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S W Homans

Publications and source records attributed to S W Homans.

At least 55 records · Page 3Linked to original sources
PubMed1992

Reducing the overlap problem in the proton NMR spectra of oligosaccharides by application of pseudo-four-dimensional homonuclear HOHAHA-HOHAHA-COSY.

A new homonuclear NMR experiment is described for the assignment of the proton NMR spectra of oligosaccharides, namely HOHAHA-HOHAHA-COSY (where HOHAHA is homonuclear Hartmann-Hahn spectroscopy and COSY is correlated spectroscopy). While this technique may formally be thought of as a four-dimensional NMR experiment, by use of selective pulses it is demonstrated that the analogous pseudo-four-dimensional experiment is a valuable improvement over conventional three-dimensional HOHAHA-COSY, in that the degree of resonance overlap is markedly reduced by the dispersion of resonances into a fourth effective dimension. The technique is demonstrated by application to the biantennary nonasaccharide Gal beta 1-4-GlcNAc beta 1-2Man alpha 1-6(Gal beta 1-4GlcNAc beta 1-2Man alpha 1-3)Man-beta 1-4GlcNAc beta 1-4GlcNAc.

Carbohydrate Conformation↗
PubMed1991

Complete structure of the glycan of lipopeptidophosphoglycan from Trypanosoma cruzi Epimastigotes.

The lipopeptidophosphoglycan is the major cell surface glycoconjugate of the epimastigote forms of the parasitic protozoan Trypanosoma cruzi. A detailed partial structure for this molecule has been reported (Previato, J. O., Gorin, P. A. J., Mazurek, M., Xavier, M. T., Fournet, B., Wieruszesk, J. M., and Mendonca-Previato, L. (1990) J. Biol. Chem. 265, 2518-2526). In this study, we complete the primary structure assignments and describe the microheterogeneity found in the lipopeptidophosphoglycan glycan, using a combination of 1H and 31P NMR, fast atom bombardment mass spectrometry, methylation linkage analysis, and exoglycosidase sequencing. The lipopeptidophosphoglycan is a glycosylated inositol-phosphoceramide with striking homology to glycosylphosphatidylinositol membrane anchors found attached to a wide variety of plasma membrane proteins throughout the eukaryotes.

Acylation↗
PubMed1991

Evolutionary aspects of GPI metabolism in kinetoplastid parasites.

There is a growing, but still very patchy, data base of GPI structure, biosynthesis and function. In this article we speculate freely on the function of GPI anchors, and the origins of GPI-related molecules, primarily with reference to the protozoan parasites Trypanosoma brucei and the Leishmania. The views expressed draw on fairly wild extrapolations and some will, no doubt, not stand the tests of time. Several of the hypotheses presented should therefore be taken with a pinch of salt, some lemon, and large quantities of tequila!

Animals↗
PubMed1990

Structure of the lipophosphoglycan from Leishmania major.

The major cell surface glycoconjugate of the parasitic protozoan Leishmania major is a heterogeneous lipophosphoglycan. It has a tripartite structure, consisting of a phosphoglycan (Mr 5,000-40,000), a variably phosphorylated hexasaccharide glycan core, and a lysoalkylphosphatidylinositol (lysoalkyl-PI) lipid anchor. The structures of the phosphoglycan and the hexasaccharide core were determined by monosaccharide analysis, methylation analysis, fast atom bombardment-mass spectrometry, one- and two-dimensional 500-MHz (correlated spectroscopy (COSY), homonuclear Hartmann-Hahn spectroscopy (HOHAHA] 1H NMR spectroscopy, and exoglycosidase digestions. The phosphoglycan consists of eight types of phosphorylated oligosaccharide repeats which have the general structure, [formula: see text] where R = H, Galp(beta 1-3), Galp(beta 1-3)Galp(beta 1-3), Arap(alpha 1-2)Galp(beta 1-3), Glcp(beta 1-3)Galp(beta 1-3), Galp(beta 1-3)Galp(beta 1-3)Galp(beta 1-3), Arap(alpha 1-2)Galp(beta 1-3)Galp(beta 1-3), or Arap(alpha 1-2)Galp(beta 1-3)Galp(beta 1-3)Galp(beta 1-3)Galp(beta 1-3), and where all the monosaccharides, including arabinose, are in the D-configuration. The average number of repeat units/molecule (n) is 27. Data are presented which suggest that the nonreducing terminus of the phosphoglycan is capped exclusively with the neutral disaccharide Manp(alpha 1-2)Manp alpha 1-. The structure of the glycan core was determined to be, [formula: see text] where approximately 60% of the mannose residues distal to the glucosamine are phosphorylated and where the inositol is part of the lysoalkyl-PI lipid moiety containing predominantly 24:0 and 26:0 alkyl chains. The unusual galactofuranose residue is in the beta-configuration, correcting a previous report where this residue was identified as alpha Galf. Although most of the phosphorylated repeat units are attached to the terminal galactose 6-phosphate of the core to form a linear lipophosphoglycan (LPG) molecule, some of the mannose 6-phosphate residues may also be substituted to form a Y-shaped molecule. The L. major LPG is more complex than the previously characterized LPG from Leishmania donovani, although both LPGs have the same repeating backbone structure and glycolipid anchor. Finally we show that the LPG anchor is structurally related to the major glycolipid species of L. major, indicating that some of these glycolipids may have a function as precursors to LPG.

Animals↗
PubMed1990

Structure of the glycosyl-phosphatidylinositol membrane anchor of the Leishmania major promastigote surface protease.

In common with many other plasma membrane glycoproteins of eukaryotic origin, the promastigote surface protease (PSP) of the protozoan parasite Leishmania contains a glycosyl-phosphatidylinositol (GPI) membrane anchor. The GPI anchor of Leishmania major PSP was purified following proteolysis of the PSP and analyzed by two-dimensional 1H-1H NMR, compositional and methylation linkage analyses, chemical and enzymatic modifications, and amino acid sequencing. From these results, the structure of the GPI-containing peptide was found to be Asp-Gly-Gly-Asn-ethanolamine-PO4-6Man alpha 1-6Man alpha 1-4GlcN alpha 1-6myo-inositol-1-PO4-(1-alkyl-2-acyl-glycerol). The glycan structure is identical to the conserved glycan core regions of the GPI anchor of Trypanosoma brucei variant surface glycoprotein and rat brain Thy-1 antigen, supporting the notion that this portion of GPIs are highly conserved. The phosphatidylinositol moiety of the PSP anchor is unusual, containing a fully saturated, unbranched 1-O-alkyl chain (mainly C24:0) and a mixture of fully saturated unbranched 2-O-acyl chains (C12:0, C14:0, C16:0, and C18:0). This lipid composition differs significantly from those of the GPIs of T. brucei variant surface glycoprotein and mammalian erythrocyte acetylcholinesterase but is similar to that of a family of glycosylated phosphoinositides found uniquely in Leishmania.

Amino Acid Sequence↗
PubMed1990

A molecular mechanical force field for the conformational analysis of oligosaccharides: comparison of theoretical and crystal structures of Man alpha 1-3Man beta 1-4GlcNAc.

A molecular mechanical force field is described for the conformational analysis of oligosaccharides. This force field has been derived by the addition of new parameters to the AMBER force field and is compatible with simulations of proteins. This new parametrization is assessed by comparison of the theoretically predicted conformations of Man alpha 1-3Man beta 1-4GlcNAc with the corresponding crystal structure. Molecular dynamics simulation data are presented for this structure both in vacuo and with the explicit inclusion of water molecules. While the former demonstrate significant torsional oscillations about glycosidic linkages at physiological temperature, in the latter these oscillations are highly damped due to the stabilizing influence of a "cage" of solvent-solvent and solvent-solute hydrogen bonds.

Carbohydrate Conformation↗
PubMed1990

Structures of the glycoinositolphospholipids from Leishmania major. A family of novel galactofuranose-containing glycolipids.

Structures of the major glycolipids isolated from the protozoan parasite Leishmania major (strains V121 and LRC-L119), were elucidated by fast atom bombardment-mass spectrometry, two-dimensional proton NMR, methylation analysis, exoglycosidase digestions and mild acid hydrolysis. These glycolipids belong to a family of glycoinositolphospholipids (GIPLs), which contain 4-6 saccharide residues linked to alkylacylphosphatidylinositol (alkylacyl-PI) or lyso alkyl-PI. The general structure of the elucidated GIPLs can be expressed as follows: R-3Galf(alpha 1-3)Manp(alpha 1-3)Manp(alpha 1-4)GlcNp(alpha 1-6) alkylacyl-PI or lyso alkyl-PI where R = OH for GIPL-1; R = Galp(alpha 1- for GIPL-2; R = Galp(alpha 1-6)Galp (alpha 1- for GIPL-3 and R = Galp(alpha 1-3)Galf(alpha 1- for GIPL-A. The alkylacyl-PI lipid moieties are unusual in containing predominantly 18:0, 22:0, 24:0, or 26:0 alkyl chains and 12:0, 14:0, or 16:0 acyl chains. Remodeling of the lipid moieties may occur based on the finding that 1) lyso derivatives account for approximately 35% of the GIPL-3 fraction in strain V121 and 2) there is an increase in the proportion of 24:0 and 26:0 alkyl chains with elongation of the carbohydrate chain. Together with the elucidated structures, these properties are consistent with some of the GIPLs having a role as biosynthetic precursors to the major cell surface glycoconjugate, lipophosphoglycan. In particular, the saccharide sequences of GIPL-3, lyso-GIPL-3, and the glycan core of lipophosphoglycan (Turco, S. J., Orlandi, P. A., Homans, S. W., Ferguson, M. A. J., Dwek, R. A., and Rademacher, T. W. (1989) J. Biol. Chem. 264, 6711-6715) are identical. Finally, immunostaining of thin layer chromatograms with antibodies from patients with cutaneous leishmaniasis suggests that the major GIPLs are highly immunogenic and that the elevated anti-Gal antibodies, commonly seen in leishmaniasis patients, may be directed against terminal Galp(alpha 1-3)Galf residues.

Animals↗
PubMed1989

Structure of the phosphosaccharide-inositol core of the Leishmania donovani lipophosphoglycan.

The phosphosaccharide-inositol core of the lipophosphoglycan of Leishmania donovani was generated by treatment of the glycoconjugate with mild acid and digestion with phosphatidylinositol-specific phospholipase C. The core was purified and examined by one- and two-dimensional 1H-1H NMR and by methylation analysis. From the results, the carbohydrate core was elucidated as a phosphosaccharide attached to the inositol residue of the lyso-alkylphosphatidylinositol anchor of lipophosphoglycan as follows: PO4----6GalP(alpha 1----6)GalP(alpha 1----3)Galf(alpha 1----3)ManP(alpha 1----3)ManP(alpha 1----4)GlcNP(alpha 1----6)myo-inositol. The presence of an internal galactofuranose residue is highly unusual and the ManP(alpha 1----4)GlcNP(alpha 1----6)myo-inositol sequence is homologous to the respective portion of the glycosylphosphatidylinositol anchors reported for both the Trypanosoma brucei variant surface glycoprotein and the rat brain Thy-1 glycoprotein.

Animals↗
PubMed1989

Solution structure of the glycosylphosphatidylinositol membrane anchor glycan of Trypanosoma brucei variant surface glycoprotein.

The average solution conformation of the glycosylphosphatidylinositol (GPI) membrane anchor of Trypanosoma brucei variant surface glycoprotein (VSG) has been determined by using a combination of two-dimensional 1H-1H NMR methods together with molecular orbital calculations and restrained molecular dynamics simulations. This allows the generation of a model to describe the orientation of the glycan with respect to the membrane. This shows that the glycan exists in an extended configuration along the plane of the membrane and spans an area of 600 A2, which is similar to the cross-sectional area of a monomeric N-terminal VSG domain. Taken together, these observations suggest a possible space-filling role for the GPI anchor that may maintain the integrity of the VSG coat. The potential importance of the GPI glycan as a chemotherapeutic target is discussed in light of these observations.

Animals↗
PubMed1988

Complete structure of the glycosyl phosphatidylinositol membrane anchor of rat brain Thy-1 glycoprotein.

Glycosyl-phosphatidylinositol (GPI) anchors have recently been identified as alternatives to hydrophobic amino acid sequences for the attachment of a variety of eukaryotic cell surface molecules to the lipid bilayer. In single cell eukaryotes the GPI group appears to be the predominant form of membrane attachment, and in vertebrates a substantial minority of molecules have this anchor including cell surface hydrolytic enzymes, antigens and cell adhesion molecules. Analysis of different GPI anchors suggests they share common structural features including linkage to the COOH group of the terminal amino acid via ethanolamine phosphate, the presence of phosphatidylinositol lipid and a glycan between the bridging ethanolamine phosphate and the lipid. In the case of the Trypanosoma brucie variant surface glycoprotein (VSG) the full structure of the GPI anchor has been determined and this provides a prototype for comparison with other molecules. We now report the structure of the GPI anchor of rat brain Thy-1 glycoprotein. It has an identical backbone to the VSG anchor but shows significant differences in side chain moieties.

Animals↗
PubMed1988

Glycosyl-phosphatidylinositol moiety that anchors Trypanosoma brucei variant surface glycoprotein to the membrane.

Two forms of protein-membrane anchor have been described for the externally disposed glycoproteins of eukaryotic plasma membranes; namely, the hydrophobic transmembrane polypeptide and the complex glycosylphosphatidylinositol (G-PI) moiety. The chemical structures of the major species of G-PI anchors found on a single variant surface glycoprotein (VSG) of the parasitic protozoan Trypanosoma brucei were determined by a combination of nuclear magnetic resonance spectroscopy, mass spectrometry, chemical modification, and exoglycosidase digestions. The G-PI anchor was found to be heterogeneous with respect to monosaccharide sequence, and several novel glycosidic linkages were present. The results are pertinent to the mechanism of the biosynthesis of G-PI anchors.

Animals↗
PubMed1988

Parasite glycoconjugates: towards the exploitation of their structure.

The parasitic protozoa express many unusual complex carbohydrates at the cell surface in the form of glycoproteins and glycophospholipids. In several cases, such molecules have been shown to be involved in parasite survival, infectivity and host-cell recognition. The carbohydrate chains of these glycoconjugates are often highly immunogenic, and can in some cases elicit protective immunity. The immunogenicity of some parasite glycans is a function of their unusual chemical structure as compared with mammalian glycans. This suggests differences in the glycosylation pathways between host and parasite due to their evolutionary distance. This article describes how a combination of biophysical and biochemical techniques can be used to determine the primary and three-dimensional structures of parasite carbohydrate and how this information might be exploited towards the development of new selective chemotherapeutic agents and synthetic vaccines.

Animals↗
PubMed1987

Structure and dynamics in oligomannose-type oligosaccharides.

Using a combination of 1H NMR nuclear Overhauser effect measurements, molecular orbital calculations, and molecular dynamics simulations, we have determined the tertiary structure and dynamic properties of the oligomannose oligosaccharide Man alpha 6(Man alpha 3)Man alpha 6(Man alpha 3)Man beta 4GlcNAc beta 4GLcNAc. While the calculated potential surfaces for the majority of the glycosidic linkages could be described by a single deep potential well, similar calculations for the Man alpha 1-6Man alpha and Man alpha 1-6Man beta linkages described a diffuse, shallow well, suggesting that a larger degree of flexibility exists about the latter. These conclusions are supported by the results of molecular dynamics simulations, which suggest that the NMR data should be interpreted in terms of a degree of flexibility about the Man alpha 1-6Man beta and Man alpha 1-6Man alpha linkages. In contrast, a similar series of investigations suggests that the conformation of the Man alpha 1-6Man beta linkage in Man alpha 2Man6(Man alpha 2Man alpha 3)Man alpha 6 (Man alpha 2Man alpha 2Man alpha 3)Man beta 4GlcNAc beta 4GlcNAc is more restricted, resulting in an overall structure that is "restrained".

Carbohydrate Conformation↗
PubMed1987

Tertiary structure in N-linked oligosaccharides.

Distance constraints derived from two-dimensional nuclear Overhauser effect measurements have been used to define the orientation of the Man alpha 1-3Man beta linkage in seven different N-linked oligosaccharides, all containing the common pentasaccharide core Man alpha 1-6(Man alpha 1-3)Man beta 1-4GlcNAc beta 1-4GlcNAc. Conformational invariance of the Man alpha 1-3Man beta linkage was found for those structures bearing substitutions on the Man alpha 1-3Man beta antenna. However, the presence of either a GlcNAc residue in the beta 1-4 linkage to Man beta ("bisecting GlcNAc") or a xylose residue in the beta 1-2 linkage to Man beta of the trimannosyl core was found to generate conformational transitions that were similar. These transitions were accompanied by characteristic chemical shift perturbations of proton resonances in the vicinity of the Man alpha 1-3Man beta linkage. Molecular orbital energy calculations suggest that the conformational transition between the unsubstituted and substituted cores arises from energetic constraints in the vicinity of the Man alpha 1-3Man beta linkage, rather than specific long-range interactions. These data taken together with our previous results on the Man alpha 1-6Man beta linkage [Homans, S. W., Dwek R. A., Boyd, J., Mahmoudian, M., Richards, W. G., & Rademacher, T. W. (1986) Biochemistry 25, 6342] allow us to discuss the consequences of the modulation of oligosaccharide solution conformations.

Animals↗
PubMed1987

Structure of the major carbohydrate fragment of the Leishmania donovani lipophosphoglycan.

The major carbohydrate fragment from the lipophosphoglycan of Leishmania donovani was generated by mild acid hydrolysis (0.02 N HCl, 5 min, 100 degrees C) and purified by chromatography on DE-52 cellulose and thin layer. By a combination of analyses including gas-liquid chromatography-mass spectrometry and 1H NMR, the structure of the fragment was elucidated as PO4----6Gal(beta 1----4)Man. Approximately 16 of these phosphorylated disaccharide units occur in the overall glycoconjugate structure. NMR analysis of an alkaline phosphatase treated phosphorylated tetrasaccharide generated from lipophosphoglycan showed that the phosphorylated disaccharide units are linked together via alpha-glycosidic linkages. Complete characterization of the phosphorylated disaccharide units of lipophosphoglycan provides the first example of a defined carbohydrate anchored in membranes by a derivative of phosphatidylinositol.

Animals↗