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S W Homans

Publications and source records attributed to S W Homans.

At least 37 records · Page 2Linked to original sources
PubMed1994

Glycosyl-phosphatidylinositol molecules of the parasite and the host.

The glycosyl-phosphatidylinositol (GPI) protein-membrane anchors are ubiquitous among the eukaryotes. However, while mammalian cells typically express in the order of 100 thousand copies of GPI-anchor per cell, the parasitic protozoa, particularly the kinetoplastids, express up to 10-20 million copies of GPI-anchor and/or GPI-related glycolipids per cell. Thus GPI-family members dominate the cell surface molecular architecture of these organisms. In several cases, GPI-anchored proteins, such as the variant surface glycoprotein (VSG) of the African trypanosomes, or GPI-related glycolipids, such as the lipophosphoglycan (LPG) of the Leishmania, are known to be essential for parasite survival and infectivity. The highly elevated levels and specialised nature of GPI metabolism in the kinetoplastid parasites suggest that the GPI biosynthetic pathways might be good targets for the development of chemotherapeutic agents. This article introduces the range of GPI structures found in protozoan parasites, and their mammalian hosts, and discusses some aspects of GPI biosynthesis.

Animals↗
PubMed1994

5 nanosecond molecular dynamics and NMR study of conformational transitions in the sialyl-Lewis X antigen.

The range of internal motions of the sialyl Lewis-X (SLe(x)) tetrasaccharide (NeuNAc alpha 2-->3Gal beta 1-->4(Fuc alpha 1-->3)GlcNAc) [where Fuc is L-fucopyranose, Gal is D-galactopyranose, GlcNAc is 2-acetamido-2-deoxy-D-glucopyranose and NeuNAc is D-neuraminic acid (sialic acid)] was studied by restrained simulated annealing and restrained molecular dynamics (MD) calculations. Transitions between predominantly two conformational states were observed for the NeuNAc alpha (2-->3)Gal linkage, consistent with previous observations for this linkage in sialyl-N-acetyllactosamine. The MD trajectory was simulated for 5 ns of real time, in order to observe a statistically significant number of these relatively low-frequency transitions. The Fuc alpha(1-->3)GlcNAc and Gal beta (1-->4)GlcNAc linkages, however, showed more restricted flexibility within a single energy well (RMS differences for the time-averaged glycosidic torsion angles, and , were 50% lower than for the NeuNAc alpha(2-->3)Gal linkage), and approximate to a rigid conformation. NMR parameters [relative rotating-frame Overhauser enhancement (r.O.e.) and inter-glycosidic 3JCH] back-calculated from the MD simulation were in close agreement with experimentally measured values for the free reducing oligosaccharide in D2O solution.

Carbohydrate Conformation↗
PubMed1994

A novel 13C isotopic labelling strategy for probing the structure and dynamics of glycan chains in situ on glycoproteins.

A protocol is described for uniform 13C labelling of terminal galactose residues of the glycan chains of glycoproteins, using an enzymatic method which does not perturb the protein. The technique is illustrated by application to the biantennary N-linked glycan chains attached at Asn 297 of immunoglobulin G (IgG). Isotope-edited NMR experiments on this glycoprotein yield data which suggest that the galactose residues on the glycan exist in two discrete environments, with the galactose in one environment having greater mobility than that in the other. These data are qualitatively consistent with crystallographic data on an Fc fragment, which suggest that one arm of the glycan is in contact with the protein, while the other projects into the space between the C gamma 2 domains. Quantitatively, however, these data cannot be rationalized with the crystallographic data, which implies subtle differences in oligosaccharide structure and dynamics between the solution and crystal states of Fc.

Carbohydrate Sequence↗
PubMed1994

Novel GPI structures of the membrane anchor of acetylcholinesterase from the electric organ of Torpedo californica.

The structure of the glycan moiety of the glycosylphosphatidylinositol (GPI) membrane anchor from Torpedo californica electric organ acetylcholinesterase was solved using nuclear magnetic resonance (NMR), methylation analysis, and chemical and enzymic microsequencing. Two structures were found to be present: Glc alpha 1-2 Man alpha 1-2 Man alpha 1-6 Man alpha 1-4 GlcN alpha 1-6myo-inositol, and Glc alpha 1-2 Man alpha 1-2 Man alpha 1-6 (GalNAc beta 1-4) Man alpha 1-4 GlcN alpha 1-6myo-inositol. The presence of glucose in this GPI anchor structure is a novel feature. The anchor was also shown to contain 2.3 residues of ethanolamine per molecule.

Acetylcholinesterase↗
PubMed1993

Structures of glycosylphosphatidylinositol membrane anchors from Saccharomyces cerevisiae.

Metabolic labeling studies suggest that Saccharomyces cerevisiae contains many glycoproteins that are anchored in the lipid bilayer by glycosylphosphatidylinositol membrane anchors. Membrane anchors were purified from a crude yeast membrane protein fraction and analyzed by two-dimensional 1H-1H NMR, fast atom bombardment-mass spectrometry, compositional and methylation linkage analyses, as well as chemical and enzymatic modifications. The yeast glycosylphosphatidylinositol anchors consist of the following structures: ethanolamine-PO4-6(R-2)Man alpha 1-2Man alpha 1-6Man alpha 1-4Glc-NH2 alpha 1-6myo-inositol-1-PO4-lipid, where R is mainly Man alpha 1- (80%) with some Man alpha 1-2Man alpha 1- (15%) and Man alpha 1-3Man alpha 1- (5%). The core region of the yeast anchors (ethanolamine-PO4-6Man alpha 1-2Man alpha 1-6Man alpha 1-4GlcNH2 alpha 1-6myo-inositol-1-PO4) is identical to the conserved core region found in glycosylphosphatidylinositol anchors from protozoa and mammals. The lipid moieties of the total yeast glycosylphosphatidylinositol anchors are mainly ceramides, consisting mostly of C18:0 phytosphingosine and C26:0 fatty acid. However, the lipid moiety of the glycosylphosphatidylinositol anchor of the purified ggp125 protein is a lyso- or diacylglycerol, containing C26:0 fatty acids. This suggests that yeast adds different lipid components to the glycosylphosphatidylinositol anchors of different proteins.

Carbohydrate Conformation↗
PubMed1993

Structure of the glycosyl-phosphatidylinositol membrane anchor of acetylcholinesterase from the electric organ of the electric-fish, Torpedo californica.

The structure of the glycan moiety of the glycosyl-phosphatidylinositol (GPI) membrane anchor from Torpedo californica (electric fish) electric-organ acetylcholinesterase was solved using n.m.r., methylation analysis and chemical and enzymic micro-sequencing. Two structures were found to be present: Glc alpha 1-2Man alpha 1-2Man alpha 1-6Man alpha 1-4GlcN alpha 1-6myo-inositol and Glc alpha 1-2Man alpha 1-2Man alpha 1-6(GalNAc beta 1-4)Man alpha 1-4GlcN alpha 1-6myo-inositol. The presence of glucose in this GPI anchor structure is a novel feature. The anchor was also shown to contain 2.3 residues of ethanolamine per molecule.

Acetylcholinesterase↗
PubMed1993

Characterization of the extent of internal motions in oligosaccharides.

A detailed investigation has been undertaken on the extent and nature of torsional fluctuations about the glycosidic linkage of the model disaccharide Man alpha 1-3Man alpha 1-OMe. In particular, we sought to determine whether the three nuclear Overhauser effects and the two long-range heteronuclear 3JCH spin coupling constants measurable across the glycosidic linkage were consistent with a single conformation or multiple conformations about that linkage. Within experimental error, we have found that these five parameters can be interpreted in terms of a single, rigid geometry. Alternatively, the data are also consistent with a model in which the glycosidic torsional angles exhibit significant but restricted fluctuations about the global minimum energy conformation. Evidence from restrained molecular dynamics simulations both in vacuo and with explicit inclusion of solvent water and from 13C relaxation measurements upon an oligomannose glycan in covalent association with protein suggests that the latter model is the most accurate representation of the conformational behavior of oligosaccharides in solution.

Carbohydrate Conformation↗
PubMed1993

Nuclear magnetic resonance study of a deoxyoligonucleotide duplex containing a three base bulge.

The three-dimensional structures of a DNA oligonucleotide containing three extra unpaired adenosine residues (dGCCAGGAAATGGAC+dGTCCGACCTGGC) and that of the perfect duplex analogue (dGCCAGGTCGGAC+dGTCCGACCTGGC) have been studied in solution by 1H and 13C nuclear magnetic resonance. All non-exchangeable aromatic and H-1', H-2', H-2" sugar protons were assigned using standard assignment pathways for B-DNA. All cross-peaks within these pathways were present for the perfect duplex molecule as would be expected for a right-handed A or B-form duplex. However, a few cross-peaks which would be expected in the standard case are extremely weak in the nuclear Overhauser enhancement spectroscopy (NOESY) spectrum of the bulged duplex even at long mixing times (250 ms). For example, almost no cross-relaxation is observed between the H-6 proton of C22 and the H-1' of A21, directly across from the three base bulge. Yet the continuity of assignment pathways through the three base bulge argues against any discontinuous "looping out" of one or more of the extra adenosine residues. Double quantum-filtered correlated spectroscopy experiments demonstrate very little deviation from south sugar conformations for residues at or near the bulge. The perfect duplex contains three A.T basepairs as expected, resulting in three very intense T imino-AH2 cross-peaks in the H2O NOESY experiment. In contrast, only two such intense cross-peaks are observed in the same experiment using the bulged duplex sample. Assignments of the two T imino peaks using one-dimensional NOEs are consistent with disruption of the T.A base-pair immediately 3' to the bulge; this is consistent with our earlier observation of chemical reactivity at a T 3' to an An or Tn bulge. We also find evidence of disruption of the G.C base-pair immediately 5' to the bulge.

Base Sequence↗
PubMed1992

Refined structure of the lipophosphoglycan of Leishmania donovani.

The primary structure of the major surface glycoconjugate of Leishmania donovani parasites, a lipophosphoglycan, has been further characterized. The repeating PO4-6Galp beta 1-4Man disaccharide units, which are a salient feature of the molecule, are shown to terminate with one of several neutral structures, the most abundant of which is the branched trisaccharide Galp beta 1-4(Manp alpha 1-2)Man. The phosphosaccharide core of lipophosphoglycan, which links the disaccharide repeats to a lipid anchor, contains 2 phosphate residues. One of the core phosphates has previously been localized on O-6 of the galactosyl residue distal to the lipid anchor; the second phosphate is now shown to be on O-6 of the mannosyl residue distal to the anchor and to bear an alpha-linked glucopyranosyl residue. Also, the anomeric configuration of the unusual 3-substituted Galf residue in the phosphosaccharide core is established as beta. The complete structure of the core is thus PO4-6Galp alpha 1-6Galp alpha 1-3Galf beta 1-3[Glcp alpha 1-PO4-6]Manp alpha 1-3Manp alpha 1-4GlcN alpha 1-. This further clarification of the structure of lipophosphoglycan may prove beneficial in determining the structure-function relationships of this highly unusual glycoconjugate.

Animals↗
PubMed1992

Structure of Leishmania mexicana lipophosphoglycan.

Lipophosphoglycan (LPG) was isolated from the culture supernatant of Leishmania mexicana promastigotes and its structure elucidated by a combination of 1H NMR, fast atom bombardment mass spectrometry, methylation analysis, and chemical and enzymatic modifications. It consists of the repeating phosphorylated oligosaccharides PO4-6Gal beta 1-4Man alpha 1- and PO4-6[Glc beta 1-3]Gal beta 1-4Man alpha 1-, which are linked together in linear chains by phosphodiester linkages. Each chain of repeat units is linked to a phosphosaccharide core with the structure PO4-6Gal alpha 1-6Gal alpha 1-3Galf beta 1- 3[Glc alpha 1-PO4-6]Man alpha 1-3Man alpha 1-4GlcNH2 alpha 1-6 myo-inositol, where the myo-inositol residue forms the head group of a lyso-alkylphosphatidylinositol moiety. The nonreducing terminus of the repeat chains appear to be capped with the neutral oligosaccharides Man alpha 1-2Man, Man alpha 1-2Man alpha 1-2Man, or Man alpha 1-2[Gal beta 1-4]Man. Cellular LPG, isolated from promastigotes, has a very similar structure to the culture supernatant LPG. However, it differs from culture supernatant LPG in the average number of phosphorylated oligosaccharide repeat units (20 versus 28) and in alkyl chain composition. Although culture supernatant LPG contained predominantly C24:0 alkyl chains, cellular LPG contained approximately equal amounts of C24:0 and C26:0 alkyl chains. It is suggested that culture supernatant LPG is passively shed from promastigotes and that it may contribute significantly, but not exclusively, to the "excreted factor" used for serotyping Leishmania spp. Comparison of L. mexicana LPG with the LPGs of Leishmania major and Leishmania donovani indicate that these molecules are highly conserved but that species-specific differences occur in the phosphorylated oligosaccharide repeat branches and in the relative abundance of the neutral cap structures.

Animals↗
PubMed1992

Identification of the defect in lipophosphoglycan biosynthesis in a non-pathogenic strain of Leishmania major.

The major macromolecule on the surface of the protozoan parasite, Leishmania major, is a complex lipophosphoglycan (LPG), which is anchored to the plasma membrane by an inositol-containing phospholipid. A defect in LPG biosynthesis is thought to be responsible for the avirulence of the L. major strain LRC L119 in mice. In order to identify the nature of this defect we have characterized two truncated forms of LPG, which are accumulated in this strain, by one- and two-dimensional 500-MHz 1H NMR spectroscopy, two-dimensional heteronuclear 1H-31P NMR spectroscopy, methylation analysis, and exoglycosidase digestions. The structures of these glycoinositolphospholipids, termed GIPL-4 and -6, are as follows: [formula: see text] The glycan moieties of GIPL-4 and -6 are identical to the anchor region of LPG, which is also substituted with a Glc-1-PO4 residue in approximately 60% of the structures. However, instead of being capped with chains of phosphorylated oligosaccharide repeat units, both glycan moieties terminate in Man alpha 1-PO4, suggesting that the defect in LPG biosynthesis is in the transfer of galactose to this residue to form the disaccharide backbone of the first repeat unit. These results indicate that the phosphoglycan moiety of LPG is essential for intracellular survival of the parasite and have implications for LPG biosynthesis.

Animals↗
PubMed1992

Purification and characterization of an extracellular phosphoglycan from Leishmania donovani.

An extracellular phosphoglycan (exPG), present in the culture medium of the promastigote form of Leishmania donovani, was purified and structurally characterized. The purification scheme included ethanol precipitation of the culture medium, anion exchange chromatography, hydrophobic chromatography on phenyl-Sepharose, and preparative polyacrylamide gel electrophoresis. Structural analysis by 1H-1H NMR, methylation linkage analysis, and glycosidase digestion revealed that the exPG consisted of the following structure: (CAP)----[PO4-6Galp beta 1-4Manp alpha 1]10-11-PO4-6Galp beta 1-4Man. The cap was found to be one of several small, neutral oligosaccharides, the most abundant of which was the trisaccharide Galp beta 1-4(Manp alpha 1-2)Man. The results indicated structural analogy to the cellular-derived lipophosphoglycan (LPG) from L. donovani. The important exceptions are a lack of the lipid anchor, the entire phosphosaccharide core, and several of the repeating disaccharide units. Although the function of exPG is presently unknown, it may play a protective role for the promastigote in the insect vector or during infection of a mammalian host.

Animals↗
PubMed1992

Solution structure of the lipophosphoglycan of Leishmania donovani.

The three-dimensional solution structure of the repeating -PO4-6Gal beta 1-4Man alpha 1- disaccharide fragment of the lipophosphoglycan (LPG) derived from Leishmania donovani has been determined by use of a combination of homo- and heteronuclear NMR spin coupling constant measurements together with restrained molecular mechanical minimization and molecular dynamics simulations. The fragment exists with limited mobility in solution about the Gal beta 1-4Man linkages, whereas in contrast a variety of stable rotamers exist about the Man alpha 1-PO4-6Gal linkages. These rotamers define several major stable conformers in solution, which are discussed in terms of the proposed biological role of LPG.

Animals↗
PubMed1992

Application of restrained minimization, simulated annealing and molecular dynamics simulations for the conformational analysis of oligosaccharides.

The purpose of the present study was to determine the confidence with which the small number of 1H NMR nuclear Overhauser effect (NOE) distance constraints measurable across glycosidic linkages in oligosaccharides could be used for solution conformational analysis. This was assessed by use of these constraints in restrained molecular mechanical minimization of the tetrasaccharide Gal beta 1----4(Fuc alpha 1----3)Glc-NAc beta 1----3Gal, a model compound of the Lewis-X antigenic determinant. This presents a particularly severe test case in view of extreme resonance overlap and a dearth of inter-residue distance constraints. It is concluded that these constraints, when used in conventional restrained minimization, result in the generation of 'virtual conformations' and local minima about glycosidic linkages. However, these restraints are nevertheless found to be useful in the initial stages of a conformational analysis strategy involving restrained minimization combined with dynamical simulated annealing to define more accurately the global minimum energy configuration, together with molecular dynamics simulation to explore conformational mobility about this minimum. Theoretical ROE values calculated over the time course of the MD simulation, using a formalism appropriate for the time scale of the internal motion, are compared with those obtained experimentally in the oligosaccharide.

Carbohydrate Conformation↗
PubMed1992

Homonuclear three-dimensional NMR methods for the complete assignment of proton NMR spectra of oligosaccharides--application to Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1-3Gal beta 1-4Glc.

Two new homonuclear three-dimensional NMR techniques are described for the simplification of proton resonance assignment in oligosaccharides, namely HOHAHA-COSY and ROESY-COSY. The former technique is of value in the resonance assignment of gluco-configuration monosaccharide residues, whereas the latter is more suited to resonance assignment of galacto-configuration monosaccharide residues. The value of these techniques is illustrated by application to the proton resonance assignment of the pentasaccharide Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1-3 Gal beta 1-4Glc, a compound which exhibits a variety of assignment problems due to severe cross-peak overlap in conventional COSY or HOHAHA spectra.

Carbohydrate Conformation↗