Structural analysis of a novel sialic-acid-containing trisaccharide from Rhodobacter capsulatus 37b4 lipopolysaccharide

Biological functions of sialic acid as a component of bacterial endotoxin

Lipopolysaccharide (endotoxin, LPS) is an important Gram-negative bacteria antigen. LPS of some bacteria contains sialic acid (Neu5Ac) as a component of O-antigen (O-Ag), in this review we present an overview of bacteria in which the presence of Neu5Ac has been confirmed in their outer envelope and the possible ways that bacteria can acquire Neu5Ac. We explain the role of Neu5Ac in bacterial pathogenesis, and also involvement of Neu5Ac in bacterial evading the host innate immunity response and molecular mimicry phenomenon. We also highlight the role of sialic acid in the mechanism of bacterial resistance to action of serum complement. Despite a number of studies on involvement of Neu5Ac in bacterial pathogenesis many aspects of this phenomenon are still not understood.

Exploration of the Sialic Acid World

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

Structural analysis of lipopolysaccharides from Gram-negative bacteria

This review covers data on composition and structure of lipid A, core, and O-polysaccharide of the known lipopolysaccharides from Gram-negative bacteria. The relationship between the structure and biological activity of lipid A is discussed. The data on roles of core and O-polysaccharide in biological activities of lipopolysaccharides are presented. The structural homology of some oligosaccharide sequences of lipopolysaccharides to gangliosides of human cell membranes is considered.

Per-O-methylation reaction for structural analysis of carbohydrates by mass spectrometry

Per-O-methylation of carbohydrates is an important sample preparation step in structural analysis of complex carbohydrates, which has generated considerable interest as shown by thousands of citations in the last 10 years. This article provides a critical overview of the per-O-methylation methods applied for structural analysis of carbohydrates by mass spectrometry. The understanding of the O-methylation mechanism can help the researchers to apply the adequate O-methylation method and can generate new ideas in the effort of improving this reaction. The per-O-methylation of carbohydrates is relied upon stepwise reactions. The parameters that affect the reaction are discussed for the most important methods and are critically commented for each reaction step. The limits of each method are emphasized. The improvements of the per-O-methylation reaction are described in detail with their advantages and disadvantages and some illustrative examples are given. The methods that give complete O-methylation in non-hazardous conditions with high yields within minutes at room temperature with a very low amount of side-products are especially highlighted.

Aldolase-Catalyzed Synthesis of β-d-Galp-(1→9)-d-KDN: A Novel Acceptor for Sialyltransferases

[reaction: see text] beta-D-Galp-(1-->9)-D-KDN, a disaccharide component of the cell wall of Streptomyces sp. MB-8, was synthesized from beta-D-Galp-(1-->6)-D-Manp and pyruvate using a sialic acid aldolase. The obtained KDN-containing compound was a novel acceptor for bacterial sialyltransferases. Unusual alpha2,3- and alpha2,6-linked sialyltrisaccharides and a tetrasaccharide were synthesized using a one-pot two-enzyme system containing a Neisseria meningitidis CMP-sialic acid synthetase and a Pasteurella multocida sialyltransferase or a Photobacterium damsela alpha2,6-sialyltransferase.

Lipopolysaccharides from Campylobacter jejuni O:41 strains associated with Guillain-Barré syndrome exhibit mimicry of GM1 ganglioside

Three Campylobacter jejuni, biotype 2, serotype O:41 strains that were isolated from patients who developed Guillain-Barré syndrome (GBS) and one C. jejuni isolate from a patient who developed enteritis only were examined. The aim of the study was to determine the structure of the core oligosaccharide (OS) of the lipopolysaccharide (LPS) of C. jejuni serotype O:41, a serotype rarely associated with the development of GBS, and to determine if the LPS shares similar epitopes with any of the major human gangliosides. Electrophoretic analysis with silver staining or immunoblotting demonstrated that the strains had LPS profiles characteristic of low-molecular-weight LPS. Colorimetric analysis detected N-acetylneuraminic (sialic) acid in the core OSs of all the strains. Thin-layer chromatography with immunostaining showed that antisera raised against the GBS strains reacted with the GM1 ganglioside, suggesting that C. jejuni serotype O:41 LPSs and the GM1 ganglioside have similar epitopes. Furthermore, polyclonal anti-GM1 and anti-asialoGM1 antibodies cross-reacted with each C. jejuni O:41 LPS tested, suggesting that the serotype O:41 core OS has a GM1- and asialoGM1-like structure. LPSs extracted from C. jejuni serostrains O:2, O:3, and O:19 were also used in the study. Cholera toxin (a GM1 ligand) and peanut agglutinin (a Galbeta1-3GalNAc ligand) recognized all serotype O:41 LPSs and the serostrain O:2 LPS. Immunoadsorption results confirmed GM1 relatedness. Moreover, the core OS was isolated from a GBS-associated C. jejuni O:41 LPS by gel permeation chromatography. An analysis by gas-liquid chromatography (GLC), GLC-mass spectrometry, and nuclear magnetic resonance showed the core OS of one of the C. jejuni O:41 GBS isolates to have a tetrasaccharide structure consistent with GM1 mimicry.

NMR investigations of the role of the sugar moiety in glycosylated recombinant human granulocyte-colony-stimulating factor

Human granulocyte-colony-stimulating factor (G-CSF) is a hematopoietic growth factor that plays a major role in the stimulation of the proliferation and maturation of granulocyte neutrophil cells. With the recent increased understanding of its biological properties in vivo together with available preparations of recombinant human G-CSF, this growth factor has become an essential agent for clinical applications. The presence of an O-linked carbohydrate chain at position 133 greatly improves the physical stability of the protein. To clarify the molecular basis for the stabilisation effect of saccharide moieties on human G-CSF the whole glycoprotein expressed in CHO cells has been investigated by means of two 1H-NMR-spectroscopy and two 1H-detected-heteronuclear 1H-13C experiments at natural abundance, and compared with the non-glycosylated form. The present NMR study reports assignments of 1H and 13C resonances of the bound saccharidic chain NeuNAc(alpha2-3)Gal(beta1-3)[NeuNAc(alpha2-6)]GalNAc, where NeuNAc represents N-acetylneuraminic acid, and demonstrates the alpha-anomeric configuration of the N-acetylgalactosamine-threonine linkage. It also provides results suggesting that the carbohydrate moiety reduces the local mobility around the glycosylation site, which could be responsible for the stabilising effect observed on the glycoprotein.

NMR reinvestigation of two N-acetylneuraminic acid-containing O-specific polysaccharides (O56 and O24) of Escherichia coli

Structures for the N-acetylneuraminic acid (Neu5Ac)-containing O56 and O24 polysaccharides of Escherichia coli have been reported previously. During these studies unusual chemical shifts had been observed for the NMR signals for H-3eq and C-3 of the Neu5Ac residues of both polysaccharides. In further pursuing this phenomenon, we have reinvestigated the O56 and O24 polysaccharides as well as derived oligosaccharides by one- and two-dimensional NMR spectroscopy. The results showed that structures of both polysaccharides (PSs) had to be modified and formulated as [formula: see text] 2D ROESY spectra revealed a strong NOE between H-3eq of Neu5Ac and the protons of the side-chain sugar (H-3 and H-5 of alpha-D-Gal p in the O56 PS and H-3 of alpha-D-Glc p in the O24 PS) and also between H-3ax of Neu5Ac and H-3 of beta-D-Glc p in the main chain. This indicated a close spatial association of the seven-linked alpha-Neu5Ac and the side-chain residues alpha-D-Gal p (O56 PS) and alpha-D-Glc p (O25 PS), respectively. The strong long-range spatial contacts caused the unusual chemical shifts of H-3eq and C-3 of Neu5Ac.

Occurrence of N-acetyl- and N-O-diacetyl-neuraminic acid derivatives in wild and mutant Crithidia fasciculata

The cell-surface expression of sialic acids in wild-type Crithidia fasciculata and three drug-resistant mutants (FU(R)11, TR3, and TFRR1) was analyzed using fluorescein-labeled Limulus polyphemus agglutinin (LPA) binding, glycosidase of known sugar specificity, and thin-layer chromatography (TLC). Gas-liquid chromatography-mass spectrometry (GC-MS) analysis using both electron-impact (EI-MS) and chemical ionization (CI-MS) by isobutane with selected ion monitoring (SIM) was also used. The surface location of sialic acid was inferred from LPA binding to whole cells abrogated by previous treatment with neuraminidase. An exception occurred with the TFRR1 strain, which after incubation with neuraminidase showed increased reactivity with the fluorescent lectin. Both N-acetyl- and N-O-diacetyl-neuraminic acids were identified in the flagellates by TLC, with a clear predominance being noted for the former derivative. However, the content of N-O-diacetyl-neuraminic acid was preferentially found in the TFRR1 strain. The GC-MS analysis of the acidic component of the TFRR1 mutant strain confirmed the occurrence of N-acetyl-neuraminic acid (Neu5Ac) by the presence of the diagnostic ions (m/z values: 684 and 594 for CI-MS and 478, 298, and 317 for EI-MS) and also by comparison with the standard Neu5Ac retention time. GC-MS analysis also showed fragments (m/z values: 654 and 564 for CI-MS and 594, 478, 298, and 317 for EI-MS) expected for the 7-O- and 9-O-acetyl-N-acetyl-neuraminic acids (Neu5,7Ac2 and Neu 5,9Ac2, respectively).

The sialic acid-containing lipopolysaccharides of Salmonella djakarta and Salmonella isaszeg (serogroup O: 48): chemical characterization and reactivity with a sialic acid-binding lectin from Cepaea hortensis

Lipopolysaccharides (LPS) of Salmonella djakarta and Salmonella isaszeg, as well as of a spontaneous R-mutant of S. djakarta were investigated as to their content in neuraminic acid (Neu) and its individual linkage. The two Salmonella serovars both belong to the O:48 serogroup of Salmonella, but to two different subgroups. LPS of both S-forms contained high amounts of Neu, although in different quantities, whereas the R-form was completely devoid of it. Methylation analysis indicated that Neu is exclusively terminally linked in S. djakarta whereas both terminal and 4-linked Neu were recognized in S. isaszeg. Although terminally linked, a sialidase from Arthrobacter ureafaciens was unable to split Neu even after prolonged incubation from both S-type LPSs. When LPS was first treated by mild alkali, however, the total amount of Neu from S. djakarta LPS and about 50% from that of LPS of S.isaszeg could be removed. In contrast, alkali-treated LPS, but also the non-treated one, proved to be effective inhibitors for a sialic acid-binding lectin from Cepaea hortensis. The resistance of terminal Neu towards sialidase may be due to the presence of an O-acetyl group which would be removed during the methylation analysis but would, especially when linked to C-4, not interfere with the reactivity of the lectin.

Biochemical characterization of Campylobacter fetus lipopolysaccharides

Lipopolysaccharides (LPS) of five strains of the human and animal pathogen Campylobacter fetus were electrophoretically and chemically characterized. Analysis with sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that all the strains produced smooth-form LPS with O side chains of relatively constant chain length. Upon extraction, LPS partitioned into both the water and phenol phases of phenol-water extracts, which showed that two chemical species of LPS were present in each C. fetus strain. Constituents common to all the LPS, though differing in molar ratios, were L-rhamnose, L-fucose, D-mannose, D-glucose, D-galactose, L-glycero-D-manno-heptose, and D-glycero-D-manno-heptose. L-Acofriose (3-O-methyl-L-rhamnose) was present in only two of the C. fetus strains. On the basis of these differences, it was possible to distinguish between LPS from strains of different serotypes and biotypes. Furthermore, chemical analysis indicated that the phenol phase LPS had a lower level of substitution by certain neutral sugars than did water phase LPS. N-Acetylneuraminic (sialic) acid and D-galactosamine were present in all the C. fetus LPS. Constituents normally found in the core and lipid A regions of LPS, 3-deoxy-D-manno-2-octulosonic acid, D-glucosamine, ethanolamine and its phosphorylated derivatives, and fatty acids [14:0, 16:0 14:0(3-OH), and 16:0(3-OH)] were detected. Unlike Campylobacter jejuni, in which 2,3-diamino-2,3-dideoxy-D-glucose occurs as a constituent of the lipid A backbone, this amino sugar was absent from C. fetus LPS, indicating major structural differences in the lipid A's of these species.

Analysis of 7-substituted sialic acid in some enterobacterial lipopolysaccharides

Sialic acid-containing lipopolysaccharides (LPS) were isolated from six bacterial strains of the family Enterobacteriaceae. Sialic acid was released from permethylated LPS by methanolysis, and partially O-methylated N-acetyl-N-methyl-neuraminic acid methyl ester methyl glycosides were analyzed by gas-liquid chromatography-electron ionization mass spectrometry. It was proved that all LPS contain N-acetylneuraminic acid (NeuAc). The occurrence of 7-substituted NeuAc in Escherichia coli serotypes O24 and O56 and in Citrobacter freundii O37 LPS was documented. The LPS preparations also contained terminal NeuAc. LPS of E. coli O104 had exclusively 4-substituted sialic acid. The remaining LPS studied, namely, from Salmonella toucra O48 and Hafnia alvei 2, had 4-linked and terminally localized NeuAc residues.