Characterisation of oligosaccharides released from human-blood-group O erythrocyte glycopeptides by the endo-beta-galactosidase of Bacteroides fragilis. A study of the enzyme susceptibility of branched poly(N-acetyllactosamine) structures

Enzymatic Synthesis of Oligosaccharides and Neoglycoconjugates

Oligosaccharides involved in glycoconjugates play important roles in a number of biological events. To elucidate the biological functions of oligosaccharides, sufficient quantities of structurally defined oligosaccharides, are of limited availability by traditional purification methods, are required. Hence, chemical and enzymatic syntheses of oligosaccharides are becoming increasingly important in glycobiology and glycotechnology. In addition, oligosaccharides often occur as glycoconjugates attached to proteins or lipids. Hence, the development of simple and effective methods for synthesizing neoglycoconjugates such as neoglycoprotein and neoglycolipids is essential for an understanding of the biological function of these molecules. Here we review the most recent developments in the enzymatic synthesis of oligosaccharides and neoglycoconjugates.

Enzymatic synthesis of poly-N-acetyllactosamines as potential substrates for endo-β-galactosidase-catalyzed hydrolytic and transglycosylation reactions

Enzymatic synthesis of GlcNAc-terminated poly-N-acetyllactosamine beta-glycosides GlcNAcbeta1,3(Galbeta1,4GlcNAcbeta1,3)(n)Galbeta1,4GlcNAcbeta-pNP (n=1-4) was demonstrated using a transglycosylation reaction of Escherichia freundii endo-beta-galactosidase. The enzyme catalyzed a transglycosylation reaction on GlcNAcbeta1,3Galbeta1,4GlcNAcbeta-pNP (1), which served both as a donor and an acceptor, and converted 1 into p-nitrophenyl beta-glycosides GlcNAcbeta1,3(Galbeta1,4GlcNAcbeta1,3)(1)Galbeta1,4GlcNAcbeta-pNP (2), GlcNAcbeta1,3(Galbeta1,4GlcNAcbeta1,3)(2)Galbeta1,4GlcNAcbeta-pNP (3), GlcNAcbeta1,3(Galbeta1,4GlcNAcbeta1,3)(3)Galbeta1,4GlcNAcbeta-pNP (4) and GlcNAcbeta1,3(Galbeta1,4GlcNAcbeta1,3)(4)Galbeta1,4GlcNAcbeta-pNP (5). When 2 was used as an initial substrate, it led to the preferential synthesis of nonasaccharide beta-glycoside 4 to heptasaccharide beta-glycoside 3. This suggests that 4 is directly synthesized by transferring the tetrasaccharide unit GlcNAcbeta1,3Galbeta1,4GlcNAcbeta1,3Gal to nonreducing end GlcNAc residue of 2 itself. The efficiency of production of poly-N-acetyllactosamines by E. freundii endo-beta-galactosidase was significantly enhanced by the addition of BSA and by a low-temperature condition. Resulting 2 and 3 were shown to be useful for studying endo-beta-galactosidase-catalyzed hydrolytic and transglycosylation reactions.

Kinetic studies on endo-beta-galactosidase by a novel colorimetric assay and synthesis of N-acetyllactosamine-repeating oligosaccharide beta-glycosides using its transglycosylation activity

Novel chromogenic substrates for endo-beta-galactosidase were designed on the basis of the structural features of keratan sulfate. Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAcbeta-pNP (2), which consists of two repeating units of N-acetyllactosamine, was synthesized enzymatically by consecutive additions of GlcNAc and Gal residues to p-nitrophenyl beta-N-acetyllactosaminide. In a similar manner, GlcNAcbeta1-3Galbeta1-4GlcNAcbeta-pNP (1), GlcNAcbeta1-3Galbeta1-4Glcbeta-pNP (3), Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glcbeta-pNP (4), Galbeta1-3GlcNAcbeta1-3Galbeta1-4Glcbeta-pNP (5), and Galbeta1-6GlcNAcbeta1-3Galbeta1-4Glcbeta-pNP (6) were synthesized as analogues of 2. Endo-beta-galactosidases released GlcNAcbeta-pNP or Glcbeta-pNP in an endo-manner from each substrate. A colorimetric assay for endo-beta-galactosidase was developed using the synthetic substrates on the basis of the determination of p-nitrophenol liberated from GlcNAcbeta-pNP or Glcbeta-pNP formed by the enzyme through a coupled reaction involving beta-N-acetylhexosaminidase (beta-NAHase) or beta-d-glucosidase. Kinetic analysis by this method showed that the value of Vmax/Km of 2 for Escherichia freundii endo-beta-galactosidase was 1.7-times higher than that for keratan sulfate, indicating that 2 is very suitable as a sensitive substrate for analytical use in an endo-beta-galactosidase assay. Compound 1 still acts as a fairly good substrate despite the absence of a Gal group in the terminal position. In addition, the hydrolytic action of the enzyme toward 2 was shown to be remarkably promoted compared to that of 4 by the presence of a 2-acetamide group adjacent to the p-nitrophenyl group. This was the same in the case of a comparison of 1 and 3. Furthermore, the enzyme also catalysed a transglycosylation on 1 and converted it into GlcNAcbeta1-3Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAcbeta-pNP (9) and GlcNAcbeta1-3Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAcbeta-pNP (10) as the major products, which have N-acetyllactosamine repeating units.

N-linked glycans containing linear poly-N-acetyllactosamine as sorting signals in endocytosis in Trypanosoma brucei

African trypanosomes, such as Trypanosoma brucei, are protozoan parasites that are transmitted by the tsetse fly and cause sleeping sickness in humans and Nagana in cattle. Trypanosomes evade the immune responses of their hosts by varying their surface coat protein (VSG) and restricting exocytosis and endocytosis to an invagination of the plasma membrane called the flagellar pocket (FP). The FP represents only 0.5% of the cellular surface but membrane turnover here occurs at high rates [1] [2] [3]. No model has yet been proposed to account for the sequestration of membrane proteins and the rate of membrane turnover that occur in the FP. Recent data have suggested that glycans are involved in the sorting of membrane proteins in polarized cells [4] [5] [6] [7]. Here, we show that N-linked glycans containing linear poly-N-acetyllactosamine (pNAL) are only associated with proteins of the FP/endocytic pathway in T. brucei and are present only in bloodstream forms of the parasite. These glycoproteins bind to tomato lectin (TL), a property that allowed their single-step isolation. Chito-oligosaccharides that compete specifically for pNAL binding to TL also inhibited receptor-mediated uptake of several ligands. These results suggest a model in which N-linked linear pNAL acts as a sorting signal for endocytosis in trypanosomes.

Structural characterisation of two forms of procyclic acidic repetitive protein expressed by procyclic forms of Trypanosoma brucei

A procyclic acidic repetitive protein (PARP) fraction was purified from long-term cultures of Trypanosoma brucei procyclic forms by a solvent-extraction and reverse phase chromatography procedure. The PARP fraction yielded small quantities of a single N-linked oligosaccharide with the structure Man alpha1-6(Man alpha1-3)Man alpha1-6(Man alpha1-3)Manbeta1-4GlcNAcbeta1-4GlcNAc (Man5GlcNAc2). Fractionation of PARP on Con A-Sepharose revealed that the majority (80 to 90%) of the PARP fraction did not bind to Con A and was composed of the parpA alpha gene product that contains repeats of -Glu-Pro-Pro-Thr- (GPEET-PARP) and that lacks an N-glycosylation site. This form of PARP has not been previously identified at the protein-level. The minor Con-A-binding fraction was shown to be rich in the previously described form of PARP, encoded by the parpAbeta and/or parpB alpha genes, that contains a -Glu-Pro- repeat domain (EP-PARP) and an N-glycosylation site. Analysis of longer and shorter-term cultures suggested that procyclic cells initially express predominantly EP-PARP that is gradually replaced by GPEET-PARP. Both forms of PARP were shown to contain indistinguishable glycosylphosphatidylinositol (GPI) membrane anchors, where the conserved GPI core structure is substituted by heterogeneous sialylated branched polylactosamine-like structures that are predicted to form a dense surface glycocalyx above which the polyanionic -Glu-Pro-Pro-Thr- and -Glu-Pro- repeat domains are displayed. The phosphatidylinositol (PI) component of the GPI anchor was shown to be a mixture of 2-O-acyl-myo-inositol-1-HPO4-(sn-1-stearoyl-2-lyso-glycerol) and 2-O-acyl-myo-inositol-1-HPO4-(sn-1-octadecyl-2-lyso-glycerol), where the acyl chain substituting the inositol ring showed considerable heterogeneity. Mass spectrometric and light scattering experiments both suggested an average mass of approximately 15 kDa for GPEET-PARP, with individual glycoforms ranging from about 12 kDa to 20 kDa, that is consistent with its amino acid and carbohydrate composition. A measured translational diffusion coefficient of 3.9 x 10(7) cm2 s(-1) indicates that this molecule has a highly elongated shape. The possible functions of these unusual glycoproteins are discussed.

Brain contains HNK-1 immunoreactive O-glycans of the sulfoglucuronyl lactosamine series that terminate in 2-linked or 2,6-linked hexose (mannose)

The monoclonal antibody HNK-1 originally raised to an antigenic marker of natural killer cells also binds to selected regions in nervous tissue. The antigen is a carbohydrate that has attracted much interest as its expression is developmentally regulated in nervous tissue, and it is found, and proposed to be a ligand, on several of the adhesive glycoproteins of the nervous system. It is also expressed on glycolipids and proteoglycans, and is the target of monoclonal auto-antibodies that give rise to a demyelinating disease. The epitope, as characterized on glycolipids isolated from the nervous system, is expressed on 3-sulfated glucuronic acid joined by beta1-3-linkage to a neolacto backbone. Here we exploit the neoglycolipid technology, in conjunction with immunodetection and in situ liquid secondary ion mass spectrometry, to characterize HNK-1-positive oligosaccharide chains derived by reductive alkaline release from total brain glycopeptides. The immunoreactive oligosaccharides detected are tetra- to octasaccharides that are very minor components among a heterogeneous population, each representing less than 0.1% of the starting material. Their peripheral and backbone sequences resemble those of the HNK-1-positive glycolipids. An unexpected finding is that they terminate not with N-acetylgalactosaminitol but with hexitol (2-substituted and 2,6-disubstituted). In a tetrasaccharide investigated in the greatest detail, the hexitol is identified as 2-substituted mannitol.

The glycosylation of the influenza A virus hemagglutinin by mammalian cells. A site-specific study

We have characterized the glycans at individual sites on the hemagglutinin of three influenza A variants to obtain information on the role of cell-specific glycosylation in determining the receptor binding properties of this virus. The variants differ in whether they have a glycosylation site at residue 129 on the tip of the hemagglutinin and whether amino acid 184 (near to the receptor binding site) is His or Asn. We found that all sites on each variant are glycosylated in Madin-Darby bovine kidney cells, that the glycosylation is site-specific, and that the glycans at the same site in each variant are highly similar. One site that is buried in the hemagglutinin trimer contains only oligomannose glycans. The remaining sites carry complex glycans of increasing size as the distance of the site from the viral membrane decreases. Most of these complex glycans are terminated with alpha-galactose residues, a consequence in bovine cells of the removal of terminal sialic acids by the viral neuraminidase. Although the glycans at residue 129 are among the smallest on the molecule, they are large enough to reach the receptor binding pocket on their own and adjacent monomers. The results suggest that the reduction in receptor binding observed with Madin-Darby bovine kidney cell-grown virus is due to the combined effect of large complex glycans at the tip of the hemagglutinin and a His to Asn substitution close to the receptor binding pocket.

Characterization of mouse ZP3-derived glycopeptide, gp55, that exhibits sperm receptor and acrosome reaction-inducing activity in vitro

During fertilization, free-swimming mouse sperm bind to mZP3 (approximately 83 000 Mr), one of three zona pellucida glycoproteins, and once bound undergo the acrosome reaction, a type of cellular exocytosis [Wassarman, P. M., & Litscher, E. S. (1995) Curr. Top. Dev. Biol. 30, 1-19]. Sperm recognize and bind to specific serine/threonine-linked oligosaccharides located at the mZP3 combining site for sperm. Here, we examined certain characteristics of gp55, a approximately 55 000 Mr glycopeptide derived from the carboxy-terminal half of mZP3 polypeptide to which sperm bind [Rosiere, T. K., & Wassarman, P. M. (1992) Dev. Biol. 154, 309-317]. gp55 is heterogeneous with respect to Mr (approximately 47 000-62 000 Mr) and has a relatively low pI (approximately 4.3-4.5) compared to the polypeptide portion of the glycopeptide (pI approximately 6.5). gp55 inhibits binding of sperm to eggs (i.e., exhibits sperm receptor activity) and induces sperm to undergo the acrosome reaction in vitro at about the same concentrations required for intact mZP3 (approximately 50-200 nM). Each of three different size-fractions of gp55, separated by SDS-PAGE, also exhibits bioactivity in vitro. Removal of asparagine-linked (N-linked) oligosaccharides from gp55, by extensive digestion with N-glycanase, reduces its Mr to approximately 21 000 and increases it pI to approximately 5.3, but does not significantly affect its ability to inhibit binding of sperm to eggs or to induce sperm to undergo the acrosome reaction. Similarly, digestion of gp55 with either endo-beta-galactosidase or neuraminidase alters its Mr and/or pI, but does not significantly affect either of its bioactivities. These observations are consistent with the proposal that neither N-linked oligosaccharides nor sialic acid is an essential element of the mZP3 combining site for sperm. They also indicate that a relatively small mZP3 glycopeptide is able to induce sperm to undergo the acrosome reaction (i.e., cellular exocytosis) in vitro.

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

We have constructed by enzyme-aided in vitro synthesis a hexasaccharide alditol Gal beta 1-4GlcNAc beta 1-6Gal beta 1-4GlcNAc beta 1-6(Gal beta 1-3) GalNAc-ol and shown that it resists the action of endo-beta-galactosidase from Bacteroides fragilis under conditions where a related pentasaccharide alditol, GlcNAc beta 1-3Gal beta 1-4GlcNAc beta 1-6(Gal beta 1-3)GalNAc-ol, was completely cleaved. Together with earlier results from this laboratory, our present data imply that endo-beta-galactosidase from B. fragilis, apparently, can be used to distinguish between GlcNAc beta 1-6Gal and GlcNAc beta 1-3Gal units within linear backbone sequences of all known types of oligo-(N-acetyllactosamino)glycans.

A simple purification of procyclic acidic repetitive protein and demonstration of a sialylated glycosyl-phosphatidylinositol membrane anchor

The procyclic acidic repetitive protein is the major cell-surface glycoprotein of the insect-dwelling procyclic forms of the Trypanosoma brucei species of African trypanosomes. The glycoprotein contains an acidic Glu-Pro repeat domain, a glycosyl-phosphatidylinositol membrane anchor and a putative asparagine glycosylation site. In this paper we describe a rapid purification scheme for this glycoprotein, using solvent extraction and hydrophobic interaction chromatography, and a partial characterization of the glycosylphosphatidylinositol membrane anchor. The carbohydrate composition of the anchor is extremely unusual; it contains on average nine GlcNAc, nine Gal, and five sialic acid residues. This is the first description of such a heavily substituted and negatively charged anchor. A comparison between the trypanosome procyclic surface and the Leishmania promastigote surface is also presented.

Novel sulfated ligands for the cell adhesion molecule E-selectin revealed by the neoglycolipid technology among O-linked oligosaccharides on an ovarian cystadenoma glycoprotein

E-selectin is the inducible adhesion protein on the surface of endothelial cells which has a crucial role in the initial stages of recruitment of leucocytes to sites of inflammation. In addition, it is almost certainly involved in tumor cell adhesion and metastasis. This report is concerned with identification of a new class of oligosaccharide ligand--sulfate-containing--for the human E-selectin molecule from among oligosaccharides on an ovarian cystadenoma glycoprotein. This has been achieved by application of the neoglycolipid technology to oligosaccharides released from the glycoprotein by mild alkaline beta-elimination. Oligosaccharides were conjugated to lipid, resolved by thin-layer chromatography, and tested for binding by Chinese hamster ovary cells which had been transfected to express the full-length E-selectin molecule. Several components with strong E-selectin binding activity were revealed among acidic oligosaccharides. The smallest among these was identified by liquid secondary ion mass spectrometric analysis of the neoglycolipid, in conjunction with methylation analysis of the purified oligosaccharide preparation as an equimolar mixture of the Le(a)- and Le(x)/SSEA-1-type fucotetrasaccharides sulfated at position 3 of outer galactose: [formula: see text] To our knowledge this is the first report of a sulfofucooligosaccharide ligand for E-selectin. The binding activity is substantially greater than those of lipid-linked Le(a) and Le(x)/SSEA-1 sequences and is at least equal to that of the 3'-sialyl-Le(x)/SSEA-1 glycolipid analogue.

[Characterization of murine monoclonal antibodies against fetal erythrocytes]

Balb/c mice were immunized against papain-treated fetal erythrocytes and splenocytes were fused with Sp2/0-Ag-14 myeloma cells. Several hybrids secreting antibodies directed against antigenic determinants predominantly exposed on fetal and cord cells were selected and cloned twice. Antibodies NaM61-1A2 and NaM61-768 (IgM class) were shown to be specific for an endo-beta-galactosidase-sensitive oligosaccharide chain. The antigen, strongly expressed on fetal and cord cells, was identified as the i blood group antigen. The antibodies represent powerful blood group reagents to be use in conventional agglutination techniques as well as in the gel typing system and in indirect flow cytometry. The antibody NaM46-4A8 (IgG class) is specific for an antigenic structure expressed on fetal cells and accessible only after papain, ficin, bromelin and endo-beta galactosidase treatment. The antigen was not identified.

Epitope expression and partial structural characterization of F62 lipooligosaccharide (LOS) of Neisseria gonorrhoeae: IgM monoclonal antibodies (3F11 and 1-1-M) recognize non-reducing termini of the LOS components

F62 LOS of Neisseria gonorrhoeae consists of two components. The higher molecular weight (MW) component is recognized by monoclonal antibody (MAb) 1-1-M and the smaller MW component by MAb 3F11. Epitope expression of the two LOS components and their partial structures were investigated by treating the F62 LOS with several glycosidases and then monitoring their antigenicity with the two mouse IgM MAbs. The 1-1-M-defined LOS component was cleaved with both beta-N-acetylhexosaminidase and endo-beta-galactosidase, and each cleavage resulted in the loss of expression of the 1-1-M-defined epitope. The N-acetylhexosamine (HexNAc) released by the hexosaminidase was found to be GalNAc, and the smaller oligosaccharide released by the endo enzyme was identified to be a dimer GalNAc beta----Gal. In contrast, the MAb 3F11-defined LOS component was not digested by the endo galactosidase, but it was cleaved with alpha and beta-galactosidase, and expression of the MAb 3F11-defined LOS epitope expression of the MAb 3F11-defined LOS was abolished by the treatment with each of two exo enzymes. MAb 3F11 bound to the 1-1-M-defined LOS component resulting from the removal of the beta-GalNAc residue, and the resulting LOS was further cleaved with beta-galactosidase, but not with alpha-galactosidase. From these results, we conclude the following: (1) MAbs 1-1-M and 3F11 both recognize the non-reducing termini of the LOS components; (2) the 1-1-M-defined LOS component has the GalNAc beta----Gal beta 1----4-Glc (or GlcNAc) structure, and the GalNAc beta----Gal residue is involved in the MAb 1-1-M-defined epitope; (3) the MAb 3F11-defined LOS component may not have a Gal beta 1----4GlcNAc beta 1----4Gal beta 1----4Glc structure within the molecule. However, it has beta-Gal residue at its non-reducing terminus, and this residue is involved in the MAb 3F11-defined epitope; (4) the two LOS components share a similar antigenic structure, and the 3F11-defined epitope structure is present in the MAb 1-1-M-defined LOS component. Expression of this epitope within the 1-1-M-defined LOS molecule is blocked by the beta-GalNAc residue; however, the beta-GalNAc residue at the non-reducing end may be not the only structural difference between the two components.

Synthesis and characterization of 2-O-beta-lactosylglycerol, 1,2-di-O-beta-lactosyl-(R,S)-glycerols, and 1,2,3,-tri-O-beta-lactosylglycerol

The reaction of 2,3,6,2',3',4',6'-hepta-O-acetyl-alpha-lactosyl bromide (5) and 1,3-di-O-benzylglycerol in the presence of mercury(II) cyanide in benzene-nitromethane afforded 1,3-di-O-benzyl-2-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lactosyl)glyc erol (70%), which was converted into 2-O-beta-lactosylglycerol. 1,2-Di-O-beta-lactosyl-(R,S)-glycerols were obtained by way of the coupling of 5 to either 1-O-benzyl-(R,S)-glycerol or 1-O-benzyl-2-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lactosyl)-(R,S)-gl ycerols. The most efficient route to 1,2, 3-tri-O-beta-lactosylglycerol (17) involved treatment of 2-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lactosyl)glycerol with 3 mol. equiv. of 5 followed by removal of the blocking groups, to give 17 (47%).

Single mid-chain GlcNAc beta 1-6Gal beta 1-4R sequences of linear oligosaccharides are resistant to endo-beta-galactosidase of Bacteroides fragilis

Endo-beta-galactosidase (EC 3.2.1.103) of Bacteroides fragilis, at 250 mU ml-1, did not cleave the internal galactosidic linkage of the linear radiolabelled trisaccharide GlcNAc beta 1-6Gal beta 1-4GlcNAc, or those of tetrasaccharides Gal beta 1-4GlcNAc beta 1-6Gal beta 1-4GlcNAc and Gal beta 1-4GlcNAc beta 1-6Gal beta 1-4Glc. The isomeric glycans which contained the GlcNAc beta 1-3Gal beta 1-4GlcNAc/Glc sequence were readily cleaved.

Determination of the branch location of extra N-acetyllactosamine units in sialo N-linked tetraantennary oligosaccharides

An approach is presented for the determination of the branch location of 1 or 2 extra N-acetyllactosamine units in sialo N-linked carbohydrate chains from glycoproteins. Tetraantennary oligosaccharides containing extra N-acetyllactosamine units were digested with endo-beta-galactosidase, followed by treatment with N-acetyl-beta-glucosaminidase, yielding products which could be analysed by 1H-NMR spectroscopy, thereby giving conclusive data about the location of the extra units in the intact structures.

Identification of a novel oligosaccharide backbone structure with a galactose residue monosubstituted at C-6 in human foetal gastrointestinal mucins

An oligosaccharide purified from a major penta- to hexa-saccharide fraction of human meconium glycoproteins has been shown by m.s. and n.m.r. analysis to have a novel backbone structure containing an internal galactose residue monosubstituted at C-6 by N-acetylglucosamine: (Formula: see text). This oligosaccharide may represent a biosynthetic product of a previously unrecognized N-acetylglucosaminyltransferase catalysing formation of a linear GlcNAc beta 1-6Gal sequence.

500 MHz 1H n.m.r. of oligosaccharides of N-acetyl-lactosamine-type released from human erythrocyte glycopeptides by endo-beta-galactosidase

500 MHz 1H n.m.r. spectroscopy has been used in structural studies of three linear and five branched oligosaccharides of N-acetyl-lactosamine-type that were released from desialylated blood group O erythrocyte glycopeptides by treatment with the endo-beta-galactosidase of Bacteroides fragilis followed by reduction. The following oligosaccharide alditols were characterized: (formula; see book)