Enzymatic midchain branching of polylactosamine backbones is restricted in a site-specific manner in alpha 1,3-fucosylated chains

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.

Fuc-TIX: a versatile alpha1,3-fucosyltransferase with a distinct acceptor- and site-specificity profile

alpha1,3-Fucosyltransferases (Fuc-Ts) convert N-acetyllactosamine (LN, Galbeta1-4GlcNAc) to Galbeta1-4(Fucalpha1-3)GlcNAc, the Lewis x (CD15, SSEA-1) epitope, which is involved in various recognition phenomena. We describe details of the acceptor specificity of alpha1,3-fucosyltransferase IX (Fuc-TIX). The unconjugated N- and O-glycan analogs LNbeta1-2Man, LNbeta1-6Manalpha1-OMe, LNbeta1-2Manalpha1-3(LNbeta1-2Manalpha1-6)Manbeta1-4GlcNAc, and Galbeta1-3(LNbeta1-6)GalNAc reacted well in vitro with Fuc-TIX present in lysates of appropriately transfected Namalwa cells. Fuc-TIX reacted well with the reducing end LN of GlcNAcbeta1-3'LN (underscored site reacted) and GlcNAcbeta1-3'LNbeta1-3'LN (both LNs reacted), but very poorly with the reducing end LN of LNbeta1-3'LN. However, Fuc-TIX reacted significantly better with the non-reducing end LN as compared to the other LN units in the glycans LNbeta1-3'LN and LNbeta1-3'LNbeta1-3'LNbeta1-3'LN, confirming our previous data on LNbeta1-3'LNbeta1-OR. In contrast, the sialylated glycan Neu5Acalpha2-3'LNbeta1-3'LNbeta1-3'LNbeta1-3'LN was fucosylated preferentially at the two most reducing end LN units. We conclude that Fuc-TIX is a versatile alpha1,3-Fuc-T, that (1) generates distal Lewis x epitopes from many different acceptors, (2) possesses inherent ability for the biosynthesis of internal Lewis x epitopes on growing polylactosamine backbones, and (3) fucosylates the remote internal LN units of alpha2,3-sialylated i-type polylactosamines.

Fingerprinting of large oligosaccharides linked to ceramide by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: highly heterogeneous polyglycosylceramides of human erythrocytes with receptor activity for Helicobacter pylori

Highly microheterogeneous polyglycosylceramides (PGCs) of human erythrocytes with an average composition of about 25 monosaccharides linked to ceramide were analyzed by matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS). The human gastric pathogen Helicobacter pylori was earlier shown to bind this glycosphingolipid mixture by thin-layer chromatogram binding assay. The receptor activity was present along the whole nonresolved chromatographic interval. Mass spectra of intact PGCs were compared with corresponding spectra of oligosaccharides enzymatically released from the ceramides. Two subfractions of PGCs containing less than one and more than one sialic acid residue per molecule were used. MALDI-MS spectra were recorded in both linear and reflectron mode with the accuracies of </=0.08% and </=0.02%, respectively, which allowed determination of the constituent parts of the detected ions in form of ceramide and number of hexoses, N-acetylhexosamines, fucoses and sialic acids. Molecular species were found based on ceramide with mainly sphingosine and fatty acids 24:0 and 24:1 (with less amounts of 22:0), and with a total number of monosaccharides ranging from 11 (neutral, m/z = 2605 for [M+Na](+)) to 41 (one sialic acid, m/z = 8057 for [M-H](-)). The saccharide composition obtained supported a successively extended and branched N -acetyllactosamine core with substitutions of fucoses (0 up to 8) and sialic acid (0 to 1). The reliable molecular analysis of large oligosaccharides linked to ceramide using this approach will be of great help for the further structure analysis in order to define the epitope for the sialic acid-dependent binding by the bacterium.

Enzymatic synthesis of alpha3'sialylated and multiply alpha3fucosylated biantennary polylactosamines. A bivalent [sialyl diLex]-saccharide inhibited lymphocyte-endothelium adhesion organ-selectively

Multifucosylated sialo-polylactosamines are known to be high affinity ligands for E-selectin. PSGL-1, the physiological ligand of P-selectin, is decorated in HL-60 cells by a sialylated and triply fucosylated polylactosamine that is believed to be of functional importance. Mimicking some of these saccharide structures, we have synthesized enzymatically a bivalent [sialyl diLex]-glycan, Neu5Acalpha2-3'Lexbeta1-3'Lexbeta1-3'(Neu5Acalpha2-3'Lexbeta1-3Lexbe ta1-6')LN [where Neu5Ac is N-acetylneuraminic acid, Lex is the trisaccharide Galbeta1-4(Fucalpha1-3)GlcNAc and LN is the disaccharide Galbeta1-4GlcNAc]. Several structurally related, novel polylactosamine glycans were also constructed. The inhibitory effects of these glycans on two L-selectin-dependent, lymphocyte-to-endothelium adhesion processes of rats were analysed in ex-vivo Stamper-Woodruff binding assays. The IC50 value of the bivalent [sialyl diLex]-glycan at lymph node high endothelium was 50 nm, but at the capillaries of rejecting cardiac allografts it was only 5 nm. At both adhesion sites, the inhibition was completely dependent on the presence of fucose units on the sialylated LN units of the inhibitor saccharide. These data show that the bivalent [sialyl diLex]-glycan is a high affinity ligand for L-selectin, and may reduce extravasation of lymphocytes at sites of inflammation in vivo without severely endangering the normal recirculation of lymphocytes via lymph nodes.

The centrally acting beta1,6N-acetylglucosaminyltransferase (GlcNAc to gal). Functional expression, purification, and acceptor specificity of a human enzyme involved in midchain branching of linear poly-N-acetyllactosamines

In the present experiments the cDNA coding for a truncated form of the beta1,6N-acetylglucosaminyltransferase responsible for the conversion of linear to branched polylactosamines in human PA1 cells was expressed in Sf9 insect cells. The catalytic ectodomain of the enzyme was fused to glutathione S-transferase, allowing effective one-step purification of the glycosylated 67-74-kDa fusion protein. Typically a yield of 750 microg of the purified protein/liter of suspension culture was obtained. The purified recombinant protein catalyzed the transfer of GlcNAc from UDP-GlcNAc to the linear tetrasaccharide Galbeta1-4GlcNAcbeta1-3Galbeta1-4GlcNAc, converting the acceptor to the branched pentasaccharide Galbeta1-4GlcNAcbeta1-3(GlcNAcbeta1-6)Galbeta1-4 GlcNAc as shown by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, degradative experiments, and 1H NMR spectroscopy of the product. By contrast, the recombinant enzyme did not catalyze any reaction when incubated with UDP-GlcNAc and the trisaccharide GlcNAcbeta1-3Galbeta1-4GlcNAc. Accordingly, we call the recombinant beta1,6-GlcNAc transferase cIGnT6 to emphasize its action at central rather than peridistal galactose residues of linear polylactosamines in the biosynthesis of blood group I antigens. Taken together this in vitro expression of I-branching enzyme, in combination with the previously cloned enzymes, beta1,4galactosyltransferase and beta1, 3N-acetylglucosaminyltransferase, should allow the general synthesis of polylactosamines based totally on the use of recombinant enzymes.

Purification and characterization of UDP-GlcNAc:Galbeta1-4GlcNAcbeta1-3*Galbeta1-4Glc(NAc)-R(GlcNAc to *Gal) beta1,6N-acetylglucosaminyltransferase from hog small intestine

A beta1,6N-acetylglucosaminyltransferase (beta1-6GnT) responsible for the formation of the beta1,6-branched poly-N-acetyllactosamine structure has been purified 210,000-fold in 2.4% yield from a homogenate of hog small intestine by successive column chromatographies involving CM-Sepharose FF, Ni2+-chelating Sepharose FF, and UDP-hexanolamine-agarose, using an assay wherein pyridylaminated lacto-N-neotetraose (Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc-PA) was used as an acceptor substrate, and the reaction product was Galbeta1-4GlcNAcbeta1-3(GlcNAcbeta1-6)Galbeta1-4 Glc-PA. The apparent molecular weight of the purified enzyme was 76,000 under nonreducing conditions. The enzyme has a pH optimum at 7.0 and has no requirement for any divalent metal ions. The Km values for pyridylaminated lacto-N-neotetraose and UDP-GlcNAc were 0.96 and 2. 59 mM, respectively. For its activity, this enzyme was shown to have an absolute requirement of at least a complete LacNAc (LacNAc = Galbeta1-4GlcNAc) residue bound to position 3 of the acceptor Gal residues, i.e. it is capable of acting only on the Gal residues of internal LacNAc units. The data strongly suggest that this enzyme could be involved in generating branches to central positions of preformed as well as growing polylactosamine chains, but not in synthesizing the distal branches to growing polylactosamine chains.