Enzyme-aided construction of medium-sized alditols of complete O-linked saccharides. The constructed hexasaccharide alditol Gal beta 1-4GlcNAc beta 1-6Gal beta 1-4GlcNAc beta 1-6(Gal beta 1-3)GalNAc-ol resists the action of endo-beta-galactosidase from Bacteroides fragilis

Helicobacter pylori β1,3-N-acetylglucosaminyltransferase for versatile synthesis of type 1 and type 2 poly-LacNAcs on N-linked, O-linked and I-antigen glycans

Poly-N-acetyllactosamine extensions on N- and O-linked glycans are increasingly recognized as biologically important structural features, but access to these structures has not been widely available. Here, we report a detailed substrate specificity and catalytic efficiency of the bacterial β3-N-acetylglucosaminyltransferase (β3GlcNAcT) from Helicobacter pylori that can be adapted to the synthesis of a rich diversity of glycans with poly-LacNAc extensions. This glycosyltransferase has surprisingly broad acceptor specificity toward type-1, -2, -3 and -4 galactoside motifs on both linear and branched glycans, found commonly on N-linked, O-linked and I-antigen glycans. This finding enables the production of complex ligands for glycan-binding studies. Although the enzyme shows preferential activity for type 2 (Galβ1-4GlcNAc) acceptors, it is capable of transferring N-acetylglucosamine (GlcNAc) in β1-3 linkage to type-1 (Galβ1-3GlcNAc) or type-3/4 (Galβ1-3GalNAcα/β) sequences. Thus, by alternating the use of the H. pylori β3GlcNAcT with galactosyltransferases that make the β1-4 or β1-3 linkages, various N-linked, O-linked and I-antigen acceptors could be elongated with type-2 and type-1 LacNAc repeats. Finally, one-pot incubation of di-LacNAc biantennary N-glycopeptide with the β3GlcNAcT and GalT-1 in the presence of uridine diphosphate (UDP)-GlcNAc and UDP-Gal, yielded products with 15 additional LacNAc units on the precursor, which was seen as a series of sequential ion peaks representing alternative additions of GlcNAc and Gal residues, on matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) analysis. Overall, our data demonstrate a broader substrate specificity for the H. pylori β3GlcNAcT than previously recognized and demonstrate its ability as a potent resource for preparative chemo-enzymatic synthesis of complex glycans.

The acceptor and site specificity of alpha 3-fucosyltransferase V. High reactivity of the proximal and low of the distal galbeta 1-4GlcNAc unit in i-type polylactosamines

We report here on in vitro acceptor and site specificity of recombinant alpha3-fucosyltransferase V (Fuc-TV) with 40 oligosaccharide acceptors. Galbeta1-4GlcNAc (LN) and GalNAcbeta1-4GlcNAc (LDN) reacted rapidly; Galbeta1-3GlcNAc (LNB) reacted moderately, and GlcNAcbeta1-4GlcNAc (N, N'-diacetyl-chitobiose) reacted slowly yet distinctly. In neutral and terminally alpha3-sialylated polylactosamines of i-type, the reducing end LN unit reacted rapidly and the distal (sialyl)LN group very slowly; the midchain LNs revealed intermediate reactivities. The data suggest that a distal LN neighbor enhances but a proximal LN neighbor reduces the reactivity of the midchain LNs. This implies that Fuc-TV may bind preferably the tetrasaccharide sequence Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAc for transfer at the underlined monosaccharide. Terminal alpha3-sialylation of i-type polylactosamines almost doubled the reactivities of the LN units at all positions of the chains. We conclude that, in comparison with human Fuc-TIV and Fuc-TIX, Fuc-TV reacted with a highly distinct site specificity with i-type polylactosamines. The Fuc-TV reactivity of free LNB resembled that of LNBbeta1-3'R of a polylactosamine, contrasting strongly with the dissimilarity of the reactivities of the analogous pair of LN and LNbeta1-3'R. This observation supports the notion that LN and LNB may be functionally bound at distinct sites on Fuc-TV surface. Our data show that Fuc-TV worked well with a very wide range of LN-glycans, showing weak reactivity only with distal (sialyl)LN units of i-type polylactosamines, biantennary N-glycans, and I branches of polylactosamines.

The beta 1,6-GlcNAc transferase activity present in hog gastric mucosal microsomes catalyses site-specific branch formation on a long polylactosamine backbone

We find that the beta 1,6-GlcNAc transferase activity present in hog gastric mucosal microsomes converts the linear pentasaccharide GlcNAc beta 1-3Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4GlcNAc (1) in a site-specific way to the branch-bearing hexasaccharide GlcNAc beta 1-3(GlcNAc beta 1-6)Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4GlcNAc (2). The product is a positional isomer of GlcNAc beta 1-3Gal beta 1-4GlcNAc beta 1-3(GlcNAc beta 1-6)Gal beta 1-4GlcNAc (3), reportedly formed from 1 by another polylactosamine beta 1,6-GlcNAc transferase activity present in human serum (Leppänen et al., Biochemistry, 30 (1991) 9287). Combined use of the two kinds of activities gave in the present experiments the heptasaccharide GlcNAc beta 1-3(GlcNAc beta 1-6)Gal beta 1-4GlcNAc beta 1-3(GlcNAc beta 1-6)Gal beta 1-4GlcNAc (4), in which one of the branches occupies the position of the branch in 2 and the other the position of the branch in 3.

In vitro biosynthesis of a decasaccharide prototype of multiply branched polylactosaminoglycan backbones

Multiply branched polylactosaminoglycans are expressed in glycoproteins and glycolipids of many cells. Interest in their biology stems from their abundant expression in early embryonal cells and from their ability to carry multiple lectin-binding determinants, which makes them prominent ligands and antagonists of cell adhesion proteins. A prototype of their backbones is represented by the decasaccharide LacNAc beta1-3'(LacNAc beta1-6')LacNAc beta1-3'(LacNAc beta1-6')LacNAc (5), where LacNAc is the disaccharide Gal beta1-4GlcNAc. Here, we describe in vitro biosynthesis of glycan 5. Incubation of the linear hexasaccharide LacNAc beta1-3'LacNAc beta1-3'LacNAc (1) with UDP-GlcNAc and alpha midchain beta1,6-GlcNAc transferase activity (GlcNAc to Gal), present in rat serum [Gu, J., Nishikawa, A., Fujii, S., Gasa, S., & Taniguchi, N. (1992) J. Biol. Chem. 267, 2994-2999], gave the doubly branched octasaccharide LacNAc beta1-3'(GlcNAc beta1-6')LacNAc beta1-3'(GlcNAc beta1-6')LacNAc (4). The latter was converted to 5 by enzymatic beta1,4-galactosylation. In the initial branching reaction of 1, two isomeric heptasaccharide intermediates, LacNAc beta1-3'LacNAc beta1-3'(GlcNAc beta1-6')LacNAc (2) and LacNAc beta1-3'(GlcNAc beta1-6')LacNAc beta1-3'LacNAc (3), were formed first at comparable rates. Later, both intermediates were converted to 4, revealing two distinct pathways of the reaction: 1 --> 2 --> 4 and 1 --> 3 --> 4. These data suggest that, regardless of their chain length, linear polylactosamines similar to 1 contain potential branching sites at each of the internal galactoses. The enzyme-binding epitope of 1 is probably LacNAc beta1-3'LacNAc, because the trisaccharides GlcNAc beta1-3'LacNAc and LacNAc beta1-3Gal as well as the tetrasaccharide GlcNAc beta1-3'LacNAc beta1-3Gal were poor acceptors, while LacNAc beta1-3'LacNAc was a good one. Midchain beta1,6-GlcNAc transferase activities present in serum of several mammalian species, including man, resembled closely the rat serum activity in their mode of action and in their acceptor specificity. We suggest that analogous membrane-bound Golgi enzymes are involved in the biosynthesis of multiply branched polylactosamines in vivo.

1H and 13C NMR analysis of the pentasaccharide Gal beta (1-->4)GlcNAc beta (1-->3)-[GlcNAc beta (1-->6)]Gal beta (1-->4)GlcNAc synthesized by the mid-chain beta-(1-->6)-D-N-acetylglucosaminyltransferase of rat serum

Chemical shifts and coupling constants of completely assigned 1H and 13C NMR spectra at 500 MHz, as well as ROESY and HMBC connectivities were used to establish the structure of the pentasaccharide Gal beta (1-->4)GlcNAc beta (1-->3)[GlcNAc beta (1-->6)]Gal beta (1-->4)GlcNAc, synthesized by the action of the mid-chain beta-(1-->6)-D-N-acetylglucosaminyltransferase of rat serum from UDP-GlcNAc and the linear tetrasaccharide Gal beta (1-->4)GlcNAc beta (1-->3)Gal beta (1-->4)GlcNAc.

Synthesis of a divalent sialyl Lewis x O-glycan, a potent inhibitor of lymphocyte-endothelium adhesion. Evidence that multivalency enhances the saccharide binding to L-selectin

The recognition of cell-surface L-selectin by its carbohydrate ligands causes lymphocytes to roll on capillary endothelium at sites of inflammation. As this primary contact is a prerequisite for extravasation of the leukocytes to the tissue, its inhibition by free oligosaccharides capable of competing with the natural L-selectin ligands in an attractive therapeutic possibility. The exact structures of the biological ligands of L-selectin are not yet known, but the principal carbohydrate epitopes share some structural features: they are O-glycosidically linked mucin-type oligosaccharides with N-acetyllactosamine backbone, which is 3'-sialylated or 3'-sulfated, 3-fucosylated and sometimes 6- or 6'-sulfated at the distal N-acetyllactosamine termini. Multivalency of the ligand, which is believed to enhance the binding, is achieved by a branched polylactosamine backbone or by a clustered array of O-glycans. We report here enzymic synthesis of a large oligosaccharide fulfilling several of the features characteristic to the L-selectin ligands: it is a dodecameric O-glycosidic core-2-type oligosaccharide alditol with a branched polylactosamine backbone carrying two distal alpha-2,3'-sialylated and alpha-1,3-fucosylated N-acetyl-lactosamine groups (sialyl Lewis x, sialyl Le(x)). The structure of each saccharide on the synthesis route from disaccharide Gal beta 1-3GalNAc to the dodecasaccharide alditol was established by several methods including one- and two-dimensional 1H-NMR spectroscopy. The last step of the synthesis, the alpha-1,3-fucosylation of the 6-linked arm proceeded sluggishly, and was associated with a noticeable shift in H1 resonance of the GlcNAc residue of the branch-bearing N-acetyllactosamine unit. The final synthesis product and its analogs lacking one or both of the fucose residues were tested as inhibitors of L-selectin-mediated lymphocyte-endothelium interaction in vitro in rejecting rat kidney transplant. While the non-fucosylated O-glycosidic oligosaccharide alditol did not possess any inhibitory activity, the mono-fucosylated one (i.e. monovalent sialyl Le(x)) prevented the binding significantly and the difucosylated dodecasaccharide alditol (i.e. divalent sialyl Le(x)) was a very potent inhibitor (IC50, inhibitory concentration preventing 50% of binding = 0.15 microM). Besides the multivalency, also the Gal beta 1-3GalNAc-ol sequence of the O-glycosidic core appeared to increase the affinity of the glycan to L-selectin. This was indicated by parallel inhibition experiments, where a disialylated and difucosylated branched polylactosamine decasaccharide, similar to the divalent dodecasaccharide alditol, but lacking the reduced O-glycosidic core, was a less effective inhibitor (IC50 = 0.5 microM) than the O-glycosidic dodecasaccharide alditol.