Use of the "core-2"-N-acetylglucosaminyltransferase in the chemical-enzymatic synthesis of a sialyl-LeX-containing hexasaccharide found on O-linked glycoproteins

Complete enzymic synthesis of the mucin-type sialyl Lewis x epitope, involved in the interaction between PSGL-1 and P-selectin

Sialyl Lewis x (sLe(x)) is an established selectin ligand occurring on N- and O-linked glycans. Using a completely enzymic approach starting from p-nitrophenyl N-acetyl-alpha-D-galactosaminide (GalNAc(alpha1-pNp as core substrate, the sLe(x)-oligosaccharide Neu5Ac(alpha2-3)Gal(beta1-4)[Fuc(alpha1-3)]GlcNAc(beta1-6)[Gal(bet a1-3)]GalNAc(alpha1-pNp, representing the O-linked form, was synthesized in an overall yield of 32%. In a first step, Gal(beta1-3)GalNAc(alpha1-pNp was prepared in a yield of 52% using UDP-Gal and an enriched preparation of beta3-galactosyltransferase (EC 2.4.1.122) from rat liver. UDP-GlcNAc and a recombinant affinity-purified preparation of core 2 beta6-N-acetylglucosaminyltransferase (EC 2.4.1.102) fused to Protein A were used to branch the core 1 structure, affording GlcNAc(beta1-6)[Gal(beta1-3)]GalNAc(alpha1-pNp in a yield of >85%. The core 2 structure was galactosylated using UDP-Gal and purified human milk beta4-galactosyltransferase 1 (EC 2.4.1.38) (yield of >85%), then sialylated using CMP-Neu5Ac and purified recombinant alpha3-sialyltransferase 3 (EC 2.4.99.X) (yield of 87%), and finally fucosylated using GDP-Fuc and recombinant human alpha3-fucosyltransferase 6 (EC 2.4.1.152) produced in Pichia pastoris (yield of 100%). Overall 1.5 micromol of product was prepared. MALDI TOF mass spectra, and 1D and 2D TOCSY and ROESY 1H NMR analysis confirmed the obtained structure.

The yeast expression system for recombinant glycosyltransferases

Glycosyltransferases are increasingly being used for in vitro synthesis of oligosaccharides. Since these enzymes are difficult to purify from natural sources, expression systems for soluble forms of the recombinant enzymes have been developed. This review focuses on the current state of development of yeast expression systems. Two yeast species have mainly been used, i.e. Saccharomyces cerevisiae and Pichia pastoris. Safety and ease of fermentation are well recognized for S. cerevisiae as a biotechnological expression system; however, even soluble forms of recombinant glycosyltransferases are not secreted. In some cases, hyperglycosylation may occur. P. pastoris, by contrast, secrete soluble orthoglycosylated forms to the supernatant where they can be recovered in a highly purified form. The review also covers some basic features of yeast fermentation and describes in some detail those glycosyltransferases that have successfully been expressed in yeasts. These include beta1,4galactosyltransferase, alpha2,6sialyltransferase, alpha2,3sialyltransferase, alpha1,3fucosyltransferase III and VI and alpha1,2mannosyltransferase. Current efforts in introducing glycosylation systems of higher eukaryotes into yeasts are briefly addressed.

Synthesis of poly-N-acetyllactosamine in core 2 branched O-glycans. The requirement of novel beta-1,4-galactosyltransferase IV and beta-1,3-n-acetylglucosaminyltransferase

Poly-N-acetyllactosamine is a unique carbohydrate composed of N-acetyllactosamine repeats and provides the backbone structure for additional modifications such as sialyl Lex. Poly-N-acetyllactosamines in mucin-type O-glycans can be formed in core 2 branched oligosaccharides, which are synthesized by core 2 beta-1,6-N-acetylglucosaminyltransferase. Using a beta-1, 4-galactosyltransferase (beta4Gal-TI) present in milk and the recently cloned beta-1,3-N-acetylglucosaminyltransferase, the formation of poly-N-acetyllactosamine was found to be extremely inefficient starting from a core 2 branched oligosaccharide, GlcNAcbeta1-->6(Galbeta1-->3)GalNAcalpha-->R. Since the majority of synthesized oligosaccharides contained N-acetylglucosamine at the nonreducing ends, galactosylation was judged to be inefficient, prompting us to test novel members of the beta4Gal-T gene family for this synthesis. Using various synthetic acceptors and recombinant beta4Gal-Ts, beta4Gal-TIV was found to be most efficient in the addition of a single galactose residue to GlcNAcbeta1-->6(Galbeta1-->3)GalNAcalpha-->R. Moreover, beta4Gal-TIV, together with beta-1,3-N-acetylglucosaminyltransferase, was capable of synthesizing poly-N-acetyllactosamine in core 2 branched oligosaccharides. On the other hand, beta4Gal-TI was found to be most efficient for poly-N-acetyllactosamine synthesis in N-glycans. In contrast to beta4Gal-TI, the efficiency of beta4Gal-TIV decreased dramatically as the acceptors contained more N-acetyllactosamine repeats, consistent with the fact that core 2 branched O-glycans contain fewer and shorter poly-N-acetyllactosamines than N-glycans in many cells. These results, as a whole, indicate that beta4Gal-TIV is responsible for poly-N-acetyllactosamine synthesis in core 2 branched O-glycans.

Identification of an N-acetylglucosamine-6-0-sulfotransferase activity specific to lymphoid tissue: an enzyme with a possible role in lymphocyte homing

BACKGROUND

The leukocyte adhesion molecule L-selection participates in the initial attachment of blood-borne lymphocytes to high endothelial venules (HEVs) during lymphocyte homing to secondary lymphoid organs, and contributes to leukocyte adhesion and extravasation in HEV-like vessels at sites of chronic inflammation. The L-selection ligands on lymph mode HEVs are mucin-like glycoproteins adorned with the unusual sulfated carbohydrate epitope, 6-sulfo sialyl Lewis x. Sulfation of this epitope on the N-acetylglucosamine (GlcNAc) residue confers high-avidity L-selection binding, and is thought to be restricted in the vasculature to sites of sustained lymphocyte recruitment. The GlcNAc-6-0 sulfotransferase that installs the sulfate ester may be a key modulator of lymphocyte recruitment to secondary lymphoid organs and sites of chronic inflammation and is therefore a potential target for anti-inflammatory therapy.

RESULTS

A GlcNAc-6-0-sulfotransferase activity was identified within porcine lymph nodes and characterized using a rapid, sensitive, and quantitative assay. We synthesized two unnatural oligosaccharide substrates, GlcNAc beta 1-->6Gal alpha-R and Gal beta 1-->4GlcNAc beta 1-->6Gal alpha-R, that incorporate structural motifs from the native L-selection ligands into an unnatural C-glycosyl hydrocarbon scaffold. The sulfotransferase incorporated greater than tenfold more sulfate into the disaccharide than the trisaccharide, indicating a requirement for a terminal GlcNAc. Activity across tissues was highly restricted to the HEVs within peripheral lymph node.

CONCLUSIONS

The restricted expression of the GlcNAc-6-0-sulfotransferase activity to lymph node HEVs strongly suggestions a role in the biosynthesis of L-selection ligands. In addition, similar sulfated epitopes are known to be expressed on HEV-like vessels of chronically inflamed tissues; indicating that this sulfotransferase may also contribute to inflammatory lymphocyte recruitment. We identified a concise disaccharide motif, GlcNAc beta 1-->6Gal alpha-R, that preserved both recognition and specificity determinants for the GlcNAc-6-0-sulfotransferase. The absence of activity on the trisaccharide Gal beta 1-->6Gal alpha-R indicates a requirement for a substrate with a terminal GlcNAc residue, suggesting that sulfation precedes further biosynthetic assembly of L-selection ligands.

Sequential interchange of four amino acids from blood group B to blood group A glycosyltransferase boosts catalytic activity and progressively modifies substrate recognition in human recombinant enzymes

The human blood group A and B glycosyltransferase enzymes are highly homologous and the alteration of four critical amino acid residues (Arg-176 --> Gly, Gly-235 --> Ser, Leu-266 --> Met, and Gly-268 --> Ala) is sufficient to change the enzyme specificity from a blood group A to a blood group B glycosyltransferase. To carry out a systematic study, a synthetic gene strategy was employed to obtain their genes and to allow facile mutagenesis. Soluble forms of a recombinant glycosyltransferase A and a set of hybrid glycosyltransferase A and B mutants were expressed in Escherichia coli in high yields, which allowed them to be kinetically characterized extensively for the first time. A functional hybrid A/B mutant enzyme was able to catalyze both A and B reactions, with the kcat being 5-fold higher for the A donor. Surprisingly, even a single amino acid replacement in glycosyltransferase A with the corresponding residue from glycosyltransferase B (Arg-176 --> Gly) produced enzymes with glycosyltransferase A activity only, but with very large (11-fold) increases in the kcat and increased specificity. The increases observed in kcat are among the largest obtained for a single amino acid change and are advantageous for the preparative scale synthesis of blood group antigens.

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.

Synthesis of pentasaccharide analogues of the N-glycan substrates of N-acetylglucosaminyltransferases III, IV and V using tetrasaccharide precursors and recombinant beta-(1-->2)-N-acetylglucosaminyltransferase II

Recombinant human UDP-GlcNAc: alpha-Man-(1-->6)R beta-(1-->2)-N-acetylglucosaminyltransferase II (EC 2.4.1.143, GlcNAc-T II) was produced in the Sf9 insect cell/baculovirus expression system as a fusion protein with a (His)6 tag and partially purified by affinity chromatography on a metal chelating column. The partially purified enzyme was used to catalyze the transfer of GlcNAc from UDP-GlcNAc to R-alpha-Man(1-->6)(beta-GlcNAc(1-->2)alpha-Man(1-->3))beta-Man-O-octyl to form beta-GlcNAc(1-->2)R-alpha-Man(1-->6)(beta-GlcNAc(1-->2)alpha- Man(1-->3))beta-Man-O-octyl where there is either no modification of the alpha-Man(1-->6) residue (7), or where R is 3-deoxy (8), 4-deoxy (9) or 6-deoxy (10). The yields ranged from 64-80%. Products were characterized by 1H and 13C nuclear magnetic resonance spectroscopy and fast atom bombardment mass spectrometry. Compounds 7-10 are pentasaccharide analogues of the biantennary N-glycan substrates of N-acetylglucosaminyltransferases III, IV and V.

Structure of the O-glycans in GlyCAM-1, an endothelial-derived ligand for L-selectin

L-selectin, the leukocyte selectin, mediates the carbohydrate-dependent attachment of circulating leukocytes to endothelium, preceding emigration into tissues. It functions in inflammatory leukocyte trafficking and in lymphocyte homing to lymph nodes. From previous work, the binding of L-selectin to endothelial-associated glycoprotein ligands, GlyCAM-1 and CD34, requires oligosaccharide sialylation, sulfation, and probably fucosylation. We have recently identified a major capping group in GlyCAM-1 as 6' sulfated sialyl Lewis x, a novel structure which potentially satisfies all of these requirements. In the present study, we define the complete structure of beta-eliminated chains of GlyCAM-1 using metabolic radiolabeling, plant lectin binding, and glycosidase digestions in conjunction with high pH anion-exchange chromatography. The majority of the O-glycans in GlyCAM-1 contain the T-antigen, i.e. Gal beta 1-->3GalNAc, which is incorporated into the core-2 structure, i.e. Gal beta 1-->3[GlcNAc beta 1-->6]GalNAc or larger core structures with additional GlcNAc residues. The structures of two O-glycans, based on core-2, were determined to be: [sequence: see text] The implications of these structures and more complex O-glycans for binding by L-selectin are discussed.

Synthesis of tetrasaccharide analogues of the N-glycan substrate of beta-(1-->2)-N-acetylglucosaminyltransferase II using trisaccharide precursors and recombinant beta-(1-->2)-N-acetylglucosaminyltransferase I

Recombinant rabbit UDP-GlcNAc: alpha-Man-(1-->3R) beta-(1-->2)-N-acetylglucosaminyl-transferase I (EC 2.4.1.101, GlcNAc-T I) produced in the Sf9 insect cell/baculovirus expression system has been used to convert compounds of the form 3-R-alpha-Man(1-->6)(alpha-Man(1-->3)) beta-Man-O-octyl to 3-R-alpha-Man(1-->6)(beta-GlcNAc(1-->2)alpha-Man(1-->3)) beta-Man-O-octyl where R is OH (14), O-methyl (17), O-pentyl (18), O-(4,4-azo)pentyl (19), O-(5-iodoacetamido)pentyl (20) and O-(5-amino)pentyl (21); 2-deoxy-alpha-Man(1-->6)(beta-GlcNAc(1-->2) alpha-Man(1-->3)) beta-Man-O-octyl (16), 4-O-methyl-alpha-Man(1-->6) (beta-GlcNAc(1-->2) alpha-Man(1-->3)) beta-Man-O-octyl (22), 6-O-methyl-alpha-Man(1-->6)(beta-GlcNAc(1-->2) alpha Man(1-->3)) beta-Man-O-octyl (23) and alpha-Man(1-->6)[beta-GlcNAc(1-->2)(4-O-methyl) alpha-Man(1-->3)] beta-Man-O-octyl (15) were also synthesized by this procedure. The yields ranged from 80 to 99%. Products were characterized by high resolution 1H and 13C nuclear magnetic resonance spectroscopy and fast atom bombardment mass spectrometry. Compounds 14, 15, 17, 22, and 23 are excellent substrates for UDP-GlcNAc: alpha-Man(1-->6R) beta-(1-->2)-N-acetylglucosaminyltransferase II and the other compounds are inhibitors of this enzyme.

Expression of recombinant rabbit UDP-GlcNAc: alpha 3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I catalytic domain in Sf9 insect cells

UDP-GlcNAc: alpha 3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GnT I; EC 2.4.1.101) catalyses a key reaction in the conversion of oligomannose to complex and hybrid N-glycans. The cytoplasmic tail and transmembrane segment of rabbit GnT I cDNA were replaced with an in-frame cleavable signal sequence and the hybrid construct was inserted into the genome of Autographa californica nuclear polyhedrosis virus (AcMNPV) under the control of the polyhedrin promoter. Sf9 insect cells were infected with the recombinant baculovirus and the enzymatically active and soluble catalytic domain of GnT I was purified from the medium (1-5 mg l-1) in two steps to a specific activity of about 2 mumol min-1 mg-1 protein. Recombinant GnT I has been used for the chemical-enzymatic synthesis of analogues of Man alpha 1-6]GlcNAc beta 1-2Man alpha 1-3]Man beta-O-octyl.

Conversion of GalNAc beta(1-4)GlcNAc beta-OMe into GalNAc beta (1-4)-[Fuc alpha(1-3)]GlcNAc beta-OMe using human milk alpha 3/4-fucosyltransferase. Synthesis of a novel terminal element in glycoprotein glycans

Incubation of GalNAc beta(1-4)GlcNAc beta-OMe with GDP-Fuc in the presence of human milk alpha 3/4-fucosyltransferase resulted in the formation of GalNAc beta(1-4)[Fuc alpha(1-3)]GlcNAc beta-OMe. Under conditions that led to complete alpha 3-fucosylation of Gal beta(1-4)GlcNAc beta-OEt, GalNAc beta(1-4)GlcNAc beta-OMe was fucosylated for more than 85%. For the identification of the isolated fucosylated products one- and two-dimensional 1H-NMR spectroscopy was applied. In vacuo molecular dynamics simulations of Gal beta(1-4)[Fuc alpha(1-3)]GlcNAc beta-OEt and GalNAc beta(1-4)[Fuc alpha(1-3)]GlcNAc beta-OMe using the CHARMm based force field CHEAT, demonstrated only small differences between the conformations of these compounds. This illustrates the minor conformational influence of the substituent at C-2', i.e. a hydroxyl function versus a N-acetyl group.