Substrate and donor specificity of glycosyl transferases

Facile chemoenzymatic synthesis of Lewis a (Lea) antigen in gram-scale and sialyl Lewis a (sLea) antigens containing diverse sialic acid forms

An efficient streamlined chemoenzymatic approach has been developed for gram-scale synthesis of Lewis a angtigen (Le^aβProN₃) and a library of sialyl Lewis a antigens (sLe^aβProN₃) containing different sialic acid forms. Intially, commercially available inexpensive N-acetylglucosamine (GlcNAc) was converted to its N'-glycosyl p-toluenesulfonohydrazide in one step. Followed by chemical glycosylation, GlcNAcβProN₃ was synthesized using this protecting group-free method in high yield (82%). Sequential one-pot multienzyme (OPME) β1-3-galactosylation of GlcNAcβProN₃ followed by OPME α1-4-fucosylation reactions produced target Le^aβProN₃ in gram-scale. Structurally diverse sialic acid forms was successfully introduced using a OPME sialylation reation containing a CMP-sialic acid synthetase and Pasteurella multocida α2-3-sialyltransferase 1 (PmST1) mutant PmST1 M144D with or without a sialic acid aldolase to form sLe^aβProN₃ containing naturally occurring or non-natural sialic acid forms in preparative scales.

The logic of automated glycan assembly

Carbohydrates are the most abundant biopolymers on earth and part of every living creature. Glycans are essential as materials for nutrition and for information transfer in biological processes. To date, in few cases a detailed correlation between glycan structure and glycan function has been established. A molecular understanding of glycan function will require pure glycans for biological, immunological, and structural studies. Given the immense structural complexity of glycans found in living organisms and the lack of amplification methods or expression systems, chemical synthesis is the only means to access usable quantities of pure glycan molecules. While the solid-phase synthesis of DNA and peptides has become routine for decades, access to glycans has been technically difficult, time-consuming and confined to a few expert laboratories. In this Account, the development of a comprehensive approach to the automated synthesis of all classes of mammalian glycans, including glycosaminoglycans and glycosylphosphatidyl inositol (GPI) anchors, as well as bacterial and plant carbohydrates is described. A conceptual advance concerning the logic of glycan assembly was required in order to enable automated execution of the synthetic process. Based on the central glycosidic bond forming reaction, a general concept for the protecting groups and leaving groups has been developed. Building blocks that can be procured on large scale, are stable for prolonged periods of time, but upon activation result in high yields and selectivities were identified. A coupling-capping and deprotection cycle was invented that can be executed by an automated synthesis instrument. Straightforward postsynthetic protocols for cleavage from the solid support as well as purification of conjugation-ready oligosaccharides have been established. Introduction of methods to install selectively a wide variety of glycosidic linkages has enabled the rapid assembly of linear and branched oligo- and polysaccharides as large as 30-mers. Fast, reliable access to defined glycans that are ready for conjugation has given rise to glycan arrays, glycan probes, and synthetic glycoconjugate vaccines. While an ever increasing variety of glycans are accessible by automated synthesis, further methodological advances in carbohydrate chemistry are needed to make all possible glycans found in nature. These tools begin to fundamentally impact the medical but also materials aspects of the glycosciences.

Automated synthesis of sialylated oligosaccharides

Sialic acid-containing glycans play a major role in cell-surface interactions with external partners such as cells and viruses. Straightforward access to sialosides is required in order to study their biological functions on a molecular level. Here, automated oligosaccharide synthesis was used to facilitate the preparation of this class of biomolecules. Our strategy relies on novel sialyl α-(2→3) and α-(2→6) galactosyl imidates, which, used in combination with the automated platform, provided rapid access to a small library of conjugation-ready sialosides of biological relevance.

Synthesis and photolytic activation of 6''-O-2-nitrobenzyl uridine-5'-diphosphogalactose: a 'caged' UDP-Gal derivative

Placing an 2-nitrobenzyl group on O-6 of the galactosyl residue in uridine-5'-diphosphogalactose (UDP-Gal) gives 6''-O-2-nitrobenzyl-UDP-Gal that is shown to be inactive as a donor substrate for beta-(1-->4)-galactosyltransferase (GalT). On irradiation at 365 nm, the nitrobenzyl group is completely removed yielding native UDP-Gal that then transfers normally to produce the expected betaGal-(1-->4)-betaGlcNAc disaccharidic linkage. 6''-O-2-Nitrobenzyl-UDP-Gal thus fulfils the minimum requirements of a 'caged' UDP-Gal for application in time-resolved crystallographic studies of beta-(1-->4)-GalT.

Synthesis of a sialic acid alpha(2-3) galactose building block and its use in a linear synthesis of sialyl Lewis X

[reaction: see text] The ubiquity of the sialic acid alpha(2-3) galactose linkage in oligosaccharides of biological relevance necessitates a building block for the incorporation of this motif into oligosaccharides prepared by modular synthesis. The linear synthesis of the sialyl Lewis X tumor-associated antigen (1) has been accomplished in good yield using a sialic acid alpha(2-3) galactose disaccharide building block. The disaccharide building block was synthesized efficiently from readily available galactal by a high-yielding and selective sialylation reaction.

Formation of lactones from sialylated MUC1 glycopeptides

The tumor-associated carbohydrate antigens TN, T, sialyl TN and sialyl T are expressed on mucins in several epithelial cancers. This has stimulated studies directed towards development of glycopeptide-based anticancer vaccines. Formation of intramolecular lactones involving sialic acid residues and suitably positioned hydroxyl groups in neighboring saccharide moieties is known to occur for glycolipids such as gangliosides. It has been suggested that these lactones are more immunogenic and tumor-specific than their native counterparts and that they might find use as cancer vaccines. We have now investigated if lactonization also occurs for the sialyl TN and T antigens of mucins. It was found that the model compound sialyl T benzyl glycoside , and the glycopeptide Ala-Pro-Asp-Thr-Arg-Pro-Ala from the tandem repeat of the mucin MUC1, in which Thr stands for the 2,3-sialyl-T antigen, lactonized during treatment with glacial acetic acid. Compound gave the 1''--> 2' lactone as the major product and the corresponding 1''--> 4' lactone as the minor product. For glycopeptide the 1''--> 4' lactone constitued the major product, whereas the 1''--> 2' lactone was the minor one. When lactonized was dissolved in water the 1''--> 4' lactone underwent slow hydrolysis, whereas the 1''--> 2' remained stable even after a 30 days incubation. In contrast the corresponding 2,6-sialyl-TN glycopeptide did not lactonize in glacial acetic acid.

Rational design, synthesis, and characterization of novel inhibitors for human beta1,4-galactosyltransferase

An affinity labeling reagent, uridine 5'-(6-amino-{2-[(7-bromomethyl-2-naphthyl)methoxycarbonylmethoxy]ethoxy}acetyl-6-deoxy-alpha-D-galactopyranosyl) diphosphate (1a), was designed on the basis of 3D docking simulation and synthesized to investigate the functional role of Trp310 residue located in the small loop near the active site of human recombinant galactosyltransferase (betaGalT-1). Mass spectrometric analysis revealed that the Trp310 residue of betaGalT1 can be selectively modified with the naphthylmethyl group of compound 1a at the C-3 position of the indole ring. This result motivated us to synthesize novel uridine-5'-diphosphogalactose (UDP-Gal) analogues as candidates for mechanism-based inhibitors for betaGalT-1. We found that uridine 5'-(6-O-[10-(2-naphthyl)-3,6,9-trioxadecanyl]-alpha-d-galactopyranosyl) diphosphate (2) is the strongest inhibitor (K(i) = 1.86 microM) against UDP-Gal (Km = 4.91 microM) among compounds reported previously. A cold spray ionization time-of-flight mass spectrometry study demonstrated that the complex of this inhibitor and betaGalT-1 cannot interact with an acceptor substrate in the presence of Mn2+.

Molecular cloning and functional expression of a novel Helicobacter pylori α-1,4 fucosyltransferase

Helicobacter pylori is an important human pathogen which causes both gastric and duodenal ulcers and is associated with gastric cancer and lymphoma. This microorganism synthesizes fucosylated oligosaccharides, predominantly the Galb-1,4GlcNAc (Type II) blood group antigens Lewis X and Y, whereas a small population also expresses the Galb-1,3GlcNAc (Type I) blood group antigens Lewis A and B. These carbohydrate structures are known to mimic host cell antigens and permit the bacteria to escape from the host immune response. Here, we report the cloning and characterization of a novel H. pylori alpha-1,4 fucosyltransferase (FucT). In contrast to the family members characterized to date, this enzyme shows exclusively Type I acceptor substrate specificity. The enzyme consisting of 432 amino acids (MW 50,502 Da) was cloned using a polymerase chain reaction (PCR)-based approach. It exhibits a high degree of identity (75-87%) and similar structural features, for example, in the heptamer repeat pattern, with other H. pylori FucTs. The kinetic characterization revealed a very efficient transferase (k(cat)/Km = 229 mM(-1) s(-1)) for the Type I acceptor substrate (Gal)-1,3 GlcNAc-Lem (1). Additionally, the enzyme possesses a broad tolerance toward nonnatural Type I acceptor substrate analogs and therefore represents a valuable tool for the chemoenzymatic synthesis of Lewis A, sialyl Lewis A as well as mimetics thereof.

C-terminal amino acids of Helicobacter pylori alpha1,3/4 fucosyltransferases determine type I and type II transfer

The alpha1,3/4 fucosyltransferase (FucT) enzyme from Helicobacter pylori catalyzes fucose transfer from donor GDP-beta-l-fucose to the GlcNAc group of two series of acceptor substrates in H. pylori lipopolysaccharide: betaGal1,3betaGlcNAc (Type I) or betaGal1,4betaGlcNAc (Type II). Fucose is added either in alpha1,3 linkage of Type II acceptor to produce Lewis X or in alpha1,4 linkage of Type I acceptor to produce Lewis A, respectively. H. pylori FucTs from different strains have distinct Type I or Type II substrate specificities. FucT in H. pylori strain NCTC11639 has an exclusive alpha1,3 activity because it recognizes only Type II substrates, whereas FucT in H. pylori strain UA948 can utilize both Type II and Type I acceptors; thus it has both alpha1,3 and alpha1,4 activity, respectively. To identify elements conferring substrate specificity, 12 chimeric FucTs were constructed by domain swapping between 11639FucT and UA948FucT and characterized for their ability to transfer fucose to Type I and Type II acceptors. Our results indicate that the C-terminal region of H. pylori FucTs controls Type I and Type II acceptor specificity. In particular, the highly divergent C-terminal portion, seven amino acids DNPFIFC at positions 347-353 in 11639FucT, and the corresponding 10 amino acids CNDAHYSALH at positions 345-354 in UA948FucT, controls the Type I and Type II acceptor recognition. This is the opposite of mammalian FucTs where acceptor preference is determined primarily by the N-terminal residues in the hypervariable stem domain.

Chemical and biological strategies for engineering cell surface glycosylation

Oligosaccharides play a crucial role in many of the recognition, signaling, and adhesion events that take place at the surface of cells. Abnormalities in the synthesis or presentation of these carbohydrates can lead to misfolded and inactive proteins, as well as to several debilitating disease states. However, their diverse structures, which are the key to their function, have hampered studies by biologists and chemists alike. This review presents an overview of techniques for examining and manipulating cell surface oligosaccharides through genetic, enzymatic, and chemical strategies.

An inhibitor of the human UDP-GlcNAc 4-epimerase identified from a uridine-based library: a strategy to inhibit O-linked glycosylation

The biological study of O-linked glycosylation is particularly problematic, as chemical tools to control this modification are lacking. An inhibitor of the UDP-GlcNAc 4-epimerase that synthesizes UDP-GalNAc, the donor initiating O-linked glycosylation, would be a powerful reagent for reversibly inhibiting O-linked glycosylation. We synthesized a 1338 member library of uridine analogs directed to the epimerase by virtue of substrate mimicry. Screening of the library identified an inhibitor with a K(i) value of 11 microM. Tests against related enzymes confirmed the compound's specificity for the UDP-GlcNAc 4-epimerase. Inhibitors of a key step of O-linked glycan biosynthesis can be discovered from a directed library screen. Progeny thereof may be powerful tools for controlling O-linked glycosylation in cells.

Molecular recognition of sialyl Lewis(x) and related saccharides by two lectins

The interaction of sialyl Lewis(x), Lewis(x), and alpha-L-Fuc-(1-->3)-beta-D-GlcNAc with isolectin A from Lotus tetragonolobus (LTL-A), and with Aleuria aurantia agglutinin (AAA) was studied using NMR experiments and surface plasmon resonance. Both lectins are specific for fucose residues. From NMR experiments it was concluded that alpha-L-Fuc-(1-->3)-beta-D-GlcNAc and Lewis(x) bound to both lectins, whereas sialyl Lewis(x) only bound to AAA. Increased line broadening of 1H NMR signals of the carbohydrate ligands upon binding to AAA and LTL-A suggested that AAA bound to the ligands more tightly. Further comparison of line widths showed that for both lectins binding strengths decreased from alpha-L-Fuc-(1-->3)-beta-D-GlcNAc to Lewis(x) and were lowest for sialyl Lewis(x). Surface plasmon resonance measurements were then employed to yield accurate dissociation constants. TrNOESY, QUIET-trNOESY, and trROESY experiments delivered bioactive conformations of the carbohydrate ligands, and STD NMR experiments allowed a precise epitope mapping of the carbohydrates bound to the lectins. The bioactive conformation of Lewis(x) bound to LTL-A, or AAA revealed an unusual orientation of the fucose residue, with negative values for both dihedral angles, phi and psi, at the alpha(1-->3)-glycosidic linkage. A similar distortion of the fucose orientation was also observed for sialyl Lewis(x) bound to AAA. From STD NMR experiments it followed that only the L-fucose residues are in intimate contact with the protein. Presumably steric interactions are responsible for locking the sialic acid residue of sialyl Lewis(x) in one out of many orientations that are present in aqueous solution. The sialic acid residue of sialyl Lewis(x) bound to AAA adopts an orientation similar to that in the corresponding sialyl Lewis(x)/E-selectin complex.

Synthesis of the milk oligosaccharide 2'-fucosyllactose using recombinant bacterial enzymes

The enzymatic synthesis of GDP-beta-L-fucose and its enzymatic transfer reaction using recombinant enzymes from bacterial sources was examined. The GDP-D-mannose 4,6-dehydratase and the GDP-4-keto-6-deoxy-D-mannose 3,5-epimerase-4-reductase from Escherichia coli K-12, respectively, were used to catalyse the conversion of GDP-alpha-D-mannose to GDP-beta-L-fucose with 78% yield. For the transfer of the L-fucose to an acceptor, we cloned and overproduced the alpha-(1-->2)-fucosyltransferase (FucT2) protein from Helicobacter pylori. We were able to synthesise 2'-fucosyllactose using the overproduced FucT2 enzyme, enzymatically synthesised GDP-L-fucose and lactose. The isolation of 2'-fucosyllactose was accomplished by anion-exchange chromatography and gel filtration to give 65% yield.

Large-scale production of oligosaccharides using engineered bacteria

Rapid advances in the cloning and expression of glycosyltransferase genes, especially from bacteria, could open the way to overcoming difficulties in the mass production of oligosaccharides. The large-scale production of oligosaccharides using either glycosyltransferases isolated from engineered microorganisms or whole cells as an enzyme source could promote a new era in the field of carbohydrate synthesis.

Substrate specificity of fucosyl transferase III: An efficient synthesis of sialyl Lewisx-, sialyl Lewisa-derivatives and mimetics thereof

Fucosyl transferase III (FucT III) has previously been characterized as the most general enzyme of the FucT family, as judged from its ability to catalyze the transfer of fucose to both Galβ(1-3)GlcNAc and Galβ(1-4)GlcNAc. In order to explore the synthetic potential of FucT III for the enzymatic synthesis of sialyl Lewisx and sialyl Lewisa derivatives, its substrate specificity has been probed using a number of natural substrate mimetics. A remarkable range of acceptor substrates was found when N-acetyl glucosamine was replaced by D-glucal, (R,R)-1,2-cyclohexanediol and (R,R)-butan-2,3-diol. Although the reaction rates were low compared to the reaction with the natural substrates, they proved to be sufficient for the synthesis of preparative amounts.Key words: fucosyl transferase III, sialyl Lewisa, sialyl Lewisx, carbohydrate mimetics.