Structures of Capsular Polysaccharides

Some of the fundamental principles involved in these structural analyses are outlined here for the group A polysaccharide.33 The 13C-n.m.r. spectrum thereof is shown in Fig. 1 although complex, due to the presence of O-acetyl substituents, it is considerably simplified, to an eight-resonance spectrum (carbonyl signal at 175.8 p.p.m. not 

Experience has indicated that the chemical shifts of the monosaccharides are similar to those of the monosaccharide residues within the polysaccharide, except for substituent effects.37,38 These effects, caused by the attachment of any substituent to a sugar moiety, cause an increase in the chemical shift of the carbon atom directly involved in the linkage this increase is usually accompanied by a decrease of smaller magnitude (or, sometimes, an increase) in the chemical shifts 

The O-acetyl substituents of the native, group C polysaccharide were located by comparing its 13C-n.m.r. spectrum with that of the O-deacetylated, group C polysaccharide (see Fig. 2). The appearance of characteristic multiplets in some of the signals of the native polysaccharide showed that the O-acetyl groups are distributed exclusively between 0-7 and 0-8 of its sialic acid residues. 

The group Y and W-135 capsular polysaccharides also contain sialic acid. However, unlike the groups B and C polysaccharides, they are not homopolymers, as they also contain, respectively, D-glucosyl and D-galactosyl residues.35 Structural studies indicated that the group Y 

The group 29-e polysaccharide is composed of an alternating sequence of 3-deoxy-/3-D-manno-2-oetulosylonic acid and 2-acetamido-2-deoxy-a-D-glucopyranosyl residues linkage is to 0-7 of the former and to 0-3 of the latter. O-Acetyl substituents were also located on both 0-4 and 0-5 of the KDO residues.26 It is of interest that, whereas sialic acid has only been found in bacterial polysaccharides, and elsewhere in Nature, as its a-D anomer, there is strong evidence to suggest that KDO probably exists in bacterial polysaccharides in both of its an-omeric forms.28,40 

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By 1945, Stacey speculated about the possibility of a structural relationship between Pneumococcus capsular polysaccharides and those produced by other organisms. With Miss Schliichterer, he had examined the capsular polysaccharide of Rhizobium radicicolum. This polysaccharide gave a precipitin reaction in high dilution, not only with Type III Pneumococcus antiserum, but also mixed with antisera from other Pneumococcus types. The chemical evidence indicated that the polysaccharide resembled the specific polysaccharides of Types I and II Pneumococcus. A decade later, the acidic capsular polysaccharide from Azoto-bacter chroococcum, a soil organism, was studied. It, too, produced serological cross-reactions with certain pneumococcal specific antisera. Although the molecular structure of the polysaccharide was not established, adequate evidence was accumulated to show a structural relationship to Type III Pneumococcus-specific polysaccharide. This was sufficiently close to account for the Type III serological cross-relationship. 

One clear example of the importance of capsular polysaccharides is in the symbiotic relationship between nitrogen-fixing rhizobial bacteria and leguminous plants, where MS has played a central role in helping elucidate structures. Here we aim to highlight, using the specific example of bacterial K-antigen studies, the central role of modern mass-spectrometric techniques in polysaccharide analysis. 

III. Other Important Structural and Physical Features of Capsular Polysaccharides... 

Wzz proteins are in the plasma membrane, and all have two TM helices flanking a periplasmic loop with a predicted coiled-coil structure. Wzz belongs to a family of proteins called polysaccharide copolymerases . Some of its members occur in the synthesis of capsules and they have a larger cytosolic C-terminal region that contains ATP-binding sites and several tyrosine residues that can become phosphorylated. Phosphorylation and dephosphorylation of these proteins are important for the export of capsular polysaccharides (reviewed in Whitfield ), but the specific mechanism of export is still not well understood. 

Fructose occurs rarely in the bacterial polysaccharide. It has been found in the LPS of several Vibrio species [108] and in the K4 [109] and Kll [110] capsular antigens of Escherichia coli. In all cases, fructose appears as a terminal residue. In several plant species, fructans consisting of p-D-fructofuranosyl units are present as important storage polymers. The structure of the polysaccharide isolated from B. caryophylli is different from those of the above polysaccharides however, as levan can be isolated as bacterial exopolysaccharides, this polysaccharide may be a side-product of levan biosynthesis. 

A book dealing with the surface carbohydrates of prokaryotic cells has discussed, among other topics, capsular polysaccharide antigens, immunological expression, the structures of bacterial polysaccharides,and the biosynthesis of capsular polysaccharides. ... 

Most bacterial capsules are polysaccharides with a specific structure. In many respects, the capsular polysaccharide can be considered an identifying feature of the organism. The pneumococci are divided into types (or groups) that are defined by a particular immunological precipitin reaction of the capsular polysaccharide with an antibody preparation obtained by inoculation of the polysaccharide into an animal host such as a rabbit. The reaction is usually highly specific and depends on the structure of the polysaccharide. S. pneumoniae has been found to have over 80 different immunological types and, therefore, 80 different polysaccharide structures. 

In 1967, Heidelberger, Stacey et al. reported the purification, some structural features, and the chemical modification of the capsular polysaccharide from Pneumococcus Type I. Difficulties of direct hydrolysis of the polysaccharide were overcome and it was possible to identify some of the fragments in the hy-drolyzate. At least six products resulted from nitrous acid deamination. Two were disaccharides, which were identified, and sequences of linked sugar units were proposed. As modification of the polysaccharide decreased the amounts of antibody precipitated by anti-pneumococcal Type I sera, the importance of the unmodified structural features in contributing to the specificity of the polysaccharide was indicated. 

Structure of Pneumococcus Capsular Polysaccharides, S. A. Barker and M. Stacey, Biochem. /., 82... 

In some polysaccharides, the reducing terminal is linked, through a phosphoric diester linkage, to O-1 of a 2,3-di-6 -acylglycerol. This structural feature has been demonstrated for some capsular polysaccharides from E. coli and Neisseria species, - but is probably more common than that. Non-covalent linkage between the lipid part and the cell membrane may explain why extracellular polysaccharides often occur as capsules, and the high (apparent) molecular weight observed for these polysaccharides may be due to micelle formation in aqueous solution. 

There are solitary examples of other alditol phosphates as components of this class of polymers. Arabinitol 1-phosphate is part of the S. pneumoniae type 17F capsular polysaccharide. o-Glucitol 6-phosphate is a component of the group-specific polysaccharide from group B Streptococcus, which has a most unusual, ramified structure. In a polysaccharide from Nocardia... 

A structural study on lipid A and the O-specific polysaccharide of the lipopoly-saccharide from a clinical isolate of Bacteroides vulgatus from a patient with Crohn s disease was conducted by Hashimoto and coworkers [39]. They separated two potent virulence factors, capsular polysaccharide (CPS) and lipopolysaccharide (LPS), from a clinical isolate of B. vulgatus and characterized the structure of CPS. Next, they elucidated the strucmres of O-antigen polysaccharide (OPS) and lipid A in the LPS. LPS was subjected to weak acid hydrolysis to produce the lipid A fraction and polysaccharide fraction. Lipid A was isolated by PLC, and its structure was determined by MS and NMR. 

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