Preferred conformations and dynamics of five core structures of mucin type O-glycans determined by NMR spectroscopy and force field calculations.

Glycosyltransferases acting on O-glycans have been shown to exhibit distinct specificity for the carbohydrate and the peptide moiety of their substrates. As an approach to study the 3-dimensional interactions between enzymes and O-glycan substrates, we determined the preferred conformations of five oligosaccharide-core structures of mucin type glycoproteins by NMR spectroscopy and by static and dynamic force field calculations. Seven oligosaccharides, representing five basic core structures, were investigated: Gal beta (1-3)GalNAc alpha Bzl (1, core 1), GlcNAc beta (1-6)[Gal beta (1-3)]GalNAc alpha Bzl (2, core 2), GlcNAc beta (1-3)GalNAc alpha Bzl (3, core 3), GlcNAc beta (1-6)[GlcNAc beta (1-3)]GalNAc alpha Bzl (4, core 4), GlcNAc beta (1-6)GalNAc alpha Bzl (5, core 6), the elongated core 2, Gal beta (1-4)GlcNAc beta (1-6)[Gal beta (1-3)]GalNAc alpha pNp (6) and GalNAc alpha-Bzl (7). The dynamic behaviour of the molecules was studied by Metropolis Monte Carlo (MMC) simulations. Experimental coupling constants, chemical shift changes, and NOEs were compared with results from static energy minimizations and dynamic MMC simulations and show a good agreement. MMC simulations show that the (1-6) linkage is much more flexible than the (1-3) or the (1-4) linkages. The preferred conformations of the disaccharides (1) and (3) show only slight differences due to the additional N-acetyl group in (3). The conformational equilibrium of beta (1-3) glycosidic bonds of 1 and 3 was not affected by attaching a beta (1-6) linked GlcNAc unit to the GalNAc residue in 2 and 4. However, experimental and theoretical data show that the beta (1-6) linkages of the trisaccharides 2 and 4, which carry an additional beta (1-3) linked glycosyl residue, change their preferred conformations when compared with (5). The 6-branch also shows significant interactions with the benzyl aglycon altering the preferred conformation of the hydroxymethyl group of the GalNAc to a higher proportion of the gt conformer. The (1-6) linkage of 2, 4, and 6 can have two different families of conformations of which the lower energy state is populated only to about 20% of the time whereas the other state with a relative enthalpy of approximately 4 kcal mol-1 is populated to 80%. This fact demonstrates that the two conformational states have different entropy contents. Entropy is implicitly included in MMC simulations but cannot be derived from energy minimizations.

Control of O-glycan synthesis: specificity and inhibition of O-glycan core 1 UDP-galactose:N-acetylgalactosamine-alpha-R beta 3-galactosyltransferase from rat liver.

The specificity of glycosyltransferases is a major control factor in the biosynthesis of O-glycans. The enzyme that synthesizes O-glycan core 1, i.e., UDP-galactose:N-acetylgalactosamine-alpha-R beta 3-galactosyltransferase (beta 3-Gal-T; EC 2.4.1.122), was partially purified from rat liver. The enzyme preparation, free of pyrophosphatases, beta 4-galactosyltransferase, beta-galactosidase, and N-acetylglucosaminyltransferase I, was used to study the specificity and inhibition of the beta 3-Gal-T. beta 3-Gal-T activity is sensitive to changes in the R-group of the GalNAc alpha-R acceptor substrate and is stimulated when the R-group is a peptide or an aromatic group. Derivatives of GalNAc alpha-benzyl were synthesized and tested as potential substrates and inhibitors. Removal or substitution of the 3-hydroxyl or removal of the 4-hydroxyl of GalNAc abolished beta 3-Gal-T activity. Compounds with modifications of the 3- or 4-hydroxyl of GalNAc alpha-benzyl did not show significant inhibition. Removal or substitution of the 6-hydroxyl of GalNAc reduced activity slightly and these derivatives acted as competitive substrates. derivatives with epoxide groups attached to the 6-position of GalNAc acted as substrates and not as inhibitors, with the exception of the photosensitive 6-O-(4,4-azo)pentyl-GalNAc alpha-benzyl, which inhibited Gal incorporation into GalNAc alpha-benzyl. The results indicate that the enzyme does not require the 6-hydroxyl of GalNAc, but needs the 3- and the axial 4-hydroxyl as essential requirements for binding and activity. In the usual biochemical O-glycan pathway, core 2 (GlcNAc beta 6[Gal beta 3] GalNAc alpha-) is formed from core 1 (Gal beta 3GalNAc-R). We have now demonstrated an alternate pathway that may be of importance in human tissues.

[Conformational analysis of N-terminal O-glycopeptide sequences of interleukin-2]. / Konformationsanalytische Untersuchungen von N-terminalen O-glycopeptidsequenzen des Interleukin-2.

The preferred conformations of eight O-glycopeptide sequences from the N-terminus of interleukin-2 containing two to ten amino acids, monoglycosylated at Thr3 with a 2-acetamido-2-deoxy-alpha-D-galactopyranosyl group, were determined by means of n.m.r. spectroscopic methods. The preferred conformation of the N-terminal sequence, L-Ala-L-Pro-[alpha-D-GalpNAc-(1----3)]-L-Thr-L-Ser, including the O-glycosidically linked 2-acetamido-2-deoxy-alpha-D-galactopyranosyl group is not substantially influenced by the linkage of additional amino acids at the C-terminal end. Extended conformations were observed for all peptide units. Measurements of the relaxation times of the 13C atoms showed that the 2-acetamido-2-deoxy-D-galactose bound to the central amino acids has the lowest mobility, whereas the terminal amino acid residues and peptide side-chains are flexible. Calculations with the force-field program AMBER yielded conformations of minimized energies that were in good agreement with the n.m.r. spectroscopic data. This was only true when n.m.r. parameters that can be used as starting values for the calculations were available. Comparison with a nonglycosylated, N-terminal tetrapeptide sequence analog did not suggest changes in the peptide conformation when Thr3 is glycosylated with a 2-acetamido-2-deoxy-alpha-D-galactopyranosyl group.

[Conformation analysis of D-xylose-containing O-glycopeptide sequences]. / Konformationsanalytische Untersuchungen von D-Xylose-haltigen O-Glycopeptidsequenzen.

The preferred conformations of the sequences of O-glycopeptides containing a beta-D-xylosyl group linked to an L-seryl residue, found in the N-terminus of proteodermatan sulfate, show an almost extended peptide chain with the sugar unit in a specific position. The results of force-field calculations with the AMBER program have been confirmed, by n.m.r.-spectroscopic experiments, for a minimum conformation.

Increased UDP-GlcNAc:Gal beta 1-3GaLNAc-R (GlcNAc to GaLNAc) beta-1, 6-N-acetylglucosaminyltransferase activity in metastatic murine tumor cell lines. Control of polylactosamine synthesis.

Malignant transformation of rodent cell lines by polyoma virus and by activated ras genes is associated with increased UDP-GlcNAc:Man alpha-R beta-1,6-N-acetylglucosaminyltransferase V (GlcNAc-transferase V) activity and it product -GlcNAc beta 1-6Man alpha 1-6Man beta 1-branched Asn-linked oligosaccharides. In this report, we have compared beta 1-6GlcNAc branching of core O- and N-linked oligosaccharides in three experimental models of malignancy, namely (a) rat2 fibroblasts and their malignant T24H-ras-transfected counterpart; (b) benign SP1 mammary carcinoma cells and two metastic sublines of SP1; and (c) the metastatic MDAY-D2 lymphoma cell line and its poorly metastatic glycosylation mutant KBL-1. In addition to the previously reported increase in GlcNAc-transferase V activity, UDP-GlcNAc:Gal beta 1-3GalNAc alpha-R (GlcNAc to GalNAc) beta-1,6-N-acetylglucosaminyltransferase (core 2 GlcNAc-transferase, EC 2.4.1.102) activity was found to be elevated by 70% in the malignant rat2 and SP1 cell lines while several other glycosyltransferase activities were not significantly different. The action of core 2 GlcNAc-transferase followed by beta 1-4Gal-transferase provides an N-acetyllactosamine antenna that can be extended with polylactosamine (i.e. repeating Gal beta 1-4GlcNAc beta 1-3) provided UDP-GlcNAc:Gal beta-R beta 1-3GlcNAc-transferase (GlcNAc-transferase) (i)) activity is present. Polylactosamine content in microsomal membrane glycoproteins was quantitated by labeling the GlcNAc termini resulting from the action of Escherichia freundii endo-beta-galactosidase with bovine galactosyltransferase/UDP-[3H] Gal. Glycopeptidase F- sensitive and -insensitive fractions were measured to assess the N- and O-linked components. In the SP1 tumor model, the metastatic sublines showed increased core 2 GlcNAc-transferase and GlcNAc-transferase V activities but no change in GlcNAc-transferase (i) activity, yet polylactosamine was increased in both O- and N-linked oligosaccharides. In rat2 cells, down-regulation of GlcNAc-transferase (i) following transformation was associated with decreased polyactosamine even though core 2 GlcNAc-transferase and GlcNAc-transferase V were elevated in the cells. Finally, a 3-fold decrease in GlcNAc-transferase V in KBL-1, the glycosylation mutant of MDAY-D2 cells, resulted in complete loss of polylactosamine in N-linked but no change in O-linked polylactosamine content. These results suggest that, provided GlcNAc-transferase (i) is not limiting, the beta 1-6-branching enzymes core 2 GlcNAc-transferase and GlcNAc-transferase V regulate the levels of polyactosamine in O- and N-linked oligosaccharides, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)