Variants of the β1,3-Galactosyltransferase CgtB from the Bacterium Campylobacter Jejuni have Distinct Acceptor Specificities

Host-Guest Chemistry-Mediated Biomimetic Chemoenzymatic Synthesis of Complex Glycosphingolipids

Glycosphingolipids (GSLs) are amphipathic complex biomolecules constituted of hydrophilic glycans covalently linked to hydrophobic lipids via glycosidic bonds. GSLs are widely distributed in cells and tissues, where they play crucial roles in various biological functions and disease processes. However, the heterogeneity and complexity of GSLs make it difficult to explore their precise biofunctions due to obstacles in obtaining well-defined structures. Herein, we report a host-guest-chemistry-mediated biomimetic chemoenzymatic approach for the efficient synthesis of diverse complex GSLs. A key feature of this approach is that the use of methyl-β-cyclodextrin enables amphipathic glycolipids forming water-soluble inclusion complexes to improve their solubility in aqueous media, thereby facilitating enzyme-catalyzed reactions. The power and applicability of our approach are demonstrated by the streamlined synthesis of biologically important globo-, ganglio-, neolacto-, and lacto-series GSLs library containing 20 neutral and acidic glycolipids with different fucosylation and sialylation patterns. The developed method will open new avenues to easily access a wide range of complex GSLs for biomedical applications.

Comprehensive Modular Synthesis of Ganglioside Glycans and Evaluation of their Binding Affinities to Siglec-7 and Siglec-9

In the present work, bacterial glycosyltransferases are utilized to construct ganglioside glycans in a convergent approach via a sugar‒nucleotide regeneration system and one-pot multienzyme reactions. Starting from β-lactoside enables the diversification of both the glycan moieties and the linkages in the lower α-arm and upper β-arm. Overall, a comprehensive panel of 24 natural a-series (GM3, GM2, GM1a, GD1a, GT1a, and fucosyl-GM1), b-series (GD3, GD2, GD1b, GT1b, and GQ1b), c-series (GT3, GT2, GT1c, GQ1c, and GP1c), α-series (GM1α, GD1aα, and GT1aα), and o-series (GA2, GA1, GM1b, GalNAc-GM1b, and GD1c) ganglioside glycans are prepared, which are suitable for biological studies and further applications. Moreover, a microarray is constructed with these synthesized ganglioside glycans to investigate their binding specificity with recombinant Fc-fused Siglec-7 and Siglec-9, which are immune checkpoint-like glycan recognition proteins on natural killer cells. The microarray binding results reveal that GD3 and GT1aα are specific ligands for Siglec-7 and Siglec-9, respectively, and this discovery can lead to the identification of appropriate ligands for investigating the roles of these Siglecs in immunomodulation.

A previously uncharacterized O-glycopeptidase from Akkermansia muciniphila requires the Tn-antigen for cleavage of the peptide bond

Akkermansia muciniphila is key member of the human gut microbiota, impacting many features of host health. A major characteristic of this bacterium is its interaction with host mucin, which is abundant in the gut environment, and its ability to metabolize mucin as a nutrient source. The machinery deployed by A. muciniphila to enable this interaction appears to be extensive and sophisticated, yet it is incompletely defined. The uncharacterized protein AMUC_1438 is encoded by a gene that was previously shown to be upregulated when the bacterium is grown on mucin. This uncharacterized protein has features suggestive of carbohydrate-recognition and peptidase activity, which led us to hypothesize that it has a role in mucin depolymerization. Here we provide structural and functional support for the assignment of AMUC_1438 as a unique O-glycopeptidase with mucin degrading capacity. O-glycopeptidase enzymes recognize glycans but hydrolyze the peptide backbone and are common in host-adapted microbes that colonize or invade mucus layers. Structural, kinetic, and mutagenic analyses point to a metzincin metalloprotease catalytic motif but specific recognition of a GalNAc residue α-linked to serine or threonine (i.e. the Tn-antigen) within the AMUC_1438 active site. The enzyme catalyzes hydrolysis of the bond immediately N-terminal to the glycosylated residue. Additional modelling analyses suggest the presence of a carbohydrate-binding module that may assist in substrate recognition. We anticipate that these results will be fundamental to a wider understanding of the O-glycopeptidase class of enzymes and how they may contribute to host-adaptation.

Mammalian sialyltransferases allow efficient E. coli-based production of mucin-type O-glycoproteins but can also transfer Kdo

The prospect of producing human-like glycoproteins in bacteria is becoming attractive as an alternative to already-established but costly mammalian cell expression systems. We previously described an E. coli expression platform that uses a dual-plasmid approach to produce simple mucin type O-glycoproteins: one plasmid encoding the target protein and another the O-glycosylation machinery. Here, we expand the capabilities of our platform to carry out sialylation and demonstrate the high-yielding production of human interferon α2b and human growth hormone bearing mono- and disialylated T-antigen glycans. This is achieved through engineering an E. coli strain to produce CMP-Neu5Ac and introducing various α-2,3- and α-2,6 mammalian or bacterial sialyltransferases into our O-glycosylation operons. We further demonstrate that mammalian sialyltransferases, including porcine ST3Gal1, human ST6GalNAc2, and human ST6GalNAc4, are very effective in vivo and outperform some of the bacterial sialyltransferases tested, including Campylobacter jejuni Cst-I and Cst-II. In the process we came upon a way of modifying T-Antigen with Kdo, using a previously uncharacterised Kdo-transferase activity of porcine ST3Gal1. Ultimately, the heterologous expression of mammalian sialyltransferases in E. coli shows promise for the further development of bacterial systems in therapeutic glycoprotein production.

Discovery and Development of Promiscuous O-Glycan Hydrolases for Removal of Intact Sialyl T-Antigen

Mucin-type O-glycosylation (O-glycosylation) is a common post-translational modification that confers distinct biophysical properties to proteins and plays crucial roles in intercellular signaling. Yet, despite the importance of O-glycans, relatively few tools exist for their analysis and modification. In particular, there is a need for enzymes that can cleave the wide range of O-glycan structures found on protein surfaces, to facilitate glycan profiling and editing. Through functional metagenomic screening of the human gut microbiome, we discovered endo-O-glycan hydrolases from CAZy family GH101 that are capable of slowly cleaving the intact sialyl T-antigen trisaccharide (a ubiquitous O-glycan structure in humans) in addition to their primary activity against the T-antigen disaccharide. We then further explored this sequence space through phylogenetic profiling and analysis of representative enzymes, revealing large differences in the levels of this promiscuous activity between enzymes within the family. Through structural and sequence analysis, we identified active site residues that modulate specificity. Through subsequent rational protein engineering, we improved the activity of an enzyme identified by phylogenetic profiling sufficiently that substantial removal of the intact sialyl T-antigen from proteins could be readily achieved. Our best sialyl T-antigen hydrolase mutant, SpGH101 Q868G, is further shown to function on a number of proteins, tissues, and cells. Access to this enzyme opens up improved methodologies for unraveling the glycan code.

Investigation of sequon engineering for improved O-glycosylation by the human polypeptide N-acetylgalactosaminyl transferase T2 isozyme and two orthologues

We have been developing bacterial expression systems for human mucin-type O-glycosylation on therapeutic proteins, which is initiated by the addition of α-linked GalNAc to serine or threonine residues by enzymes in the GT-27 family of glycosyltransferases. Substrate preference across different isoforms of this enzyme is influenced by isoform-specific amino acid sequences at the site of glycosylation, which we have exploited to engineer production of Core 1 glycan structures in bacteria on human therapeutic proteins. Using RP-HPLC with a novel phenyl bonded phase to resolve intact protein glycoforms, the effect of sequon mutation on O-glycosylation initiation was examined through in vitro modification of the naturally O-glycosylated human interferon α-2b, and a sequon engineered human growth hormone. As part of the development of our glycan engineering in the bacterial expression system we are surveying various orthologues of critical enzymes to ensure complete glycosylation. Here we present an in vitro enzyme kinetic profile of three related GT-27 orthologues on natural and engineered sequons in recombinant human interferon α2b and human growth hormone where we show a significant change in kinetic properties with the amino acid changes. It was found that optimizing the protein substrate amino acid sequence using Isoform Specific O-Glycosylation Prediction (ISOGlyP, http://isoglyp.utep.edu/index.php) resulted in a measurable increase in kcat/KM, thus improving glycosylation efficiency. We showed that the Drosophila orthologue showed superior activity with our human growth hormone designed sequons compared with the human enzyme.

Architecturally complex O-glycopeptidases are customized for mucin recognition and hydrolysis

A challenge faced by peptidases is the recognition of highly diverse substrates. A feature of some peptidase families is the capacity to specifically use post-translationally added glycans present on their protein substrates as a recognition determinant. This is ultimately critical to enabling peptide bond hydrolysis. This class of enzyme is also frequently large and architecturally sophisticated. However, the molecular details underpinning glycan recognition by these O-glycopeptidases, the importance of these interactions, and the functional roles of their ancillary domains remain unclear. Here, using the Clostridium perfringens ZmpA, ZmpB, and ZmpC M60 peptidases as model proteins, we provide structural and functional insight into how these intricate proteins recognize glycans as part of catalytic and noncatalytic substrate recognition. Structural, kinetic, and mutagenic analyses support the key role of glycan recognition within the M60 domain catalytic site, though they point to ZmpA as an apparently inactive enzyme. Wider examination of the Zmp domain content reveals noncatalytic carbohydrate binding as a feature of these proteins. The complete three-dimensional structure of ZmpB provides rare insight into the overall molecular organization of a highly multimodular enzyme and reveals how the interplay of individual domain function may influence biological activity. O-glycopeptidases frequently occur in host-adapted microbes that inhabit or attack mucus layers. Therefore, we anticipate that these results will be fundamental to informing more detailed models of how the glycoproteins that are abundant in mucus are destroyed as part of pathogenic processes or liberated as energy sources during normal commensal lifestyles.

Enzymatic Synthesis of Glycans and Glycoconjugates

Glycoconjugates have great potential to improve human health in a multitude of different ways and fields. Prominent examples are human milk oligosaccharides and glycosaminoglycans. The typical choice for the production of homogeneous glycoconjugates is enzymatic synthesis. Through the availability of expression and purification protocols, recombinant Leloir glycosyltransferases are widely applied as catalysts for the synthesis of a wide range of glycoconjugates. Extensive utilization of these enzymes also depends on the availability of activated sugars as building blocks. Multi-enzyme cascades have proven a versatile technique to synthesize and in situ regenerate nucleotide sugar.In this chapter, the functions and mechanisms of Leloir glycosyltransferases are revisited, and the advantage of prokaryotic sources and production systems is discussed. Moreover, in vivo and in vitro pathways for the synthesis of nucleotide sugar are reviewed. In the second part, recent and prominent examples of the application of Leloir glycosyltransferase are given, i.e., the synthesis of glycosaminoglycans, glycoconjugate vaccines, and human milk oligosaccharides as well as the re-glycosylation of biopharmaceuticals, and the status of automated glycan assembly is revisited.

Expression and purification of the full-length N-acetylgalactosaminyltransferase and galactosyltransferase from Campylobacter jejuni in Escherichia coli

The successful enzymatic synthesis of various ganglioside-related oligosaccharides requires many available glycan-processing enzymes. However, the number of available glycan-processing enzymes remains limited. In this study, the full-length CgtA43456 (β-(1→4)-N-acetylgalactosaminyltransferase) and CgtB11168 (β-(1→3)-galactosyltransferase) were successfully produced from Escherichia coli through the optimization of E. coli-preferable codon usage, selection of E. coli strain, and use of the molecular chaperone GroEL-GroES (GroEL/ES). The CgtA43456 enzyme was produced as a soluble form in E. coli C41(DE3) co-expressed with codon-optimized CgtA43456 and GroEL/ES. However, soluble CgtB11168 was well expressed in E. coli C41(DE3) with only the codon-optimized CgtB11168. Rather, when co-expressed with GroEL/ES, total production of CgtB11168 was reduced. Using immobilized-metal affinity chromatography, the CgtA43456 and CgtB11168 proteins were obtained with approximately 75-78 % purity. The purified CgtA43456 showed a specific activity of 21 mU/mg using UDP-N-acetylgalactosamine and GM3 trisaccharide as donor and acceptor, respectively. The purified CgtB11168 catalyzed the transfer of galactose from UDP-Gal to GM2 tetrasaccharide with a specific activity of 16 mU/mg. We propose that they could be used as catalysts for enzymatic synthesis of GM1 ganglioside-related oligosaccharides.

In vitro Production and Immunogenicity of a Clostridium Difficile Spore-Specific BclA3 Glycopeptide Conjugate Vaccine

Abstract: The BclA3 glycoprotein is a major component of the exosporangial layer of Clostridium difficile spores and in this study we demonstrate that this glycoprotein is a major spore surface associated antigen. Here, we confirm the role of SgtA glycosyltransferase (SgtA GT) in BclA3 glycosylation and recapitulate this process by expressing and purifying SgtA GT fused to MalE, the maltose binding protein from Escherichia coli. In vitro assays using the recombinant enzyme and BclA3 synthetic peptides demonstrated that SgtA GT was responsible for the addition of β-O-linked GlcNAc to threonine residues of each synthetic peptide. These peptide sequences were selected from the central, collagen repeat region of the BclA3 protein. Following optimization of SgtA GT activity, we generated sufficient glycopeptide (10 mg) to allow conjugation to KLH (keyhole limpet hemocyanin) protein. Glycosylated and unglycosylated versions of these conjugates were then used as antigens to immunize rabbits and mice. Immune responses to each of the conjugates were examined by Enzyme Linked Immunosorbent Assay ELISA. Additionally, the BclA3 conjugated peptide and glycopeptide were used as antigens in an ELISA assay with serum raised against formalin-killed spores. Only the glycopeptide was recognized by anti-spore polyclonal immune serum demonstrating that the glycan moiety is a predominant spore-associated surface antigen. To determine whether antibodies to these peptides could modify persistence of spores within the gut, animals immunized intranasally with either the KLH-glycopeptide or KLH-peptide conjugate in the presence of cholera toxin, were challenged with R20291 spores. Although specific antibodies were raised to both antigens, immunization did not provide any protection against acute or recurrent disease.

Genetics behind the Biosynthesis of Nonulosonic Acid-Containing Lipooligosaccharides in Campylobacter coli

Campylobacter jejuni and Campylobacter coli are the most common causes of bacterial gastroenteritis in the world. Ganglioside mimicry by C. jejuni lipooligosaccharide (LOS) is the triggering factor of Guillain-Barré syndrome (GBS), an acute polyneuropathy. Sialyltransferases from glycosyltransferase family 42 (GT-42) are essential for the expression of ganglioside mimics in C. jejuni Recently, two novel GT-42 genes, cstIV and cstV, have been identified in C. coli Despite being present in ∼11% of currently available C. coli genomes, the biological role of cstIV and cstV is unknown. In the present investigation, mutation studies with two strains expressing either cstIV or cstV were performed and mass spectrometry was used to investigate differences in the chemical composition of LOS. Attempts were made to identify donor and acceptor molecules using in vitro activity tests with recombinant GT-42 enzymes. Here we show that CstIV and CstV are involved in C. coli LOS biosynthesis. In particular, cstV is associated with LOS sialylation, while cstIV is linked to the addition of a diacetylated nonulosonic acid residue.IMPORTANCE Despite the fact that Campylobacter coli a major foodborne pathogen, its glycobiology has been largely neglected. The genetic makeup of the C. coli lipooligosaccharide biosynthesis locus was largely unknown until recently. C. coli harbors a large set of genes associated with lipooligosaccharide biosynthesis, including genes for several putative glycosyltransferases involved in the synthesis of sialylated lipooligosaccharide in Campylobacter jejuni In the present study, C. coli was found to express lipooligosaccharide structures containing sialic acid and other nonulosonate acids. These findings have a strong impact on our understanding of C. coli ecology, host-pathogen interaction, and pathogenesis.

A Bacterial Expression Platform for Production of Therapeutic Proteins Containing Human-like O-Linked Glycans

We have developed an Escherichia coli strain for the in vivo production of O-glycosylated proteins. This was achieved using a dual plasmid approach: one encoding a therapeutic protein target, and a second encoding the enzymatic machinery required for O-glycosylation. The latter plasmid encodes human polypeptide N-acetylgalactosaminyl transferase as well as a β1,3-galactosyl transferase and UDP-Glc(NAc)-4-epimerase, both from Campylobacter jejuni, and a disulfide bond isomerase of bacterial or human origin. The effectiveness of this two-plasmid synthetic operon system has been tested on three proteins with therapeutic potential: the native and an engineered version of the naturally O-glycosylated human interferon α-2b, as well as human growth hormone with one engineered site of glycosylation. Having established proof of principle for the addition of the core-1 glycan onto proteins, we are now developing this system as a platform for producing and modifying human protein therapeutics with more complex O-glycan structures in E. coli.

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.

Crossroads between Bacterial and Mammalian Glycosyltransferases

Bacterial glycosyltransferases (GT) often synthesize the same glycan linkages as mammalian GT; yet, they usually have very little sequence identity. Nevertheless, enzymatic properties, folding, substrate specificities, and catalytic mechanisms of these enzyme proteins may have significant similarity. Thus, bacterial GT can be utilized for the enzymatic synthesis of both bacterial and mammalian types of complex glycan structures. A comparison is made here between mammalian and bacterial enzymes that synthesize epitopes found in mammalian glycoproteins, and those found in the O antigens of Gram-negative bacteria. These epitopes include Thomsen–Friedenreich (TF or T) antigen, blood group O, A, and B, type 1 and 2 chains, Lewis antigens, sialylated and fucosylated structures, and polysialic acids. Many different approaches can be taken to investigate the substrate binding and catalytic mechanisms of GT, including crystal structure analyses, mutations, comparison of amino acid sequences, NMR, and mass spectrometry. Knowledge of the protein structures and functions helps to design GT for specific glycan synthesis and to develop inhibitors. The goals are to develop new strategies to reduce bacterial virulence and to synthesize vaccines and other biologically active glycan structures.

Characterization of two UDP-Gal:GalNAc-diphosphate-lipid β1,3-galactosyltransferases WbwC from Escherichia coli serotypes O104 and O5

Escherichia coli displays O antigens on the outer membrane that play an important role in bacterial interactions with the environment. The O antigens of enterohemorrhagic E. coli O104 and O5 contain a Galβ1-3GalNAc disaccharide at the reducing end of the repeating unit. Several other O antigens contain this disaccharide, which is identical to the mammalian O-glycan core 1 or the cancer-associated Thomsen-Friedenreich (TF) antigen. We identified the wbwC genes responsible for the synthesis of the disaccharide in E. coli serotypes O104 and O5. To functionally characterize WbwC, an acceptor substrate analog, GalNAcα-diphosphate-phenylundecyl, was synthesized. WbwC reaction products were isolated by high-pressure liquid chromatography and analyzed by mass spectrometry, nuclear magnetic resonance, galactosidase and O-glycanase digestion, and anti-TF antibody. The results clearly showed that the Galβ1-3GalNAcα linkage was synthesized, confirming WbwCECO104 and WbwCECO5 as UDP-Gal:GalNAcα-diphosphate-lipid β1,3-Gal-transferases. Sequence analysis revealed a conserved DxDD motif, and mutagenesis showed the importance of these Asp residues in catalysis. The purified enzymes require divalent cations (Mn(2+)) for activity and are specific for UDP-Gal and GalNAc-diphosphate lipid substrates. WbwC was inhibited by bis-imidazolium salts having aliphatic chains of 18 to 22 carbons. This work will help to elucidate mechanisms of polysaccharide synthesis in pathogenic bacteria and provide technology for vaccine synthesis.

Biochemical characterization of a polysialyltransferase from Mannheimia haemolytica A2 and comparison to other bacterial polysialyltransferases

Polysialic acids are bioactive carbohydrates found in eukaryotes and some bacterial pathogens. The bacterial polysialyltransferases (PSTs), which catalyze the synthesis of polysialic acid capsules, have previously been identified in select strains of Escherichia coli and Neisseria meningitidis and are classified in the Carbohydrate-Active enZYmes Database as glycosyltransferase family GT-38. In this study using DNA sequence analysis and functional characterization we have identified a novel polysialyltransferase from the bovine/ovine pathogen Mannheimia haemolytica A2 (PST_Mh). The enzyme was expressed in recombinant form as a soluble maltose-binding-protein fusion in parallel with the related PSTs from E. coli K1 and N. meningitidis group B in order to perform a side-by-side comparison. Biochemical properties including solubility, acceptor preference, reaction pH optima, thermostability, kinetics, and product chain length for the enzymes were compared using a synthetic fluorescent acceptor molecule. PST_Mh exhibited biochemical properties that make it an attractive candidate for chemi-enzymatic synthesis applications of polysialic acid. The activity of PST_Mh was examined on a model glycoprotein and the surface of a neuroprogenitor cell line where the results supported its development for use in applications to therapeutic protein modification and cell surface glycan remodelling to enable cell migration at implantation sites to promote wound healing. The three PSTs examined here demonstrated different properties that would each be useful to therapeutic applications.

One-pot multi-enzyme (OPME) chemoenzymatic synthesis of sialyl-Tn-MUC1 and sialyl-T-MUC1 glycopeptides containing natural or non-natural sialic acid

A series of STn-MUC1 and ST-MUC1 glycopeptides containing naturally occurring and non-natural sialic acids have been chemoenzymatically synthesized from Tn-MUC1 glycopeptide using one-pot multienzyme (OPME) approaches. In situ generation of the sialyltransferase donor cytidine 5'-monophosphate-sialic acid (CMP-Sia) using a CMP-sialic acid synthetase in the presence of an extra amount of cytidine 5'-triphosphate (CTP) and removal of CMP from the reaction mixture by flash C18 cartridge purification allow the complete consumption of Tn-MUC1 glycopeptide for quantitative synthesis of STn-MUC1. A Campylobacter jejuni β1-3GalT (CjCgtBΔ30-His6) mutant has been found to catalyze the transfer of one or more galactose residues to Tn-MUC1 for the synthesis of T-MUC1 and galactosylated T-MUC1. Sialylation of T-MUC1 using Pasteurella multocida α2-3-sialyltransferase 3 (PmST3) with Neisseria meningitidis CMP-sialic acid synthetase (NmCSS) and Escherichia coli sialic acid aldolase in one pot produced ST-MUC1 efficiently. These glycopeptides are potential cancer vaccine candidates.

A single N-acetylgalactosamine residue at threonine 106 modifies the dynamics and structure of interferon α2a around the glycosylation site

Enzymatic addition of GalNAc to isotopically labeled IFNα2a produced in Escherichia coli yielded the O-linked glycoprotein GalNAcα-[(13)C,(15)N]IFNα2a. The three-dimensional structure of GalNAcα-IFNα2a has been determined in solution by NMR spectroscopy at high resolution. Proton-nitrogen heteronuclear Overhauser enhancement measurements revealed that the addition of a single monosaccharide unit at Thr-106 significantly slowed motions of the glycosylation loop on the nanosecond time scale. Subsequent addition of a Gal unit produced Gal(β1,3)GalNAcα-[(13)C,(15)N]IFNα2a. This extension resulted in a further decrease in the dynamics of this loop. The methodology used here allowed the first such description of the structure and dynamics of an O-glycoprotein and opens the way to the study of this class of proteins.

Characterization of α2,3- and α2,6-sialyltransferases from Helicobacter acinonychis

Genome sequence data were used to clone and express two sialyltransferase enzymes of the GT-42 family from Helicobacter acinonychis ATCC 51104, a gastric disease isolate from Cheetahs. The deposited genome sequence for these genes contains a large number of tandem repeat sequences in each of them: HAC1267 (RQKELE)(15) and HAC1268 (EEKLLEFKNI)(13). We obtained two clones with different numbers of repeat sequences for the HAC1267 gene homolog and a single clone for the HAC1268 gene homolog. Both genes could be expressed in Escherichia coli and sialyltransferase activity was measured using synthetic acceptor substrates containing a variety of terminal sugars. Both enzymes were shown to have a preference for N-acetyllactosamine, and they each made a product with a different linkage to the terminal galactose. HAC1267 is a mono-functional α2,3-sialyltransferase, whereas HAC1268 is a mono-functional α2,6-sialyltransferase and is the first member of GT-42 to show α2,6-sialyltransferase activity.

Identification and characterization of a lipopolysaccharide α,2,3-sialyltransferase from the human pathogen Helicobacter bizzozeronii

Terminal sialic acid in the lipopolysaccharides (LPSs) of mucosal pathogens is an important virulence factor. Here we report the characterization of a Helicobacter sialyltransferase involved in the biosynthesis of sialylated LPS in Helicobacter bizzozeronii, the only non-pylori gastric Helicobacter species isolated from humans thus far. Starting from the genome sequences of canine and human strains, we identified potential sialyltransferases downstream of three genes involved in the biosynthesis of N-acetylneuraminic acid. One of these candidates showed monofunctional α,2,3-sialyltransferase activity with a preference for N-acetyllactosamine as a substrate. The LPSs from different strains were shown by SDS-PAGE and high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) to contain sialic acid after neuraminidase treatment. The expression of this sialyltransferase and sialyl-LPS appeared to be a phase-variable characteristic common to both human and canine H. bizzozeronii strains. The sialylation site of the LPSs of two H. bizzozeronii strains was determined to be NeuAc-Hex-HexNAc, suggesting terminal 3'-sialyl-LacNAc. Moreover, serological typing revealed the possible presence of sialyl-Lewis X in two additional strains, indicating that H. bizzozeronii could also mimic the surface glycans of mammalian cells. The expression of sialyl-glycans may influence the adaptation process of H. bizzozeronii during the host jump from dogs to humans.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2007-2008

This review is the fifth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2008. The first section of the review covers fundamental studies, fragmentation of carbohydrate ions, use of derivatives and new software developments for analysis of carbohydrate spectra. Among newer areas of method development are glycan arrays, MALDI imaging and the use of ion mobility spectrometry. The second section of the review discusses applications of MALDI MS to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, biopharmaceuticals, glycated proteins, glycolipids, glycosides and various other natural products. There is a short section on the use of MALDI mass spectrometry for the study of enzymes involved in glycan processing and a section on the use of MALDI MS to monitor products of the chemical synthesis of carbohydrates with emphasis on carbohydrate-protein complexes and glycodendrimers. Corresponding analyses by electrospray ionization now appear to outnumber those performed by MALDI and the amount of literature makes a comprehensive review on this technique impractical. However, most of the work relating to sample preparation and glycan synthesis is equally relevant to electrospray and, consequently, those proposing analyses by electrospray should also find material in this review of interest.

Structural and kinetic analysis of substrate binding to the sialyltransferase Cst-II from Campylobacter jejuni

Sialic acids play important roles in various biological processes and typically terminate the oligosaccharide chains on the cell surfaces of a wide range of organisms, including mammals and bacteria. Their attachment is catalyzed by a set of sialyltransferases with defined specificities both for their acceptor sugars and the position of attachment. However, little is known of how this specificity is encoded. The structure of the bifunctional sialyltransferase Cst-II of the human pathogen Campylobacter jejuni in complex with CMP and the terminal trisaccharide of its natural acceptor (Neu5Ac-α-2,3-Gal-β-1,3-GalNAc) has been solved at 1.95 Å resolution, and its kinetic mechanism was shown to be iso-ordered Bi Bi, consistent with its dual acceptor substrate specificity. The trisaccharide acceptor is seen to bind to the active site of Cst-II through interactions primarily mediated by Asn-51, Tyr-81, and Arg-129. Kinetic and structural analyses of mutants modified at these positions indicate that these residues are critical for acceptor binding and catalysis, thereby providing significant new insight into the kinetic and catalytic mechanism, and acceptor specificity of this pathogen-encoded bifunctional GT-42 sialyltransferase.

Site-specific enzymatic polysialylation of therapeutic proteins using bacterial enzymes

The posttranslational modification of therapeutic proteins with terminal sialic acids is one means of improving their circulating half-life, thereby improving their efficiency. We have developed a two-step in vitro enzymatic modification of glycoproteins, which has previously only been achieved by chemical means [Gregoriadis G, Jain S, Papaioannou I, Laing P (2005) Int J Pharm 300:125-130). This two-step procedure uses the Campylobacter jejuni Cst-II α2,8-sialyltransferase to provide a primer on N-linked glycans, followed by polysialylation using the Neisseria meningitidis α2,8-polysialyltransferase. Here, we have demonstrated the ability of this system to modify three glycoproteins with varying N-linked glycan compositions: the human therapeutic proteins alpha-1-antitrypsin (A1AT) and factor IX, as well as bovine fetuin. The chain length of the polysialic acid addition was optimized by controlling reaction conditions. After demonstrating the ability of this system to modify a variety of proteins, the effect of polysialylation on the activity and serum half-life of A1AT was examined. The polysialylation of A1AT did not adversely affect its in vitro inhibition activity against human neutrophil elastase. The polysialylation of A1AT resulted in a significantly improved pharmacokinetic profile when the modified proteins were injected into CD-1 mice. Together, these results suggest that polysialylated A1AT may be useful for improved augmentation therapy for patients with a deficiency in this protein and that this modification may be applied to other therapeutic proteins.

Michael F, Karwaski MF, Brochu D, Jarrell HC, Parker CT, Yuki N, Mandrell RE, Gilbert M. Lipooligosaccharide of Campylobacter jejuni: similarity with multiple types of mammalian glycans beyond gangliosides

Campylobacter jejuni is well known for synthesizing ganglioside mimics within the glycan component of its lipooligosaccharide (LOS), which have been implicated in triggering Guillain-Barré syndrome. We now confirm that this pathogen is capable of synthesizing a much broader spectrum of host glycolipid/glycoprotein mimics within its LOS. P blood group and paragloboside (lacto-N-neotetraose) antigen mimicry is exhibited by RM1221, a strain isolated from a poultry source. RM1503, a gastroenteritis-associated strain, expresses lacto-N-biose and sialyl-Lewis c units, the latter known as the pancreatic tumor-associated antigen, DU-PAN-2 (or LSTa). C. jejuni GC149, a Guillain-Barré syndrome-associated strain, expresses an unusual sialic acid-containing hybrid oligosaccharide with similarity to both ganglio and Pk antigens and can, through phase variation of its LOS biosynthesis genes, display GT1a or GD3 ganglioside mimics. We show that the sialyltransferase CstII and the galactosyltransferase CgtD are involved in the synthesis of multiple mimic types, with LOS structural diversity achieved through evolving allelic substrate specificity.

Fluorescence activated cell sorting as a general ultra-high-throughput screening method for directed evolution of glycosyltransferases

Glycosyltransferases (GTs) offer very attractive approaches to the synthesis of complex oligosaccharides. However, the limited number of available GTs, together with their instability and strict substrate specificity, have severely hampered the broad application of these enzymes. Previous attempts to broaden the range of substrate scope and to increase the activity of GTs via protein engineering have met with limited success, partially because of the lack of effective high-throughput screening methods. Recently, we reported an ultra-high-throughput screening method for sialyltransferases based on fluorescence-activated cell sorting (Aharoni et al. Nat. Methods 2006, 3, 609-614). Here, we considerably improve this method via the introduction of a two-color screening protocol to minimize the probability of false positive mutants and demonstrate its generality through directed evolution of a neutral sugar transferase, beta-1,3-galactosyltransferase CgtB. A variant with broader substrate tolerance than the wild-type enzyme and 300-fold higher activity was identified rapidly from a library of >10(7) CgtB mutants. Importantly, the variant effected much more efficient synthesis of G(M1a) and asialo G(M1) oligosaccharides, the building blocks of important therapeutic glycosphingolipids, than did the parent enzyme. This work not only establishes a new methodology for the directed evolution of galactosyltransferases, but also suggests a powerful strategy for the screening of almost all GT activities, thereby facilitating the engineering of glycosyltransferases.

Genetic diversity in Campylobacter jejuni is associated with differential colonization of broiler chickens and C57BL/6J IL10-deficient mice

Previous studies have demonstrated that Campylobacter jejuni, the leading causative agent of bacterial food-borne disease in the USA, exhibits high-frequency genetic variation that is associated with changes in cell-surface antigens and ability to colonize chickens. To expand our understanding of the role of genetic diversity in the disease process, we analysed the ability of three C. jejuni human disease isolates (strains 11168, 33292 and 81-176) and genetically marked derivatives to colonize Ross 308 broilers and C57BL/6J IL10-deficient mice. C. jejuni colonized broilers at much higher efficiency (all three strains, 23 of 24 broilers) than mice (11168 only, 8 of 24 mice). C. jejuni 11168 genetically marked strains colonized mice at very low efficiency (2 of 42 mice); however, C. jejuni reisolated from mice colonized both mice and broilers at high efficiency, suggesting that this pathogen can adapt genetically in the mouse. We compared the genome composition in the three wild-type C. jejuni strains and derivatives by microarray DNA/DNA hybridization analysis; the data demonstrated a high degree of genetic diversity in three gene clusters associated with synthesis and modification of the cell-surface structures capsule, flagella and lipo-oligosaccharide. Finally, we analysed the frequency of mutation in homopolymeric tracts associated with the contingency genes wlaN (GC tract) and flgR (AT tracts) in culture and after passage through broilers and mice. C. jejuni adapted genetically in culture at high frequency and the degree of genetic diversity was increased by passage through broilers but was nearly eliminated in the gastrointestinal tract of mice. The data suggest that the broiler gastrointestinal tract provides an environment which promotes outgrowth and genetic variation in C. jejuni; the enhancement of genetic diversity at this location may contribute to its importance as a human disease reservoir.

Mechanistic investigation of the endo-alpha-N-acetylgalactosaminidase from Streptococcus pneumoniae R6

The large (1767-amino acid) endo-alpha-N-acetylgalactosaminidase from Streptococcus pneumoniae (SpGH101) specifically removes an O-linked disaccharide Gal-beta-1,3-GalNAc-alpha from glycoproteins. While the enzyme from natural sources has been used as a reagent for many years, very few mechanistic studies have been performed. Using the recently determined three-dimensional structure of the recombinant protein as a background, we report here a mechanistic investigation of the SpGH101 retaining alpha-glycoside hydrolase using a combination of synthetic and natural substrates. On the basis of a model of the substrate complex of SpGH101, we propose D764 and E796 as the nucleophile and general acid-base residues, respectively. These roles were confirmed by kinetic and mechanistic analysis of mutants at those positions using synthetic substrates and anion rescue experiments. pK(a) values of 5.3 and 7.2 were assigned to D764 and E796 on the basis of the pK(a) values derived from the bell-shaped dependence of k(cat)/K(m) upon pH. The enzyme contains several putative carbohydrate binding modules whose glycan binding specificities were probed using the printed glycan array of the Consortium for Functional Glycomics using the inactive D764A and D764F mutants that had been labeled with Alexafluor 488. These studies revealed binding to galacto-N-biose, consistent with a role for these domains in localizing the enzyme near its substrates.

Complete chemoenzymatic synthesis of the Forssman antigen using novel glycosyltransferases identified in Campylobacter jejuni and Pasteurella multocida

We have identified an alpha1,4-galactosyltransferase (CgtD) and a beta1,3-N-acetylgalactosaminyltransferase (CgtE) in the lipooligosaccharide (LOS) locus of Campylobacter jejuni LIO87. Strains that carry these genes may have the capability of synthesizing mimics of the P blood group antigens of the globoseries glycolipids. We have also identified an alpha1,3-N-acetylgalactosaminyltransferase (Pm1138) from Pasteurella multocida Pm70, which is involved in the synthesis of an LOS-bound Forssman antigen mimic and represents the only known bacterial glycosyltransferase with this specificity. The genes encoding the three enzymes were cloned and expressed in Escherichia coli as soluble recombinant proteins that can be used to chemoenzymatically synthesize the Forssman antigen, and its biosynthetic precursors, in high yields.

Glycosyltransferase-catalyzed synthesis of bioactive oligosaccharides

Mammalian cell surfaces are all covered with bioactive oligosaccharides which play an important role in molecular recognition events such as immune recognition, cell-cell communication and initiation of microbial pathogenesis. Consequently, bioactive oligosaccharides have been recognized as a medicinally relevant class of biomolecules for which the interest is growing. For the preparation of complex and highly pure oligosaccharides, methods based on the application of glycosyltransferases are currently recognized as being the most effective. The present paper reviews the potential of glycosyltransferases as synthetic tools in oligosaccharide synthesis. Reaction mechanisms and selected characteristics of these enzymes are described in relation to the stereochemistry of the transfer reaction and the requirements of sugar nucleotide donors. For the application of glycosyltransferases, accepted substrate profiles are summarized and the whole-cell approach versus isolated enzyme methodology is compared. Sialyltransferase-catalyzed syntheses of gangliosides and other sialylated oligosaccharides are described in more detail in view of the prominent role of these compounds in biological recognition.

Enzymatic synthesis of tumor-associated carbohydrate antigen Globo-H hexasaccharide

We report the enzymatic synthesis of an important tumor-associated carbohydrate antigen, Globo-H hexasaccharide. Starting with Lac-OBn as the initial acceptor, this approach employs three glycosyltransferases: LgtC, an alpha1,4-galactosyltransferase; LgtD, a bifunctional beta1,3-galactosyl/beta1,3-N-acetylgalactosaminyltransferase; and WbsJ, an alpha1,2-fucosyltransferase. In addition, two epimerases, GalE and WbgU, were also employed for the generation of more expensive sugar nucleotides, UDP-Gal and UDP-GalNAc, from their corresponding inexpensive C4 epimers. This study represents a facile enzymatic synthesis of the Globo-H antigen.