PubMed Health⌕ Search

Biomedical subjects

W S York

Publications and source records attributed to W S York.

33 records · Page 2Linked to original sources
PubMed1993

The conformations of cyclic (1-->2)-beta-D-glucans: application of multidimensional clustering analysis to conformational data sets obtained by Metropolis Monte Carlo calculations.

Sets containing up to 1.3 x 10(6) energetically accessible conformations of linear (1-->2)-beta-D-glucan oligosaccharides were obtained by Metropolis Monte Carlo (MMC) calculations performed with the GEGOP (GEometry of GlycOProteins) program. Quantitative analyses of the data sets (which were expressed in terms of the glycosidic dihedral angle coordinates) were obtained by two different clustering methods: (i) the three-distance hierarchical clustering method (3-DM), published by Jure Zupan, and (ii) a nonhierarchical clustering method (Population-Density Projection, PDP) which, through a segmentation analysis of two-dimensional projections of the population-density surface, establishes a partitioning of conformational space into a set of "cluster regions", followed by a clustering step where each conformation of the data set is assigned to one of these regions. Computer programs (MCLUST and PDPCLUST) were developed to perform the 3-DM and PDP analyses, respectively. The two types of analysis provided very similar sets of conformational families (clusters), which could be expressed as combinations of distinct conformations of the glycosidic torsional angles (phi, psi) centered at (50 degrees, 10 degrees) for conformation A, (40 degrees, 160 degrees) for conformation B, (55 degrees, -160 degrees) for conformation B', and (170 degrees, 10 degrees) for conformation C. The analysis provided the populations of the families, along with relative rates for transitions between families. Examination of the frequencies of the A, B, and C glycosidic bond conformations with respect to their relative positions in the sequence revealed the tendency of the (1-->2)-beta-D-glucan to adopt conformational repeating structures of the general form [AnB], where n = 3 or 6. These repeating structures combine in an energetically cooperative fashion to give low-energy cyclic conformations having, for example C5 symmetry [AAAB]5 for the eicosamer, and C3 symmetry [AAAAAAB]3 for the heneicosamer.

Carbohydrate Conformation↗
PubMed1993

Structural analysis of tamarind seed xyloglucan oligosaccharides using beta-galactosidase digestion and spectroscopic methods.

The borohydride-reduced forms (oligoglycosyl alditols) of two isomeric octasaccharides (Glc4Xyl3Gal) that are released from xyloglucans of various plant species upon treatment with a fungal endo-(1-->4)-beta-glucanase were isolated and structurally characterized. A mixture of oligosaccharides that is released from tamarind seed xyloglucan by the endo-(1-->4)-beta- glucanase was digested with a commercially available beta-galactosidase (Aspergillus niger). The beta-galactosidase selectively hydrolyzed the galactosyl residue of one of the two isomeric octasaccharides present in the mixture. A homogeneous preparation of the beta-galactosidase-resistant octasaccharide was prepared by high-resolution gel-permeation chromatography of the enzyme-digestion products. Spectroscopic characterization of the oligoglycosyl alditol prepared by reduction of this octasaccharide confirmed the previously proposed structure that had been based on analysis of the mixture of isomeric octasaccharides. The availability of large amounts of the pure, reduced octasaccharide and of a pure, reduced pentasaccharide (Glc3Xyl2) made it possible to completely assign their 1H and 13C NMR spectra. In addition, the borohydride-reduced form of the beta-D-galactosidase-susceptible octasaccharide isomer was purified by high pH anion-exchange chromatography of the endo-(1-->4)-beta-glucanase-released octasaccharides from rape-seed xyloglucan (no beta-galactosidase treatment), and its 1H and 13C NMR spectra were assigned. Additional correlations between specific structural features of xyloglucan oligoglycosyl alditols and the positions of specific resonances in their NMR spectra were deduced and added to the extensive list that we have compiled. The effects of recording the NMR spectra of the xyloglucan oligoglycosyl alditols in the presence of borate salts, which could lead to incorrect structural assignments, are also described.

Carbohydrate Conformation↗
PubMed1993

Ultrastructural changes in follicle cell-oocyte associations during development and maturation of the ovarian follicle in Atlantic croaker.

Full-grown ovarian follicles of the marine teleost Atlantic croaker (Micropogonias undulatus) are unresponsive to steroidal maturation-inducing substance (MIS). However, responsiveness to MIS (maturational competence) can be induced in vivo or in vitro by brief stimulation with gonadotropin (e.g., human chorionic gonadotropin, hCG). The objective of this study was to examine ultrastructural changes in follicle cell-oocyte associations related to the acquisition of maturational competence and maturation in croaker ovarian follicles. Ultrathin sections of lanthanum-infiltrated samples were observed by transmission electron microscopy at various points during follicular development and hormone-induced maturation: vitellogenic oocytes (< 350 microns), full-grown oocytes (350-400 microns), hCG-stimulated (maturationally competent) oocytes, hCG/MIS-stimulated (early-maturation, hydrating) oocytes, and ovulated oocytes. In general, prior to ovulation, oocyte microvilli made contact with granulosa cells, and gap junction-like structures were seen at these points of contact. Midvitellogenic follicles (< 350 microns) had relatively high levels of follicle cell-oocyte associations and gap junctional contacts, but by late vitellogenesis (350-400 microns) these levels had declined markedly. However, high levels of intercellular associations and gap junctional contacts were reestablished upon induction of maturational competence with hCG, and these levels remained high in early-maturation oocytes. These changes seemed to be mainly due to alterations in oocyte microvilli length and surface contact with granulosa cells rather than to alterations in the numbers of microvilli. However, retraction and disappearance of microvilli occurred during ovulation. Therefore, the appearance of maturational competence coincided with the reestablishment of heterocellular gap junctional contacts in full-grown follicles. One function of these contacts may be to allow transfer of maturational signals between the granulosa cells and the oocyte during the early stages of follicular maturation.

Animals↗
PubMed1993

Measurement of inter-glycosidic 13C-1H coupling constants in a cyclic beta(1-->2)-glucan by 13C-filtered 2D (1H,1H)ROESY.

A method for measuring three-bond 13C-1H scalar coupling constants across glycosidic bonds in a cyclic beta(1-->2)-glucan icosamer is presented. This oligosaccharide molecule, with its high degree of symmetry, represents a particular challenge for NMR spectroscopy to distinguish inter-residue from intra-residue heteronuclear coupling effects. Chemically equivalent H2 protons in adjacent glucosyl residues are distinguished on the basis of their different through-space, dipolar interactions with the anomeric protons (H1). The strong NOE contact between anomeric (H1) and aglyconic (H2') protons permits the selective observation of the inter-residue heteronuclear couplings 3JC1H2' and 3JC2'H1 in a natural-abundance 13C-omega 1-half-filtered (1H,1H)ROESY experiment.

Carbohydrate Conformation↗
PubMed1992

Characterization of seven xyloglucan oligosaccharides containing from seventeen to twenty glycosyl residues.

The complete primary structures of seven oligosaccharide subunits of the xyloglucan secreted by suspension-cultured Acer pseudoplatanus cells were determined. The oligosaccharides, ranging in size from 17 to 20 glycosyl residues, were generated by treatment of the xyloglucan with an endo-beta-(1----4)-glucanase. The oligosaccharide components of a fraction obtained by Bio-Gel P-2 chromatography of enzyme-treated xyloglucan were further purified by normal-phase h.p.l.c. and then converted to the corresponding oligoglycosyl alditols by reduction with NaBH4. The oligoglycosyl alditols, after purification to near homogeneity by reversed-phase h.p.l.c., were structurally characterized by 1H-n.m.r. spectroscopy, fast-atom bombardment mass spectrometry (f.a.b.-m.s.), and analysis of their glycosyl-residue and glycosyl-linkage compositions. Novel structural elements of xyloglucans were observed in this study, including beta-D-xylopyranosyl and alpha-L-arabinofuranosyl-(1----3)-beta-D-xylopyranosyl sidechains. The results also extend our list of correlations between 1H-n.m.r. resonances and specific structural features of xyloglucans and thus enhance our ability to determine the structures of xyloglucans from various sources.

Carbohydrate Sequence↗
PubMed1991

A novel highly unsaturated fatty acid moiety of lipo-oligosaccharide signals determines host specificity of Rhizobium.

In Rhizobium leguminosarum biovar viciae, the nodABC and nodFEL operons are involved in the production of lipo-oligosaccharide signals which mediate host specificity. The structure of these metabolites and those produced in nod mutants links the nodE and nodL genes to specific chemical features of the signal molecules. A nodE-determined, highly unsaturated fatty acid and a nodL-determined O-acetyl substituent are essential for the ability of the signals to induce nodule meristems on the host plant Vicia sativa.

Fabaceae↗
PubMed1991

A new undecasaccharide subunit of xyloglucans with two alpha-L-fucosyl residues.

A new oligosaccharide subunit of xyloglucan was isolated from the beta-(1----4)-endoglucanase digestion products of the xyloglucan in what is referred to as "sycamore extracellular polysaccharides" and found to be an undecasaccharide having two terminal alpha-L-fucopyranosyl residues. The undecasaccharide was structurally characterized by 1H-n.m.r. spectroscopy, fast-atom bombardment mass spectrometry (f.a.b.-m.s.), and glycosyl-residue and glycosyl-linkage composition analyses. The structure of the undecasaccharide was confirmed by digesting it with a hydrolase that releases alpha-D-Xylp-(1----6)-D-Glc from the non-reducing end of xyloglucan oligosaccharides.

Acylation↗
PubMed1991

Simulations of the static and dynamic molecular conformations of xyloglucan. The role of the fucosylated sidechain in surface-specific sidechain folding.

The hemicellulosic polysaccharide xyloglucan binds with a strong affinity to cellulosic cell wall microfibrils, the resulting heterogeneous network constituting up to 50% of the dry weight of the cell wall in dicotyledonous plants. To elucidate the molecular details of this interaction, we have performed theoretical potential energy calculations of the static and dynamic equilibrium conformations of xyloglucan using the GEGOP software. In particular, we have evaluated the preferred sidechain conformations of hexa-, octa-, deca- and heptadecasaccharide model fragments of xyloglucan for molecules with a cellulose-like, flat, glucan backbone, and a cellobiose-like, twisted, glucan backbone conformation. For the flat backbone conformation the determination of static equilibrium molecular conformations revealed a tendency for sidechains to fold onto one surface of the backbone, defined here as the H1S face, in the fucosylated region of the polymer. This folding produces a molecule that is sterically accessible on the opposite face of the backbone, the H4S face. Typically, this folding onto the H1S surface is significantly stabilized by favorable interactions between the fucosylated, trisaccharide sidechain and the backbone, with some stabilization from adjacent terminal xylosyl sidechains. In contrast, the trisaccharide sidechain folds onto the H4S face of xyloglucan fragments with a twisted backbone conformation. Preliminary NMR data on nonasaccharide fragments isolated from sycamore suspension-cultured cell walls are consistent with the hypothesis that the twisted conformation of xyloglucan represents the solution form of this molecule. Metropolis Monte Carlo (MMC) simulations were employed to assess sidechain flexibility of the heptadecasaccharide fragments. Simulations performed on the flat, rigid, backbone xyloglucan indicate that the trisaccharide sidechain is less mobile than the terminal xylosyl sidechains. MMC calculations on a fully relaxed molecule revealed a positive correlation between a specific trisaccharide sidechain orientation and the 'flatness' of the backbone glucosyl residues adjacent to this sidechain. These results suggest that the trisaccharide sidechain may play a role in the formation of nucleation sites that initiate the binding of these regions to cellulose. Based on these conformational preferences we suggest the following model for the binding of xyloglucan to cellulose. Nucleation of a binding site is initiated by the fucosylated, trisaccharide sidechain that flattens out an adjacent region of the xyloglucan backbone. Upon contacting a cellulose microfibril this region spreads by step-wise flattening of successive segments of the backbone. Self-association of xyloglucan molecules in solution may be prevented by the low frequency of formation of these nucleation sites and the geometry of the molecules in solution.

Carbohydrate Sequence↗
PubMed1990

Structural analysis of xyloglucan oligosaccharides by 1H-n.m.r. spectroscopy and fast-atom-bombardment mass spectrometry.

A method to determine rapidly the identities and proportions of the oligosaccharide repeating-units in plant cell-wall xyloglucans by 1D 1H-n.m.r. spectroscopy was developed. Six of the most commonly found xyloglucan oligosaccharide subunits (including three subunits that had not been fully characterized previously) were prepared by endo-(1----4)-beta-D-glucanase digestion of xyloglucans from various plant species. The oligosaccharides were reduced to the corresponding oligoglycosyl-alditols, purified, and characterized by glycosyl composition and linkage analysis, 1H-n.m.r. spectroscopy, and f.a.b.-mass spectrometry. Correlations between the 1H-n.m.r. spectra and the structures of the oligoglycosyl-alditols can be used to identify oligoglycosyl-alditols derived from xyloglucans of unknown structure. The identities and relative amounts of the oligosaccharide subunits of xyloglucans isolated from tamarind seed and rapeseed hulls were determined on this basis.

Carbohydrate Conformation↗
PubMed1988

Location of the O-acetyl substituents on a nonasaccharide repeating unit of sycamore extracellular xyloglucan.

The locations of the O-acetyl substituents on the major nonasaccharide repeating unit of the xyloglucan isolated from sycamore extracellular polysaccharides were determined by a combination of analytical methods, including f.a.b.-m.s. and 1H-n.m.r. spectroscopy. The O-2-linked-beta-D-galactosyl residue of the nonasaccharide was found to be the dominant site of O-acetyl substitution. Both mono-O-acetylated and di-O-acetylated beta-D-galactosyl residues were detected. The degree of O-acetylation of the beta-D-galactosyl residue, was estimated by 1H-n.m.r. spectroscopy to be 55-60% at O-6, 15-20% at O-4, and 20-25% at O-3. 1H-n.m.r. spectroscopy also indicated that approximately 50% of the beta-D-galactosyl residues are mono-O-acetylated, 25-30% are di-O-acetylated, and 20% are not acetylated.

Carbohydrate Conformation↗
PubMed1985

Structure and function of plant cell wall polysaccharides.

Studies of the primary structures of polysaccharides of growing plant cell walls have shown that these structures are far more complex than was anticipated just a few years ago. This complexity can best be appreciated by considering xyloglucan, a hemicellulose present in the cell wall of both monocots and dicots, and rhamnogalacturonan II (RG-II) and rhamnogalacturonan I (RG-I), two structurally unrelated pectic polysaccharides. This realization led us to postulate that cell wall polysaccharides have functions beyond determining the size, shape and strength of plants. Some years ago we demonstrated that oligosaccharide fragments of a branched beta-linked glucan of fungal cell walls can elicit the production of phytoalexins (antibiotics) in plants by inducing the formation of the enzymes responsible for synthesis of the phytoalexins. It has now been ascertained and confirmed by synthesis that the elicitor activity resides in a very specific hepta-beta-D-glucoside. The heptaglucoside has been shown to elicit phytoalexins by activating the expression of specific genes, that is, by causing the synthesis of the mRNAs that encode the enzymes that synthesize phytoalexins. In other words, complex carbohydrates can be regulatory molecules. Further experiments established that oligosaccharide fragments of polysaccharides, produced by acid or base hydrolysis or by enzymolysis of primary cell walls of plants, also evoked defence responses in plants. Subsequently, we learned that defined fragments of polysaccharides, released from covalent attachment within plant cell walls, can function as regulators of various physiological processes such as morphogenesis, rate of cell growth and time of flowering and rooting, in addition to activating mechanisms for resisting potential pathogens. Examples of plant oligosaccharides with regulatory properties (called oligosaccharins) will be described.

Cell Survival↗
PubMed1984

Inhibition of 2,4-dichlorophenoxyacetic Acid-stimulated elongation of pea stem segments by a xyloglucan oligosaccharide.

Xyloglucan, isolated from the soluble extracellular polysaccharides of suspension-cultured sycamore (Acer pseudoplatanus) cells, was digested with an endo-beta-1,4-glucanase purified from the culture fluid of Trichoderma viride. A nonasaccharide-rich Bio-Gel P-2 fraction of this digest inhibited 2,4-dichlorophenoxyacetic-acid-stimulated elongation of etiolated pea stem segments. The inhibitory activity of this oligosaccharide fraction exhibited a well-defined concentration optimum between 10(-2) and 10(-1) micrograms per milliliter. Another fraction of the same xyloglucan digest, rich in a structurally related heptasaccharide, did not, at similar concentrations, significantly inhibit the elongation.

Journal Article↗
PubMed1980

Beta-2-linked glucans secreted by fast-growing species of Rhizobium.

Fast-growing species of Rhizobium were found to secrete low-molecular-weight beta-2-linked glucans when cultured in synthetic liquid medium. These glucans are quite similar to beta-2-linked glucans produced by species of Agrobacterium. No reducing terminus was detected in these glucans.

Chemical Phenomena↗