Responses of the ant Lasius niger to various compounds perceived as sweet in humans: a structure-activity relationship study

A behavioural study on the ant Lasius niger was performed by observing its feeding responses to 85 compounds presented in a two-choice situation (tested compound versus water control or sucrose solution). Among these compounds, only 21 were phagostimulating: six monosaccharides (D-glucose, 6-deoxy-D-glucose, L-galactose, L-fucose, D-fructose, L-sorbose), four derivatives of D-glucose (methyl alpha-D-glucoside, D-gluconolactone and 6-chloro- and 6-fluoro-deoxy-D-glucose), five disaccharides (sucrose, maltose, palatinose, turanose and isomaltose), one polyol glycoside (maltitol), three trisaccharides (melezitose, raffinose and maltotriose) and two polyols (sorbitol and L-iditol). None of the 16 non-carbohydrate non-polyol compounds tested, although perceived as sweet in humans, was found to be active in ants. The molar order of effectiveness of the major naturally occuring compounds (melezitose > sucrose = raffinose > D-glucose > D-fructose = maltose = sorbitol) is basically different from the molar order of their sweetness potency in humans (sucrose > D-fructose > melezitose > maltose > D-glucose = raffinose = sorbitol). On a molar basis melezitose is in L. niger about twice as effective as sucrose or raffinose, while D-glucose and D-fructose are three and four times less effective, respectively, than sucrose or raffinose. From a structure-activity relationship study it was inferred that the active monosaccharides and polyols should interact with the ant receptor through only one type of receptor, through the same binding pocket and the same binding residues, via a six-point interaction. The high effectiveness of melezitose in L. niger mirrors the feeding habits of these ants, which attend homopterans and are heavy feeders on their honeydew, which is very rich in this carbohydrate.

Synthesis of ethyl 5-O-(alpha-D-arabinofuranosyl)-6-O-(beta-D-galactofuranosyl)-beta-D- galactofuranoside present in motif E of the Mycobacterium tuberculosis cell wall

[figure: see text] The stereocontrolled synthesis of the trisaccharide ethyl 5-O-(alpha-D-arabinofuranosyl)-6-O-(beta-D-galactofuranosyl)- beta-D-galactofuranoside present in motif E of the Mycobacterium tuberculosis cell wall is described.

Structural characterization of fucose-containing oligosaccharides by high-performance liquid chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Eight pyridylamino (PA) derivatives of fucose-containing oligosaccharides, which occur as free oligosaccharides in human milk and also are derived from glycosphingolipids, have been analyzed by high-performance liquid chromatography (HPLC) on normal-phase and reversed-phase columns, and by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Six out of eight PA-oligosaccharides were clearly separated by both normal- and reversed-phase HPLC at a column temperature of 40 degrees C, but two PA-oligosaccharides, lacto-N-fucopentaose II [Gal beta1-3(Fuc alpha1-4)GlcNAc beta1-3Gal beta1-4GIcPA] and lacto-N-fucopentaose III [Gal beta1-4(Fuc alpha1-3)GlcNAc beta1-3Gal beta1-4GIcPA], were not separated. The two unresolved PA-oligosaccharides were finally separated by reversed-phase HPLC at a column temperature of 11 degrees C. MALDI-TOF mass spectra of PA-oligosaccharides demonstrated pseudo-molecular ions as the predominant signals, therefore information about the molecular mass of each PA-oligosaccharide was easily obtained. Post-source decay (PSD) MALDI-TOF mass spectra of PA-oligosaccharides gave information about the carbohydrate sequences and carbohydrate species of each PA-oligosaccharide by detecting the ions responsible for the cleavage of the glycosidic bonds. The detection limits of the PA-oligosaccharides by HPLC, MALDI-TOF mass spectrometry, and PSD MALDI-TOF mass spectrometry were 20 fmol, 20 fmol, and 2 pmol, respectively. These results suggest that a system including HPLC and MALDI-TOF mass spectrometry or HPLC and PSD MALDI-TOF mass spectrometry is quite useful for the structural characterization of sub-pmol or pmol levels of fucose-containing oligosaccharides, and that these methods could be used for the analysis of various types of oligosaccharides derived from glycoproteins and glycosphingolipids.

Synthetic heparin pentasaccharide depolymerization by heparinase I: molecular and biological implications

A synthetic pentasaccharide (SR90107/ ORG31540) representing the antithrombin III (ATIII) binding sequence in heparin is under clinical development for the prophylaxis and management of venous thromboembolism. This pentasaccharide exhibits potent anti-factor Xa (AXa) effects (>750 IU/mg) and does not exhibit any anti-factor IIa (AIIa) activity. Previous reports have suggested that synthetic heparin pentasaccharides are resistant to the digestive effects of heparinase I. To investigate the effect of heparinase I on the AXa activity of pentasaccharide SR90107/ORG31540, graded concentrations (1.25-100 microg/ml) were incubated with a fixed amount of heparinase I (0.1 U/ml). Heparinase I produced a strong neutralizing effect on this pentasaccharide, as measured by AXa activity. This observation led to further studies where high performance liquid chromatography (HPLC) analysis was employed to determine the potential breakdown products of the pentasaccharide. The experiment with the pentasaccharide included incubation (37 degrees C) at 1 mg/ml and exposure to graded concentrations of heparinase I (0.125-1 U/ml). After 30 min of incubation, the enzymatic activity was stopped by heat treatment and the mixture was analyzed using high performance size exclusion chromatography (HPSEC). Heparinase I concentration-dependent cleavage of the pentasaccharide was evident. The breakdown products exhibited a mass of 1,034 d and 743 d, respectively, suggesting the generation of a trisaccharide and a disaccharide moiety. The extinction of a disaccharide moiety in the UV region was high, indicating the presence of a double bond in this molecule. These data clearly suggest that pentasaccharide SR90107/ORG31540 is digestible by heparinase I into its two components. Furthermore, these data support the hypothesis that heparinase I can be used as a neutralizing agent for pentasaccharide overdose. Additionally, a highly methylated analog of the previously mentioned synthetic pentasaccharide. SanOrg34006, which has also been subjected to similar experiments, has shown complete resistance to the depolymerizing function of heparinase I; therefore, its use may be appropriate in chronic situations as a long-acting form of the pentasaccharide.

One-pot oligosaccharide synthesis exploiting solvent reactivity effects

[reaction: see text] One-pot syntheses of trisaccharides have been accomplished simply by changing the solvent system between the two subsequent glycosylation reactions and utilizing the difference in glycosylation rate between different solvents. By tuning the reactivity of acceptors and donors and performing the first glycosylation in Et(2)O (low glycosylation rate) and the second in CH(2)Cl(2)/Et(2)O (higher glycosylation rate), trisaccharides were synthesized in high yields (76-84%).

Inhibition of selectin-mediated cell adhesion and prevention of acute inflammation by nonanticoagulant sulfated saccharides. Studies with carboxyl-reduced and sulfated heparin and with trestatin a sulfate

Selectins play a major role in the inflammatory reaction by initiating neutrophil attachment to activated vascular endothelium. Some heparin preparations can interact with L- and P-selectin; however, the determinants required for inhibiting selectin-mediated cell adhesion have not yet been characterized. We now report that carboxyl-reduced and sulfated heparin (prepared by chemical modifications of porcine intestinal mucosal heparin leading to the replacement of carboxylates by O-sulfate groups) and trestatin A sulfate (obtained by sulfation of trestatin A, a non-uronic pseudo-nonasaccharide extracted from Streptomyces dimorphogenes) exhibit strong anti-P-selectin and anti-L-selectin activity while lacking antithrombin-mediated anticoagulant activity. In vitro experiments revealed that both compounds inhibited P-selectin- and L-selectin-mediated cell adhesion under laminar flow conditions. Moreover, carboxyl-reduced and sulfated heparin and trestatin A sulfate were also active in vivo, as assessed by experiments showing 1) that microinfusion of trestatin A sulfate reduced by 96% leukocyte rolling along rat mesenteric postcapillary venules and 2) that both compounds inhibited (by 58-81%) neutrophil migration into thioglycollate-inflamed peritoneum of BALB/c mice. These results indicate that nonanticoagulant sulfated saccharides targeted at P-selectin and L-selectin may have therapeutic potential in inflammatory disorders.

FAB CIDMS/MS analysis of partially methylated maltotrioses derived from methylated amylose: a study of the substituent distribution

Amylose was methylated with CH3I in alkaline aqueous suspension, yielding methylated amylose (MeAl) with a degree of substitution of 1.44 (s < 0.01). Determination of the monomer composition showed that HO-6 and HO-2 were highly substituted in contrast to HO-3 (7:2:5.5, HO-2:HO-3:HO-6). By using partial acid hydrolysis, oligomers were prepared that varied both in degree of polymerisation and in methyl-content. Studies on the distribution of substituents in trimers showed large deviations from random distributions. By using CID tandem mass spectrometry, the substituent distribution in these trimers was determined in more detail. Various sets of trimers with equal amounts of methyl-groups but differing in substituted positions were quantified. From the monomer composition of MeAl, the probability of each trimer was calculated and compared to the outcome of the measured distributions. It was concluded that trimers with terminal tri- or non-substituted glucose monomers at the non-reducing end were formed preferentially during partial hydrolysis and that partial hydrolysis of MeAl yielded oligomers in a non-random way. This is the first study that describes the partial hydrolysis of MeAl in such detail.

Post-source decay fragmentation analyses of linkage isomers of Lewis-type oligosaccharides in curved-field reflectron matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: combined in-source decay/post-source decay experiments and relative ion abundance analysis

Linkage isomers of Lewis(X) trisaccharide (Le(X)) and Lewis(a) trisaccharide (Le(a)) were distinguished by the post-source decay (PSD) fragment spectra obtained by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) without permethylation. Both Y- and Z-type fragmentations were observed at the C-3 position of N-acetylhexosamine. beta-Elimination at C-3 of the reducing-end N-acetylglucosamine in Le(X) formed a double bond, which conjugated to an N-acetyl group, making the chemical species stable. In contrast, the double bond formed in the reducing end glucose of 3-fucosyllactose was unstable owing to the lack of a conjugated system. Therefore, beta-elimination of N-acetylglucosamine occurred predominantly rather than that of hexose in MALDI-PSD fragmentation. The measurements of the PSD fragment mass spectra using pseudo precursor ions originating from in-source decay were useful for the analyses of the fragmentation mechanisms and for the assignments of the chemical species of the fragment ions. The combined in-source decay/post-source decay experiments revealed the formation of a double bond between C-2 and C-3 in N-acetylglucosamine of Le(X). Abundance analysis of the PSD ions indicated that the 1-3 glycosyl linkage cleaves more easily than does the 1-4 linkage in MALDI-PSD fragmentation. Ion abundance analyses were useful in estimating the degree of Y- and Z-type fragmentation at the C-3 position of hexose and N-acetylhexosamine. The analysis of the relative ion abundances was a powerful tool for the assignments of the chemical species of the PSD ions.

Differential antibacterial activity of moenomycin analogues on gram-positive bacteria

The moenomycin trisaccharide degradation product and synthetic disaccharide analogues based on the disaccharide core were bactericidal to gram-positive bacteria, inhibited lipid II polymerization, and inhibited cell wall synthesis in Enterococcus faecalis. Truncating moenomycin to the trisaccharide, and building upon the core disaccharide have both led to molecules possessing properties not shared with their respective parent structures.

Mass spectral characterization of lipooligosaccharides from Haemophilus influenzae 2019

Lipooligosaccharide (LOS) glycoforms from Haemophilus influenzae 2019 were profiled using the high-resolution and accurate mass capabilities of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Sequence and linkage for two previously unknown LOS glycoforms were subsequently obtained through MSn analyses on FT-ICR and quadrupole ion trap (qIT) instruments. MSn analysis of negative ion precursors confirmed structural details within the lipid moiety, while CID spectra of sodiated precursor ions provided monosaccharide sequence and linkage for the oligosaccharide portion of the molecule. Results obtained in this study indicate that extensive heterogeneity exists within the oligosaccharide moieties in LOS from H. influenzae 2019. More importantly, the data suggest that additional hexose moieties, which are added onto the LOS, are not simple extensions of one particular core structure but rather that structural isomers with different connectivities are present within the heterogeneous mixture.

A convergent synthesis of trisaccharides with alpha-Neu5Ac-(2 --> 3)-beta-D-gal-(1 --> 4)-beta-D-GlcNAc and alpha-Neu5Ac-(2 --> 3)-beta-D-gal-(1 --> 3)-alpha-D-GalNAc sequences

The syntheses of three trisaccharides: alpha-Neu5Ac-(2 --> 3)-beta-D-Gal-(1 --> 4)-beta-D-GlcNAc --> OMe, alpha-Neu5Ac-(2 --> 3)-beta-D-Gal6SO3Na-(1 --> 4)-beta-D-GlcNAc --> OMe, and alpha-Neu5Ac-(2 --> 3)-beta-D-Gal-(1 --> 3)-alpha-D-GalNAc --> OBn were accomplished by using either methyl (phenyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio-beta-D-glycero-D-g alacto-2-nonulopyranoside)onate or methyl (phenyl N-acetyl-5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio-beta-D-gl ycero-D-galacto-2-nonulopyranoside)onate as the sialyl donor. The N,N-diacetylamino sialyl donor appears to be more reactive than its parent acetamido sugar when allowed to react with an disaccharide acceptor under the same glycosylation conditions. The trisaccharides, as well as the intermediate products, were fully characterized by 2D DQF 1H-1H COSY and 2D ROESY spectroscopy.

Isothermal titration calorimetric studies on the binding of deoxytrimannoside derivatives with artocarpin: implications for a deep-seated combining site in lectins

The carbohydrate binding specificity of the seed lectin from Artocarpus integrifolia, artocarpin, has been elucidated by the enzyme-linked lectin absorbent assay [Misquith, S., et al (1994) J. Biol. Chem. 269, 30393-30401], wherein it was demonstrated to be a Man/Glc specific lectin with high affinity for the trisaccharide present in the core of all N-linked oligosaccharide chains of glycoproteins. As a consequence of this characterization, the binding epitopes of this trisaccharide, 3, 6-di(alpha-D-mannopyranosyl)-D-mannose, for artocarpin were investigated by isothermal titration calorimetry using its monodeoxy as well as Glc and Gal analogues. The thermodynamic data presented here implicate 2-, 3-, 4-, and 6-hydroxyl groups of the alpha(1-3) Man and alpha(1-6) Man residues, and the 2- and 4-OH groups of the central Man residue, in binding to artocarpin. Nevertheless, alpha(1-3) Man is the primary contributor to the binding affinity, unlike other Man/Glc binding lectins which exhibit a preference for alpha(1-6) Man. In addition, unlike the binding reactions of most lectins reported so far, the interaction of mannotriose involves all of its hydroxyl groups with the combining site of the lectin. Moreover, the free energy and enthalpy contributions to binding of individual hydroxyl groups of the trimannoside estimated from the corresponding monodeoxy analogues show nonlinearity, suggesting differential contributions of the solvent and protein to the thermodynamics of binding of the analogues. Thus, this study not only provides evidence for the extended site recognition of artocarpin for the trimannoside epitope but also suggests that its combining site is best described as a deep cleft as opposed to shallow indentations implicated in other lectins.

Structural basis of carbohydrate recognition by lectin II from Ulex europaeus, a protein with a promiscuous carbohydrate-binding site

Protein-carbohydrate interactions are the language of choice for inter- cellular communication. The legume lectins form a large family of homologous proteins that exhibit a wide variety of carbohydrate specificities. The legume lectin family is therefore highly suitable as a model system to study the structural principles of protein-carbohydrate recognition. Until now, structural data are only available for two specificity families: Man/Glc and Gal/GalNAc. No structural data are available for any of the fucose or chitobiose specific lectins. The crystal structure of Ulex europaeus (UEA-II) is the first of a legume lectin belonging to the chitobiose specificity group. The complexes with N-acetylglucosamine, galactose and fucosylgalactose show a promiscuous primary binding site capable of accommodating both N-acetylglucos amine or galactose in the primary binding site. The hydrogen bonding network in these complexes can be considered suboptimal, in agreement with the low affinities of these sugars. In the complexes with chitobiose, lactose and fucosyllactose this suboptimal hydrogen bonding network is compensated by extensive hydrophobic interactions in a Glc/GlcNAc binding subsite. UEA-II thus forms the first example of a legume lectin with a promiscuous binding site and illustrates the importance of hydrophobic interactions in protein-carbohydrate complexes. Together with other known legume lectin crystal structures, it shows how different specificities can be grafted upon a conserved structural framework.

Structural analysis of a chimeric bacterial alpha-amylase. High-resolution analysis of native and ligand complexes

Several chimeric alpha-amylases genes were constructed by an in vivo recombination technique from the Bacillus amyloliquefaciens and Bacillus licheniformis genes. One of the fusion amylases (hereafter BA2), consisting of residues 1-300 from B. amyloliquefaciens and 301-483 from B. licheniformis, has been extensively studied by X-ray crystallography at resolutions between 2.2 and 1.7 A. The 3-dimensional structure of the native enzyme was solved by multiple isomorphous replacement, and refined at a resolution of 1.7 A. It consists of 483 amino acids, organized similarly to the known B. lichiniformis alpha-amylase structure [Machius et al. (1995) J. Mol. Biol. 246, 545-559], but features 4 bound calcium ions. Two of these form part of a linear cluster of three ions, the central ion being attributed to sodium. This cluster lies at the junction of the A and B domains with one calcium of the cluster structurally equivalent to the major Ca(2+) binding site of fungal alpha-amylases. The third calcium ion is found at the interface of the A and C domains. BA2 contains a fourth calcium site, not observed in the B. licheniformis alpha-amylase structure. It is found on the C domain where it bridges the two beta-sheets. Three acid residues (Glu261, Asp328, and Asp231) form an active site similar to that seen in other amylases. In the presence of TRIS buffer, a single molecule of TRIS occupies the -1 subsite of the enzyme where it is coordinated by the three active-center carboxylates. Kinetic data reveal that BA2 displays properties intermediate to those of its parents. Data for crystals soaked in maltooligosaccharides reveal the presence of a maltotriose binding site on the N-terminal face of the (beta/alpha)(8) barrel of the molecule, not previously described for any alpha-amylase structure, the biological function of which is unclear. Data for a complex soaked with the tetrasaccharide inhibitor acarbose, at 1.9 A, reveal a decasaccharide moiety, spanning the -7 to +3 subsites of the enzyme. The unambiguous presence of three unsaturated rings in the (2)H(3) half-chair/(2)E envelope conformation, adjacent to three 6-deoxypyranose units, clearly demonstrates synthesis of this acarbose-derived decasaccharide by a two-step transglycosylation mechanism.

NMR investigations of protein-carbohydrate interactions: studies on the relevance of Trp/Tyr variations in lectin binding sites as deduced from titration microcalorimetry and NMR studies on hevein domains. Determination of the NMR structure of the complex between pseudohevein and N,N',N"-triacetylchitotriose

Model studies on lectins and their interactions with carbohydrate ligands in solution are essential to gain insights into the driving forces for complex formation and to optimize programs for computer simulations. The specific interaction of pseudohevein with N,N', N"-triacetylchitotriose has been analyzed by (1)H-NMR spectroscopy. Because of its small size, with a chain length of 45 amino acids, this lectin is a prime target to solution-structure determination by NOESY NMR experiments in water. The NMR-analysis was extended to assessment of the topology of the complex between pseudohevein and N, N',N"-triacetylchitotriose. NOESY experiments in water solution provided 342 protein proton-proton distance constraints. Binding of the ligand did not affect the pattern of the protein nuclear Overhauser effect signal noticeably, what would otherwise be indicative of a ligand-induced conformational change. The average backbone (residues 3-41) RMSD of the 20 refined structures was 1.14 A, whereas the heavy atom RMSD was 2.18 A. Two different orientations of the trisaccharide within the pseudohevein binding site are suggested, furnishing an explanation in structural terms for the lectin's capacity to target chitin. In both cases, hydrogen bonds and van der Waals contacts confer stability to the complexes. This conclusion is corroborated by the thermodynamic parameters of binding determined by NMR and isothermal titration calorimetry. The association process was enthalpically driven. In relation to hevein, the Trp/Tyr-substitution in the binding pocket has only a small effect on the free energy of binding in contrast to engineered galectin-1 and a mammalian C-type lectin. A comparison of the three-dimensional structure of pseudohevein in solution to those reported for wheat germ agglutinin (WGA) in the solid state and for hevein and WGA-B in solution has been performed, providing a data source about structural variability of the hevein domains. The experimentally derived structures and the values of the solvent accessibilities for several key residues have also been compared with conformations obtained by molecular dynamics simulations, pointing to the necessity to further refine the programs to enhance their predictive reliability and, thus, underscoring the importance of this kind of combined analysis in model systems.

Automated docking of alpha-(1-->4)- and alpha-(1-->6)-linked glucosyl trisaccharides and maltopentaose into the soybean beta-amylase active site

The Lamarckian genetic algorithm of AutoDock 3.0 was used to dock alpha-maltotriose, methyl alpha-panoside, methyl alpha-isopanoside, methyl alpha-isomaltotrioside, methyl alpha-(6(1)-alpha-glucopyranosyl)-maltoside, and alpha-maltopentaose into the closed and, except for alpha-maltopentaose, into the open conformation of the soybean beta-amylase active site. In the closed conformation, the hinged flap at the mouth of the active site closes over the substrate. The nonreducing end of alpha-maltotriose docks preferentially to subsites -2 or +1, the latter yielding nonproductive binding. Some ligands dock into less optimal conformations with the nonreducing end at subsite -1. The reducing-end glucosyl residue of nonproductively-bound alpha-maltotriose is close to residue Gln194, which likely contributes to binding to subsite +3. In the open conformation, the substrate hydrogen-bonds with several residues of the open flap. When the flap closes, the substrate productively docks if the nonreducing end is near subsites -2 or -1. Trisaccharides with alpha-(1-->6) bonds do not successfully dock except for methyl alpha-isopanoside, whose first and second glucosyl rings dock exceptionally well into subsites -2 and -1. The alpha-(1-->6) bond between the second and third glucosyl units causes the latter to be improperly positioned into subsite +1; the fact that isopanose is not a substrate of beta-amylase indicates that binding to this subsite is critical for hydrolysis.

Flavonoid and benzophenone glycosides from Coleogyne ramosissima

A benzophenone glucoside and two flavonol glycosides were isolated together with 27 known polyphenols from the aerial parts of Coleogyne ramosissima, and their structures were elucidated by spectroscopic and chemical methods as iriflophenone 2-O-beta-glucopyranoside, isorhamnetin 3-O-2G-rhamnopyranosylrutinoside-7-O-alpha-rhamnopyranoside and limocitrin 3-O-rutinoside-7-O-beta-glucopyranoside, respectively.

Enzyme kinetics of hevamine, a chitinase from the rubber tree Hevea brasiliensis

The enzyme kinetics of hevamine, a chitinase from the rubber tree Hevea brasiliensis, were studied in detail with a new enzyme assay. In this assay, the enzyme reaction products were derivatized by reductive coupling to a chromophore. Products were separated by HPLC and the amount of product was calculated by peak integration. Penta-N-acetylglucosamine (penta-nag) and hexa-N-acetylglucosamine (hexa-nag) were used as substrates. Hexa-nag was more efficiently converted than penta-nag, which is an indication that hevamine has at least six sugar binding sites in the active site. Tetra-N-acetylglucosamine (tetra-nag) and allosamidin were tested as inhibitors. Allosamidin was found to be a competitive inhibitor with a K(i) of 3.1 microM. Under the conditions tested, tetra-nag did not inhibit hevamine.

NMR investigations of protein-carbohydrate interactions binding studies and refined three-dimensional solution structure of the complex between the B domain of wheat germ agglutinin and N,N', N"-triacetylchitotriose

The specific interaction of the isolated B domain of wheat germ agglutinin (WGA-B) with N,N',N"-triacetylchitotriose has been analyzed by 1H-NMR spectroscopy. The association constants for the binding of WGA-B to this trisaccharide have been determined from both 1H-NMR titration experiments and microcalorimetry methods. Entropy and enthalpy of binding have been obtained. The driving force for the binding process is provided by a negative DeltaH which is partially compensated by negative DeltaS. These negative signs indicate that hydrogen bonding and van der Waals forces are the major interactions stabilizing the complex. NOESY NMR experiments in water solution provided 327 protein proton-proton distance constraints. All the experimental constraints were used in a refinement protocol including restrained molecular dynamics in order to determine the refined solution conformation of this protein/carbohydrate complex. With regard to the NMR structure of the free protein, no important changes in the protein NOEs were observed, indicating that carbohydrate-induced conformational changes are small. The average backbone rmsd of the 35 refined structures was 1.05 A, while the heavy atom rmsd was 2.10 A. Focusing on the bound ligand, two different orientations of the trisaccharide within WGA-B binding site are possible. It can be deduced that both hydrogen bonds and van der Waals contacts confer stability to both complexes. A comparison of the three-dimensional structure of WGA-B in solution to that reported in the solid state and to those deduced for hevein and pseudohevein in solution has also been performed.

The role of weak protein-protein interactions in multivalent lectin-carbohydrate binding: crystal structure of cross-linked FRIL

Binding of multivalent glycoconjugates by lectins often leads to the formation of cross-linked complexes. Type I cross-links, which are one-dimensional, are formed by a divalent lectin and a divalent glycoconjugate. Type II cross-links, which are two or three-dimensional, occur when a lectin or glycoconjugate has a valence greater than two. Type II complexes are a source of additional specificity, since homogeneous type II complexes are formed in the presence of mixtures of lectins and glycoconjugates. This additional specificity is thought to become important when a lectin interacts with clusters of glycoconjugates, e.g. as is present on the cell surface. The cryst1al structure of the Glc/Man binding legume lectin FRIL in complex with a trisaccharide provides a molecular snapshot of how weak protein-protein interactions, which are not observed in solution, can become important when a cross-linked complex is formed. In solution, FRIL is a divalent dimer, but in the crystal FRIL forms a tetramer, which allows for the formation of an intricate type II cross-linked complex with the divalent trisaccharide. The dependence on weak protein-protein interactions can ensure that a specific type II cross-linked complex and its associated specificity can occur only under stringent conditions, which explains why lectins are often found forming higher-order oligomers.

Thermodynamic binding studies of lectins from the diocleinae subtribe to deoxy analogs of the core trimannoside of asparagine-linked oligosaccharides

Lectins from seven different species of the Diocleinae subtribe have been recently isolated and characterized in terms of their carbohydrate binding specificities (Dam, T. K., Cavada, B. S., Grangeiro, T. B., Santos, C. F., de Sousa, F. A. M., Oscarson, S., and Brewer, C. F. (1998) J. Biol. Chem. 273, 12082-12088). The lectins included those from Canavalia brasiliensis, Cratylia floribunda, Dioclea rostrata, Dioclea virgata, Dioclea violacea, and Dioclea guianensis. All of the lectins exhibited specificity for Man and Glc residues, but much higher affinities for the branched chain trimannoside, 3,6-di-O-(alpha-d-mannopyranosyl)-d-mannose, which is found in the core region of all asparagine-linked carbohydrates. In the present study, isothermal titration microcalorimetry is used to determine the binding thermodynamics of the above lectins, including a new lectin from Canavalia grandiflora, to a complete series of monodeoxy analogs of the core trimannoside. From losses in the affinity constants and enthalpies of binding of certain deoxy analogs, assignments are made of the hydroxyl epitopes on the trimannoside that are involved in binding to the lectins. The pattern of binding of the deoxy analogs is similar for all seven lectins, and similar to that of concanavalin A which is also a member of the Diocleinae subtribe. However, differences in the magnitude of the thermodynamic binding parameters of the lectins are observed, even though the lectins possess conserved contact residues in many cases, and highly conserved primary sequences. The results indicate that non-contact residues in the lectins, even those distant from the binding sites, modulate their thermodynamic binding parameters.

Isoangoroside C, a phenylpropanoid glycoside from Scrophularia scorodonia roots

A new phenylpropanoid glycoside isoangoroside C was isolated from the roots of Scrophularia scorodonia. Its structure was determined on the basis of spectral data as: 3-hydroxy-4-methoxy-beta-phenylethoxy-O-alpha-L-arabinopyranosy l-(1-->6)alpha L-rhamnopyranosyl-(1-->3)-4-O-Z-feruloyl-beta-D-glucopyranoside. Additionally, one known phenylpropanoid, angoroside C, and five known iridoid glycosides, harpagoside, bartsioside, 8-O-acetyl-harpagide, aucuboside and harpagide were isolated and identified.

Determination of the solution conformation of d-gluco-dihydroacarbose, a high-affinity inhibitor bound to glucoamylase by transferred NOE NMR spectroscopy

The determination of the bound solution conformation of D-gluco-dihydroacarbose (GAC), a tight-binding inhibitor of several glycosidase and amylase enzymes, by glucoamylase is described. Transferred NOE NMR experiments and line-broadening effects indicate that GAC is bound in a conformation resembling that observed in the crystal structure. This contrasts with the predominant conformation of GAC when free in solution. The NMR results also suggest regions on the carbohydrate that are in close contact with the protein. The determination of the bound solution conformation of GAC by glucoamylase using transferred NOE (trNOE) measurements is a significant achievement given the high affinity constant (Ka = 3 x 10(7) M(-1)) for this receptor-ligand pair. It is striking that the off-rate for complexation is still sufficiently high to permit observation of trNOEs.

Molecular dynamics simulation and nuclear magnetic resonance studies of the terminal glucotriose unit found in the oligosaccharide of glycoprotein precursors

The trisaccharide alpha-d-Glcp-(1 --> 2)-alpha-d-Glcp-(1 --> 3)-alpha-d-Glcp-OMe, a model for the terminal glucotriose in Glc(3)Man(9)GlcNAc(2) in glycoprotein precursors, has been investigated by computer simulations and NMR spectroscopy. Molecular dynamics simulations were performed for 1 ns in aqueous solution and 20 ns in vacuo using the CHARMM-based force fields PARM22 and CHEAT95. An additional Monte Carlo simulation with the HSEA force field was also carried out. Experimental NMR data in water solution was obtained from measurement of long-range (1)H,(13)C heteronuclear trans-glycosidic coupling constants, (3)J(H,C), using one-dimensional Hadamard spectroscopy. Calculation of the (3)J(H,C) values from the simulations showed a varying degree of agreement to experimental data. It could be shown from simulation that the &phi; torsion angles differed, which was corroborated by the NMR measurements. Analyses were done of radial distribution functions and of hydrogen bonds. It was suggested that intermolecular hydrogen bonds were present, but in contrast to simulation the results from NMR spectroscopy did not support any major contribution. Hence, their influence on the conformation of the trisaccharide is rather small. Comparison of (1)H NMR chemical shifts for the trisaccharide and the glucotriose in Glc(3)Man(8)GlcNAc revealed high similarity. However, the derived conformation of the model substance in this work differed at one glycosidic torsion angle compared to the glucotriose on a large oligosaccharide.

Identification and synthesis of a trisaccharide produced from lactose by transgalactosylation

Enzymatic transgalactosylation of lactose by means of Streptococcus thermophilus, subspecies DN-001065, led to a mixture of D-galactose (approximately 4%), D-glucose (approximately 15%), lactose (approximately 51%), minor disaccharides (6%), trisaccharides (approximately 20%) and tetrasaccharides (3%). The major trisaccharide (approximately 16%) was identified by NMR spectroscopy and chemical synthesis as being the known beta-D-galactopyranosyl-(1-->3)-beta-D-galactopyranosyl-(1-->4)-D-glucos e (3'-beta-D-galactopyranosyl-lactose). It was purified from a mixture of peracetylated oligosaccharides by column chromatography followed by deacetylation. For the first time, 3'-beta-D-galactopyranosyl-lactose has been obtained on the 1 g scale, by resorting to simple techniques and equipment. NMR spectra have been unambiguously assigned.