Poly-N-acetyllactosamine synthesis in branched N-glycans is controlled by complemental branch specificity of I-extension enzyme and beta1,4-galactosyltransferase I.

Poly-N-acetyllactosamine is a unique carbohydrate that can carry various functional oligosaccharides, such as sialyl Lewis X. It has been shown that the amount of poly-N-acetyllactosamine is increased in N-glycans, when they contain Galbeta1-->4GlcNAcbeta1-->6(Galbeta1-->4GlcNAcbeta1 -->2)Manalpha1-->6 branched structure. To determine how this increased synthesis of poly-N-acetyllactosamines takes place, the branched acceptor was incubated with a mixture of i-extension enzyme (iGnT) and beta1, 4galactosyltransferase I (beta4Gal-TI). First, N-acetyllactosamine repeats were more readily added to the branched acceptor than the summation of poly-N-acetyllactosamines formed individually on each unbranched acceptor. Surprisingly, poly-N-acetyllactosamine was more efficiently formed on Galbeta1-->4GlcNAcbeta1-->2Manalpha-->R side chain than in Galbeta1-->4GlcNAcbeta1-->6Manalpha-->R, due to preferential action of iGnT on Galbeta1-->4GlcNAcbeta1-->2Manalpha-->R side chain. On the other hand, galactosylation was much more efficient on beta1,6-linked GlcNAc than beta1,2-linked GlcNAc, preferentially forming Galbeta1-->4GlcNAcbeta1-->6(GlcNAcbeta1-->2)Manalph a1-->6Manbeta -->R. Starting with this preformed acceptor, N-acetyllactosamine repeats were added almost equally to Galbeta1-->4GlcNAcbeta1-->6Manalpha-->R and Galbeta1-->4GlcNAcbeta1-->2Manalpha-->R side chains. Taken together, these results indicate that the complemental branch specificity of iGnT and beta4Gal-TI leads to efficient and equal addition of N-acetyllactosamine repeats on both side chains of GlcNAcbeta1-->6(GlcNAcbeta1-->2)Manalpha1-->6Manbet a-->R structure, which is consistent with the structures found in nature. The results also suggest that the addition of Galbeta1-->4GlcNAcbeta1-->6 side chain on Galbeta1-->4GlcNAcbeta1-->2Man-->R side chain converts the acceptor to one that is much more favorable for iGnT and beta4Gal-TI.     

Regulation of I-branched poly-N-acetyllactosamine synthesis. Concerted actions by I-extension enzyme, I-branching enzyme, and beta1,4-galactosyltransferase I

I-branched poly-N-acetyllactosamine is a unique carbohydrate composed of N-acetyllactosamine branches attached to linear poly-N-acetyllactosamine, which is synthesized by I-branching beta1, 6-N-acetylglucosaminyltransferase. I-branched poly-N-acetyllactosamine can carry bivalent functional oligosaccharides such as sialyl Lewisx, which provide much better carbohydrate ligands than monovalent functional oligosaccharides. In the present study, we first demonstrate that I-branching beta1, 6-N-acetylglucosaminyltransferase cloned from human PA-1 embryonic carcinoma cells transfers beta1,6-linked GlcNAc preferentially to galactosyl residues of N-acetyllactosamine close to nonreducing terminals. We then demonstrate that among various beta1, 4-galactosyltransferases (beta4Gal-Ts), beta4Gal-TI is most efficient in adding a galactose to linear and branched poly-N-acetyllactosamines. When a beta1,6-GlcNAc branched poly-N-acetyllactosamine was incubated with a mixture of beta4Gal-TI and i-extension beta1,3-N-acetylglucosaminyltransferase, the major product was the oligosaccharide with one N-acetyllactosamine extension on the linear Galbeta1-->4GlcNAcbeta1-->3 side chain. Only a minor product contained galactosylated I-branch without N-acetyllactosamine extension. This finding was explained by the fact that beta4Gal-TI adds a galactose poorly to beta1,6-GlcNAc attached to linear poly-N-acetyllactosamines, while beta1, 3-N-acetylglucosaminyltransferase and beta4Gal-TI efficiently add N-acetyllactosamine to linear poly-N-acetyllactosamines. Together, these results strongly suggest that galactosylation of I-branch is a rate-limiting step in I-branched poly-N-acetyllactosamine synthesis, allowing poly-N-acetyllactosamine extension mostly along the linear poly-N-acetyllactosamine side chain. These findings are entirely consistent with previous findings that poly-N-acetyllactosamines in human erythrocytes, PA-1 embryonic carcinoma cells, and rabbit erythrocytes contain multiple, short I-branches.     

Biosynthesis of HNK-1 glycans on O-linked oligosaccharides attached to the neural cell adhesion molecule (NCAM): the requirement for core 2 beta 1,6-N-acetylglucosaminyltransferase and the muscle-specific domain in NCAM

The HNK-1 glycan, sulfo-->3GlcAbeta1-->3Galbeta1-->4GlcNAcbeta1-->R, is highly expressed in neuronal cells and apparently plays critical roles in neuronal cell migration and axonal extension. The HNK-1 glycan synthesis is initiated by the addition of beta1,3-linked GlcA to N-acetyllactosamine followed by sulfation of the C-3 position of GlcA. The cDNAs encoding beta1,3-glucuronyltransferase (GlcAT-P) and HNK-1 sulfotransferase (HNK-1ST) have been recently cloned. Among various adhesion molecules, the neural cell adhesion molecule (NCAM) was shown to contain HNK-1 glycan on N-glycans. In the present study, we first demonstrated that NCAM also bears HNK-1 glycan attached to O-glycans when NCAM contains the O-glycan attachment scaffold, muscle-specific domain, and is synthesized in the presence of core 2 beta1,6-N-acetylglucosaminyltransferase, GlcAT-P, and HNK-1ST. Structural analysis of the HNK-1 glycan revealed that the HNK-1 glycan is attached on core 2 branched O-glycans, sulfo-->3GlcAbeta1-->3Galbeta1-->4GlcNAcbeta1-->6(Galbeta1-->3)GalNAc. Using synthetic oligosaccharides as acceptors, we found that GlcAT-P and HNK-1ST almost equally act on oligosaccharides, mimicking N- and O-glycans. By contrast, HNK-1 glycan was much more efficiently added to N-glycans than O-glycans when NCAM was used as an acceptor. These results are consistent with our results showing that HNK-1 glycan is minimally attached to O-glycans of NCAM in fetal brain, heart, and the myoblast cell line, C2C12. These results combined together indicate that HNK-1 glycan can be synthesized on core 2 branched O-glycans but that the HNK-1 glycan is preferentially added on N-glycans over O-glycans of NCAM, probably because N-glycans are extended further than O-glycans attached to NCAM containing the muscle-specific domain.     

Biosynthesis of blood group i-active polylactosaminoglycans. Partial purification and properties of an UDP-GlcNAc:N-acetyllactosaminide beta 1----3-N-acetylglucosaminyltransferase from Novikoff tumor cell ascites fluid

An N-acetylglucosaminyltransferase has been partially purified from Novikoff tumor cell ascites fluid by affinity chromatography on concanavalin A-Sepharose. The enzyme was obtained in a highly concentrated form after lyophilization. The enzyme appeared to be highly specific for acceptor oligosaccharides and glycoproteins carrying a terminal Gal beta 1----4GlcNAc beta 1----R unit. Characterization of products formed by the enzyme in vitro by methylation analysis and 1H NMR spectroscopy revealed that the enzyme catalyzed the formation of a GlcNAc beta 1----3Gal beta 1----4GlcNAc beta-R sequence. The enzyme therefore could be described as an UDP-GlcNAc:Gal beta 1----4GlcNAc beta-R beta 1----3-N-acetylglucosaminyltransferase. Acceptor specificity studies with oligosaccharides that form part of N-glycans revealed that the presence of a Gal beta 1----4GlcNAc beta 1----2(Gal beta 1----4GlcNAc beta 1----6)Man pentasaccharide in the acceptor structure is a requirement for optimal activity. Studies on the branch specificity of the enzyme showed that the branches of this pentasaccharide structure, when contained in tri- and tetraantennary oligosaccharides, are highly preferred over other branches for attachment of the 1st and 2nd mol of GlcNAc into the acceptor molecule. The enzyme also showed activity toward oligosaccharides related to blood group I- and i-active polylactosaminoglycans. In addition the enzyme together with calf thymus UDP-Gal:GlcNAc beta-R beta 1----4-galactosyltransferase was capable of catalyzing the synthesis of a series of oligomers of N-acetyllactosamine. Competition studies revealed that all acceptors were acted upon by a single enzyme species. It is concluded that the N-acetylglucosaminyltransferase functions in both the initiation and the elongation of polylactosaminoglycan chains of N-glycoproteins and possibly other glycoconjugates.     

Human ovarian cancer, lymphoma spleen, and bovine milk GlcNAc:beta1,4Gal/GalNAc transferases: two molecular species in ovarian tumor and induction of GalNAcbeta1,4Glc synthesis by alpha-lactalbumin

Affinity Gel-UDP was utilized to purify GlcNAc:beta1,4Gal/GalNAc transferases (Ts) from human lymphoma spleen, ovarian tumor, and ovarian cancer sera. Mn(2+) was found to be an absolute requirement for activity. Two molecular species containing both beta1,4Gal/GalNAc-T activities were discernible when the purified ovarian tumor microsomal enzyme was subjected to Sephacryl S-100 HR column chromatography as well as native polyacylamide gel-electrophoresis. Acceptor specificity studies of the affinity-purified lymphoma spleen and ovarian tumor microsomal enzymes and the conventionally purified, as well as the cloned, bovine milk GlcNAc:beta1,4Gal-Ts using a number of synthetic acceptors showed that the beta(1,6)-linked GlcNAc moiety to alpha-GalNAc was the most efficient acceptor. As compared to the purified milk enzyme, the recombinant form exhibited sixfold GlcNAc:beta1,4 GalNAc-T activity and up to eightfold GlcNAc6SO3beta-:beta1,4Gal-T activity. Further, the recombinant enzyme catalyzed the transfer of GalNAc to the terminal beta-linked GlcNAc6SO3 moiety. Alpha-lactalbumin (alpha-LA) inhibited up to 85%, the transfer of Gal to the GlcNAc moiety linked either to Man or GlcNAc. On the contrary, alpha-LA had no significant influence on the transfer of GalNAc to the above acceptors. alpha-LA had no appreciable effect on the recombinant enzyme, except for the transfer of Gal or GalNAc to Glc. Both alpha- and beta-glucosides, as well as alpha-N-acetylglucosaminide, did not serve as acceptors.     

Structural analysis of murine zona pellucida glycans. Evidence for the expression of core 2-type O-glycans and the Sd(a) antigen

Murine sperm initiate fertilization by binding to specific oligosaccharides linked to the zona pellucida, the specialized matrix coating the egg. Biophysical analyses have revealed the presence of both high mannose and complex-type N-glycans in murine zona pellucida. The predominant high mannose-type glycan had the composition Man(5)GlcNAc(2), but larger oligosaccharides of this type were also detected. Biantennary, triantennary, and tetraantennary complex-type N-glycans were found to be terminated with the following antennae: Galbeta1-4GlcNAc, NeuAcalpha2-3Galbeta1-4GlcNAc, NeuGcalpha2-3Galbeta1-4GlcNAc, the Sd(a) antigen (NeuAcalpha2-3[GalNAcbeta1-4]Galbeta1-4GlcNAc, NeuGcalpha2-3[GalNAcbeta1-4]Galbeta1-4GlcNAc), and terminal GlcNAc. Polylactosamine-type sequence was also detected on a subset of the antennae. Analysis of the O-glycans indicated that the majority were core 2-type (Galbeta1-4GlcNAcbeta1-6[Galbeta1-3]GalNAc). The beta1-6-linked branches attached to these O-glycans were terminated with the same sequences as the N-glycans, except for terminal GlcNAc. Glycans bearing Galbeta1-4GlcNAcbeta1-6 branches have previously been suggested to mediate initial murine gamete binding. Oligosaccharides terminated with GalNAcbeta1-4Gal have been implicated in the secondary binding interaction that occurs following the acrosome reaction. The significant implications of these observations are discussed.     

Lectin affinity high-performance liquid chromatography. Interactions of N-glycanase-released oligosaccharides with Ricinus communis agglutinin I and Ricinus communis agglutinin II

The structural determinants required for interaction of oligosaccharides with Ricinus communis agglutinin I (RCAI) and Ricinus communis agglutinin II (RCAII) have been studied by lectin affinity high-performance liquid chromatography (HPLC). Homogeneous oligosaccharides of known structure, purified following release from Asn with N-glycanase and reduction with NaBH4, were tested for their ability to interact with columns of silica-bound RCAI and RCAII. The characteristic elution position obtained for each oligosaccharide was reproducible and correlated with specific structural features. RCAI binds oligosaccharides bearing terminal beta 1,4-linked Gal but not those containing terminal beta 1,4-linked GalNAc. In contrast, RCAII binds structures with either terminal beta 1,4-linked Gal or beta 1,4-linked GalNAc. Both lectins display a greater affinity for structures with terminal beta 1,4-rather than beta 1,3-linked Gal, although RCAII interacts more strongly than RCAI with oligosaccharides containing terminal beta 1,3-linked Gal. Whereas terminal alpha 2,6-linked sialic acid partially inhibits oligosaccharide-RCAI interaction, terminal alpha 2,3-linked sialic acid abolishes interaction with the lectin. In contrast, alpha 2,3- and alpha 2,6-linked sialic acid equally inhibit but do not abolish oligosaccharide interaction with RCAII. RCAI and RCAII discriminate between N-acetyllactosamine-type branches arising from different core Man residues of dibranched complex-type oligosaccharides; RCAI has a preference for the branch attached to the alpha 1,3-linked core Man and RCAII has a preference for the branch attached to the alpha 1,6-linked core Man. RCAII but not RCAI interacts with certain di- and tribranched oligosaccharides devoid of either Gal or GalNAc but bearing terminal GlcNAc, indicating an important role for GlcNAc in RCAII interaction. These findings suggest that N-acetyllactosamine is the primary feature required for oligosaccharide recognition by both RCAI and RCAII but that lectin interaction is strongly modulated by other structural features. Thus, the oligosaccharide specificities of RCAI and RCAII are distinct, depending on many different structural features including terminal sugar moieties, peripheral branching pattern, and sugar linkages.     

Enzymatic synthesis of sulfated disaccharides using beta-D-galactosidase-catalyzed transglycosylation.

We have established a unique enzymatic approach for obtaining sulfated disaccharides using Bacillus circulans beta-D-galactosidase-catalyzed 6-sulfo galactosylation. When 4-methyl umbelliferyl 6-sulfo beta-D-galactopyranoside (S6Gal beta-4MU) was used as a donor, the enzyme induced transfer of 6-sulfo galactosyl residue to GlcNAc acceptor. As a result, the desired compound 6'-sulfo N-acetyllactosamine (S6Gal beta1-4GlcNAc) and its positional isomer 6'-sulfo N-acetylisolactosamine (S6Gal beta1-6GlcNAc) were observed by HPAEC-PAD, in 49% total yield based on the donor added, and in a molar ratio of 1:3.5. With a glucose acceptor, the regioselectivity was substantially changed and S6Gal beta1-2Glc was mainly produced along with beta-(1-1)alpha, beta-(1-3), beta-(1-6) isomers in 74% total yield. When methyl alpha-D-glucopyranoside (Glc alpha-OMe) was an acceptor, the enzyme also formed mainly S6Gal beta1-2Glc alpha-OMe with its beta-(1-6)-linked isomer in 41% total yield based on the donor added. In both cases, it led to the predominant formation of beta-(1-2)-linked disaccharides. In contrast, with the corresponding methyl beta-D-glucopyranoside (Glc beta-OMe) acceptor, S6Gal beta1-3Glc beta-OMe and S6Gal beta1-6Glc beta-OMe were formed in a low total yield of 12%. These results indicate that the regioselectivity and efficiency on the beta-D-galactosidase-mediated transfer reaction significantly depend on the anomeric configuration in the glucosyl acceptors.     

Crystal structure of beta1,4-galactosyltransferase complex with UDP-Gal reveals an oligosaccharide acceptor binding site

The crystal structure of the catalytic domain of bovine beta1,4-galactosyltransferase (Gal-T1) co-crystallized with UDP-Gal and MnCl(2) has been solved at 2.8 A resolution. The structure not only identifies galactose, the donor sugar binding site in Gal-T1, but also reveals an oligosaccharide acceptor binding site. The galactose moiety of UDP-Gal is found deep inside the catalytic pocket, interacting with Asp252, Gly292, Gly315, Glu317 and Asp318 residues. Compared to the native crystal structure reported earlier, the present UDP-Gal bound structure exhibits a large conformational change in residues 345-365 and a change in the side-chain orientation of Trp314. Thus, the binding of UDP-Gal induces a conformational change in Gal-T1, which not only creates the acceptor binding pocket for N-acetylglucosamine (GlcNAc) but also establishes the binding site for an extended sugar acceptor. The presence of a binding site that accommodates an extended sugar offers an explanation for the observation that an oligosaccharide with GlcNAc at the non-reducing end serves as a better acceptor than the monosaccharide, GlcNAc. Modeling studies using oligosaccharide acceptors indicate that a pentasaccharide, such as N-glycans with GlcNAc at their non-reducing ends, fits the site best. A sequence comparison of the human Gal-T family members indicates that although the binding site for the GlcNAc residue is highly conserved, the site that binds the extended sugar exhibits large variations. This is an indication that different Gal-T family members prefer different types of glycan acceptors with GlcNAc at their non-reducing ends.     

Drosophila beta 1,4-N-acetylgalactosaminyltransferase-A synthesizes the LacdiNAc structures on several glycoproteins and glycosphingolipids

The GalNAcbeta1,4GlcNAc (LacdiNAc or LDN) structure is a more common structural feature in invertebrate glycoconjugates when compared with the Galbeta1,4GlcNAc structure. Recently, beta1,4-N-acetylgalactosaminyltransferase (beta4GalNAcT) was identified in some invertebrates including Drosophila. However, the LDN structure has not been reported in Drosophila, and the biological function of LDN remains to be determined. In this study, we examined acceptor substrate specificity of Drosophila beta4GalNAcTA by using some N- and O-glycans on glycoproteins and neutral glycosphingolipids (GSLs). GalNAc was efficiently transferred toward N-glycans, O-glycans, and the arthro-series GSLs. Moreover, we showed that dbeta4GalNAcTA contributed to the synthesis of the LDN structure in vivo. The dbeta4GalNAcTA mRNA was highly expressed in the developmental and adult neuronal tissues. Thus, these results suggest that dbeta4GalNAcTA acts on the terminal GlcNAc residue of some glycans for the synthesis of LDN, and the LDN structure may play a role in the physiological or neuronal development of Drosophila.     

Branch specificity of bovine colostrum CMP-sialic acid: Gal beta 1----4GlcNAc-R alpha 2----6-sialyltransferase. Sialylation of bi-, tri-, and tetraantennary oligosaccharides and glycopeptides of the N-acetyllactosamine type

Using 500-MHz 1H NMR spectroscopy we have investigated the branch specificity that bovine colostrum CMP-NeuAc:Gal beta 1----4GlcNAc-R alpha 2----6-sialyltransferase shows in its sialylation of bi-, tri-, and tetraantennary glycopeptides and oligosaccharides of the N-acetyllactosamine type. The enzyme appears to highly prefer the galactose residue at the Gal beta 1----4GlcNAc beta 1----2Man alpha 1----3 branch for attachment of the 1st mol of sialic acid in all the acceptors tested. The 2nd mol of sialic acid becomes linked mainly to the Gal beta 1----4GlcNAc beta 1----2Man alpha 1----6 branch in bi- and triantennary substrates, but this reaction invariably proceeds at a much lower rate. Under the conditions employed, the Gal beta 1----4GlcNAc beta 1----6Man alpha 1----6 branch is extremely resistant to alpha 2----6-sialylation. A higher degree of branching of the acceptors leads to a decrease in the rate of sialylation. In particular, the presence of the Gal beta 1----4GlcNAc beta 1----6Man alpha 1----6 branch strongly inhibits the rate of transfer of both the 1st and the 2nd mol of sialic acid. In addition, it directs the incorporation of the 2nd mol into tetraantennary structures toward the Gal beta 1----4GlcNAc beta 1----4Man alpha 1----3 branch. In contrast, the presence of the Gal beta 1----4GlcNAc beta 1----4Man alpha 1----3 branch has only minor effects on the rates of sialylation and, consequently, on the branch preference of sialic acid attachment. Results obtained with partial structures of tetraantennary acceptors indicate that the Man beta 1----4GlcNAc part of the core is essential for the expression of branch specificity of the sialyltransferase. The sialylation patterns observed in vivo in glycoproteins of different origin are consistent with the in vitro preference of alpha 2----6-sialyltransferase for the Gal beta 1----4GlcNAc beta 1----2Man alpha 1----3 branch. Our findings suggest that the terminal structures of branched glycans of the N-acetyllactosamine type are the result of the complementary branch specificity of the various glycosyltransferases that are specific for the acceptor sequence Gal beta 1----4GlcNAc-R.     

A coupled assay for UDP-GlcNAc:Gal beta 1-3GalNAc-R beta 1,6-N-acetylglucosaminyltransferase (GlcNAc to GalNAc).

UDP-GlcNAc:Gal beta 1-3GalNAc-R beta 1,6-N-acetylglucosaminyltransferase (GlcNAc to GalNAc) (i.e., core 2 GlcNAc-T) is a developmentally regulated enzyme of the O-linked oligosaccharide biosynthesis pathway. We have developed a coupled-enzyme assay for core 2 GlcNAc-T that is approximately 100 times more sensitive than the standard assay using UDP-[3H]GlcNAc as a sugar donor. Core 2 GlcNAc-T reactions were performed using unlabeled UDP-GlcNAc donor and Gal beta 1-3GalNAc alpha-paranitrophenyl (pNp) as acceptor. The product, Gal beta 1-3(GlcNAc beta 1-6)GalNAc alpha-pNp was then further reacted with purified bovine beta 1-4Gal-T and UDP-[3H]Gal to produce Gal beta 1-3([3H]Gal beta 1-4GlcNAc beta 1-6) GalNAc alpha-pNp, which was separated on an Ultrahydrogel HPLC column. Approximately 10% of the available GlcNAc-terminating acceptor was substituted in the Gal-T reaction, allowing 1 pmol of product to be readily detected. The increased sensitivity of the coupled assay should facilitate studies of core 2 GlcNAc-T activity where material is limiting or specific activity is low.     

Oligosaccharide preferences of beta1,4-galactosyltransferase-I: crystal structures of Met340His mutant of human beta1,4-galactosyltransferase-I with a pentasaccharide and trisaccharides of the N-glycan moiety

beta-1,4-Galactosyltransferase-I (beta4Gal-T1) transfers galactose from UDP-galactose to N-acetylglucosamine (GlcNAc) residues of the branched N-linked oligosaccharide chains of glycoproteins. In an N-linked biantennary oligosaccharide chain, one antenna is attached to the 3-hydroxyl-(1,3-arm), and the other to the 6-hydroxyl-(1,6-arm) group of mannose, which is beta-1,4-linked to an N-linked chitobiose, attached to the aspargine residue of a protein. For a better understanding of the branch specificity of beta4Gal-T1 towards the GlcNAc residues of N-glycans, we have carried out kinetic and crystallographic studies with the wild-type human beta4Gal-T1 (h-beta4Gal-T1) and the mutant Met340His-beta4Gal-T1 (h-M340H-beta4Gal-T1) in complex with a GlcNAc-containing pentasaccharide and several GlcNAc-containing trisaccharides present in N-glycans. The oligosaccharides used were: pentasaccharide GlcNAcbeta1,2-Manalpha1,6 (GlcNAcbeta1,2-Manalpha1,3)Man; the 1,6-arm trisaccharide, GlcNAcbeta1,2-Manalpha1,6-Manbeta-OR (1,2-1,6-arm); the 1,3-arm trisaccharides, GlcNAcbeta1,2-Manalpha1,3-Manbeta-OR (1,2-1,3-arm) and GlcNAcbeta1,4-Manalpha1,3-Manbeta-OR (1,4-1,3-arm); and the trisaccharide GlcNAcbeta1,4-GlcNAcbeta1,4-GlcNAc (chitotriose). With the wild-type h-beta4Gal-T1, the K(m) of 1,2-1,6-arm is approximately tenfold lower than for 1,2-1,3-arm and 1,4-1,3-arm, and 22-fold lower than for chitotriose. Crystal structures of h-M340H-beta4Gal-T1 in complex with the pentasaccharide and various trisaccharides at 1.9-2.0A resolution showed that beta4Gal-T1 is in a closed conformation with the oligosaccharide bound to the enzyme, and the 1,2-1,6-arm trisaccharide makes the maximum number of interactions with the enzyme, which is in concurrence with the lowest K(m) for the trisaccharide. Present studies suggest that beta4Gal-T1 interacts preferentially with the 1,2-1,6-arm trisaccharide rather than with the 1,2-1,3-arm or 1,4-1,3-arm of a bi- or tri-antennary oligosaccharide chain of N-glycan.     

The presence of N-acetyllactosamine and lactose: beta (1-3)N-acetylglucosaminyltransferase activity in human urine

Normal human urine was found to contain beta (1-3)N-acetylglucosaminyltransferase catalyzing the transfer of N-acetylglucosamine from UDP-GlcNAc to N-acetyllactosamine and lactose. Lacto-N-tetraose which carries the terminal Gal beta (1-3)GlcNAc structure was a poor acceptor. The product of the transferase reaction with N-acetyllactosamine as acceptor was identified by methylation analysis as GlcNAc beta (1-3)Gal beta (1-4)GlcNAc. The beta-linkage of the GlcNAc in the synthesized trisaccharide was confirmed by the action of the specific beta-N-acetylhexosaminidase. The enzyme requires Mn2+ ions for its activity, shows a broad pH optimum from 7 to 9, and appears to have a molecular weight of about 200,000 as estimated by Sephadex gel filtration.     

Peptide-specific Transfer of N-Acetylgalactosamine to O-Linked Glycans by the Glycosyltransferases β1,4-N-Acetylgalactosaminyl Transferase 3 (β4GalNAc-T3) and β4GalNAc-T4

N- and O-linked oligosaccharides on pro-opiomelanocortin both bear the unique terminal sequence SO(4)-4-GalNAcβ1,4GlcNAcβ. We previously demonstrated that protein-specific transfer of GalNAc to N-linked oligosaccharides on glycoprotein substrates is dependent on the presence of both an oligosaccharide acceptor and a peptide recognition motif consisting of a cluster of basic amino acids. We characterized how two β1,4-N-acetylgalactosaminyltransferases, β4GalNAc-T3 and β4GalNAc-T4, require the presence of both the peptide recognition motif and the N-linked oligosaccharide acceptors to transfer GalNAc in β1,4-linkage to GlcNAc in vivo and in vitro. We now show that β4GalNAc-T3 and β4GalNAc-T4 are able to utilize the same peptide motif to selectively add GalNAc to β1,6-linked GlcNAc in core 2 O-linked oligosaccharide structures to form Galβ1,3(GalNAcβ1,4GlcNAcβ1,6)GalNAcαSer/Thr. The β1,4-linked GalNAc can be further modified with 4-linked sulfate by either GalNAc-4-sulfotransferase 1 (GalNAc-4-ST1) (CHST8) or GalNAc-4-ST2 (CHST9) or with α2,6-linked N-acetylneuraminic acid by α2,6-sialyltransferase 1 (ST6Gal1), thus generating a family of unique GalNAcβ1,4GlcNAcβ (LacdiNAc)-containing structures on specific glycoproteins.