN; Galbeta1-4GlcNAc) sequences on poly-N-acetyllactosamine (PL; (-3Galbeta1-4GlcNAcbeta1-)(n)) sequences, complex-type biantennary N-glycans, or novel chitin-derived glycans modified to contain terminal LN. Although terminal Gal residues are important for dGal-1 recognition, dGal-1 bound similarly to alpha3-sialylated and alpha2-fucosylated terminal LN, but not to alpha6-sialylated and alpha3-fucosylated terminal LN. The binding specificity of human recombinant dGal-1 was similar to that observed with purified bovine heart-derived dGal-1. Unexpectedly, dGal-1 bound free ligands in solution with relatively low affinity and displayed no preference for extended glycans, indicating that dGal-1 preferentially recognizes extended glycans only when they are surface-bound, such as found on cell surfaces. Human dGal-1 also bound to both native and desialylated human promyelocytic HL-60 cells with similar affinity as observed for immobilized long chain PL. Binding to these cells was reduced upon treatment with endo-beta-galactosidase, which cleaves PL sequences, indicating that cell-surface PLs are ligands. To test the role of dimerization in dGal-1 binding, we examined the binding of a mutated form of dGal-1 that weakly dimerizes (monomeric Gal-1 (mGal-1)) and a covalently dimerized (chemically cross-linked) form of mGal-1 (cd-mGal-1). dGal-1 and cd-mGal-1 had similar affinities that were both approximately 3.5-fold higher for immobilized PL than observed for mGal-1, suggesting that dGal-1 acts as a dimer to cross-link terminal LN units on immobilized PL. These results indicate that dGal-1 functions as a dimer to recognize LN units on extended PLs on cell surfaces.

A single bifunctional UDP-GlcNAc/Glc 4-epimerase supports the synthesis of three cell surface glycoconjugates in Campylobacter jejuni

The major cell-surface carbohydrates (lipooligosaccharide, capsule, and glycoprotein N-linked heptasaccharide) of Campylobacter jejuni NCTC 11168 contain Gal and/or GalNAc residues. GalE is the sole annotated UDP-glucose 4-epimerase in this bacterium. The presence of GalNAc residues in these carbohydrates suggested that GalE might be a UDP-GlcNAc 4-epimerase. GalE was shown to epimerize UDP-Glc and UDP-GlcNAc in coupled assays with C. jejuni glycosyltransferases and in sugar nucleotide epimerization equilibria studies. Thus, GalE possesses UDP-GlcNAc 4-epimerase activity and was renamed Gne. The Km(app) values of a purified MalE-Gne fusion protein for UDP-GlcNAc and UDP-GalNAc are 1087 and 1070 microm, whereas those for UDP-Glc and UDP-Gal are 780 and 784 microm. The kcat and kcat/Km(app) values were three to four times higher for UDP-GalNAc and UDP-Gal than for UDP-GlcNAc and UDP-Glc. The comparison of the kinetic parameters of MalE-Gne to those of other characterized bacterial UDP-GlcNAc 4-epimerases indicated that Gne is a bifunctional UDP-GlcNAc/Glc 4-epimerase. The UDP sugar-binding site of Gne was modeled by using the structure of the UDP-GlcNAc 4-epimerase WbpP from Pseudomonas aeruginosa. Small differences were noted, and these may explain the bifunctional character of the C. jejuni Gne. In a gne mutant of C. jejuni, the lipooligosaccharide was shown by capillary electrophoresis-mass spectrometry to be truncated by at least five sugars. Furthermore, both the glycoprotein N-linked heptasaccharide and capsule were no longer detectable by high resolution magic angle spinning NMR. These data indicate that Gne is the enzyme providing Gal and GalNAc residues with the synthesis of all three cell-surface carbohydrates in C. jejuni NCTC 11168.

Chemically defined sialoside scaffolds for investigation of multivalent interactions with sialic acid binding proteins

Four glycodendrons and a glycocluster were synthesized from carbohydrate building blocks to form paucivalent (di- to tetravalent) structures of controlled scaffold architectures. Enzymatic sialylation of the functionalized cluster and dendrons, terminated in lactose residues, generated a library of paucivalent synthetic sialosides displaying sialic acids with different dispositions. These newly constructed bioactive sialic acid-based structures were differentially recognized by sialoadhesin, a mammalian macrophage sialic acid binding protein. The binding of the sialosides to sialoadhesin was evaluated by an enzyme-linked immunosorbant assay to investigate the complementarity of scaffold structure and binding to sialoadhesin. Modulating the interaction between sialoadhesin and its sialic acid ligands has important implications in immunobiology.

Sialoside specificity of the siglec family assessed using novel multivalent probes: identification of potent inhibitors of myelin-associated glycoprotein

Ten of the 11 known human siglecs or their murine orthologs have been evaluated for their specificity for over 25 synthetic sialosides representing most of the major sequences terminating carbohydrate groups of glycoproteins and glycolipids. Analysis has been performed using a novel multivalent platform comprising biotinylated sialosides bound to a streptavidin-alkaline phosphatase conjugate. Each siglec was found to have a unique specificity for binding 16 different sialoside-streptavidin-alkaline phosphatase probes. The relative affinities of monovalent sialosides were assessed for each siglec in competitive inhibition studies. The quantitative data obtained allows a detailed analysis of each siglec for the relative importance of sialic acid and the penultimate oligosaccharide sequence on binding affinity and specificity. Most remarkable was the finding that myelin-associated glycoprotein (Siglec-4) binds with 500-10,000-fold higher affinity to a series of mono- and di-sialylated derivatives of the O-linked T-antigen (Galbeta(1-3)-GalNAc(alpha)OThr) as compared with alpha-methyl-NeuAc.

Expression patterns of alpha 2,3-sialyltransferases and alpha 1,3-fucosyltransferases determine the mode of sialyl Lewis X inhibition by disaccharide decoys

A variety of human adenocarcinomas express sialylated, fucosylated Lewis blood group antigens on cell surface and secreted mucins. Binding of these antigens to P-selectin on platelets is thought to facilitate formation of platelet-tumor emboli in the circulation, which in turn allows sequestration of the tumor cells in the microvasculature. Here we report a pharmacologic approach for blocking these interactions through metabolic inhibition of sialylation. Peracetylated forms of Galbeta1,4GlcNAcbeta-O-naphthalenemethanol and GlcNAcbeta1,3Galbeta-O-naphthalenemethanol were taken up by LS180 human colon carcinoma cells, O-deacetylated, and utilized as biosynthetic intermediates, resulting in heterogeneous oligosaccharides. The primed oligosaccharides included sialylated, sulfated, and fucosylated products based on mass spectrometry. Assembly of free oligosaccharides on the glycosides decoyed glycosylation of cellular glycoproteins, as assessed by altered binding of lectins and carbohydrate-specific antibodies. Expression of alpha2,3-sialylated oligosaccharides on the cell surface was diminished specifically, whereas alpha2,6-sialylation and fucosylation were not. In U937 lymphoma cells, the glycosides decreased fucosylation without affecting sialylation. The differential inhibitory activities correlated inversely with fucosyltransferase and sialyltransferase activity based on enzyme assays and microarray analysis. Regardless of the mechanism, the disaccharides blocked the cells from forming selectin ligands and inhibited adhesion to immobilized selectins, suggesting that the glycosides might prove useful for interfering with tumor cell adhesion and metastasis.

Efficient synthesis of globoside and isogloboside tetrasaccharides by using beta(1-->3) N-acetylgalactosaminyltransferase/UDP-N-acetylglucosamine C4 epimerase fusion protein

The beta(1-->3) N-acetylgalactosaminyltransferase/UDP-N-acetylglucosamine C4 epimerase fusion protein was constructed and used in coupled enzymatic reactions to synthesize a variety of globotetraose and isoglobotetraose derivatives from the corresponding lactoside acceptors.

Synthesis of sugar arrays in microtiter plate

1,3-Dipolar cycloadditions between azides and alkynes were exploited to attach oligosaccharides to a C(14) hydrocarbon chain that noncovalently binds to the microtiter well surface. Synthesis of sugar arrays was performed on a micromolar scale in situ in the microtiter plate. As a model study, the beta-galactosyllipid 5 was displayed on a 4-micromol scale. Formation of product was confirmed via ESI-MS, and the yield was determined via chemical and biological assays. Several complex carbohydrates (6-16) were also displayed in microtiter plates and successfully screened with various lectins. Moreover, sialyl Lewis x (17) was synthesized via the enzymatic fucosylation of a precursor displayed in the plate. Studies on inhibition of this biotransformation have been carried out, and the IC(50) value found for the known inhibitor 20 was consistent with previous studies in solution.

Phage-display selection of a human single-chain fv antibody highly specific for melanoma and breast cancer cells using a chemoenzymatically synthesized G(M3)-carbohydrate antigen

Overexpression of the cell-surface glycosphingolipid G(M3) is associated with a number of different cancers, including those of the skin, colon, breast, and lung. Antibodies against the G(M3) epitope have potential application as therapeutic agents in the treatment of these cancers. We describe the chemoenzymatic synthesis of two G(M3)-derived reagents and their use in the panning of a phage-displayed human single-chain Fv (scFv) antibody library derived from the blood of cancer patients. Three scFv-phage clones, GM3A6, GM3A8, and GM3A15, were selected for recombinant expression and were characterized using BIAcore and flow cytometry. BIAcore measurements using the purified, soluble scFvs yielded dissociation constants (K(d)) ranging from 4.2 x 10(-7) to 2.1 x 10(-5) M. Flow cytometry was used to evaluate the ability of each scFv to discriminate between normal human cells (human dermal fibroblast, HDFa), melanoma cells (HMV-1, M21, and C-8161), and breast cancer cells (BCM-1, BCM-2, and BMS). GM3A6 displayed cross-reactivity with normal cells, as well as tumor cells, and GM3A15 possessed little or no binding activity toward any of the cell lines tested. However, GM3A8 bound to five of the six tumor cell lines and showed no measurable reactivity against the HDFa cells. Hence, we have demonstrated that a synthetic G(M3) panning reagent can be used to isolate a fully human scFv that is highly specific for native G(M3) on the surface of tumor cells. The result is a significant step toward effective immunotherapies for the treatment of cancer.

Specificity and affinity of sialic acid binding by the rhesus rotavirus VP8* core

Nuclear magnetic resonance spectroscopy demonstrates that the rhesus rotavirus hemagglutinin specifically binds alpha-anomeric N-acetylneuraminic acid with a K(d) of 1.2 mM. The hemagglutinin requires no additional carbohydrate moieties for binding, does not distinguish 3' from 6' sialyllactose, and has approximately tenfold lower affinity for N-glycolylneuraminic than for N-acetylneuraminic acid. The broad specificity and low affinity of sialic acid binding by the rotavirus hemagglutinin are consistent with this interaction mediating initial cell attachment prior to the interactions that determine host range and cell type specificity.