O-specific chain

FIGURE 7-32 Bacterial lipopolysaccharides. (a) Schematic diagram of the lipopolysaccharide of the outer membrane of Salmonella ty-phimurium. Kdo is 3-deoxy-o-manno-octulosonic acid, previously called ketodeoxyoctonic acid Hep is L-glycero-D-mannoheptose AbeOAc is abequose (a 3,6-dideoxyhexose) acetylated on one of its hydroxyls. There are six fatty acids in the lipid A portion of the molecule. Different bacterial species have subtly different lipopolysaccharide structures, but they have in common a lipid region (lipid A), a core oligosaccharide, and an "O-specific" chain, which is the prin-... 

Bacteria also contain a very rich variety of glycolipids with unusual structures. Lipid A13 is the site of attachement of the O-specific chain of Gram (-) bacteria, which constitutes the antigenic lipopolysaccharide [87]. Other members of this family can be quoted, for example glycosyl glycerophospholipids in which the carbohydrate and glycerol moieties are linked by a phosphodiester bond (e.g. GPI anchor 14) [88] or carbohydrate esters (e.g. cord-factor of mycobacteria 15). 

A great diversity of monosaccharide structures is a characteristic feature of bacterial polysaccharide chains, especially of O-specific chains of lipopolysaccharides from Gram-negative bacteria. Several surveys of monosaccharide components of bacterial polysaccharides have been published.95 98... 

In bacterial lipopolysaccharides, O-specific chains composed of repeating, or modified repeating, units are linked to a unique oligosaccharide sequence of the core region which is connected to a lipid A fragment serving as a hydrophobic anchor embedded in the bacterial outer-membrane. Biosynthesis of O-specific chains was found to occur independently on formation of other structural fragments of the lipopolysaccharide molecule. Both block and monomeric mechanisms were demonstrated for the biosynthesis of these polymers. 

The material presented in previous sub-sections clearly shows that both of the possible mechanisms of polysaccharide chain-assembly may operate in the biosynthesis of bacterial polysaccharides. There is no clearcut, mechanistic difference in the biosynthesis of O-specific chains of lipopoly-saccharides, exocellular polysaccharides, and carbohydrate chains of Grampositive, cell-wall polymers for every class of polymer, the existence of both mechanisms of chain assembly was demonstrated. 

Figure 1 represents the general structure of Salmonella lipopolysaccharides. They contain an external polysaccharide, the O-antigenic chain, and an innermost component, termed lipid A. O-chain and lipid A are linked to each other by an oligosaccharide referred to as the core. O-Specific Chains. As indicated in Figure 1, O chains are in general made up of repeating units of di-, tri-, or higher oligosaccharides. In rare cases the O-chain is a homopolysaccharide. The structure of the O-chain is unique to each bacterial serotype great diversity is encountered in the structures of O-chains.

The O-specific chain is made up of a chain of repeating oligosaccharides of 3-8 units, which are specific to bacterial strains (Westphal et al., 1983 Raetz, 1990). 

Like Escherichia coli, clinical isolates of H. pylori produce high-molecular-mass (smooth-form) lipopolysaccharide (LPS) composed of an outermost saccharide moiety, divided into the O-polysaccharide or O-specific chain (O-antigen) and core oligosaccharide (OS) regions, covalently linked to lipid A (Fig. 10.1). H. pylori smooth-form LPS possesses O-polysaccharide chains of relatively constant chain length compared with those of enterobacterial LPS (Moran et al., 1992a Moran, 1999). Their length is determined by an enzymatic molecular ruler mechanism... 

The Lipid A component of LPS is embedded in the outer membrane of the bacterial cell, whereas the polysaccharide protrudes into the environment. The polysaccharide component is composed of two parts, the core oligosaccharide that is connected to Lipid A and a longer oligosaccharide O-specific chain that is attached to the core (Fig. 2). The O-specific chain is the most variable component of the complex. It consists... 

Figure 33.2 Lipopolysacchartde structure. Lipopolysaccharide. a potent activator of the innate immune system, is found on the surfaces of Gram-negative bacteria. The structure is built around lipid A, a specialized lipid that has four fatty acyl chains linked to two N-acetylglucosamine residues. Lipid A is linked to a polysaccharide chain consisting of a core and a more variable region termed the O-specific chain.

Endotoxins. Name for bacterial toxins which, in contrast to the exotoxins, are not secreted by living bacteria but are released from the cells by autolysis (e.g. in the intestines). In the case of classic E. they consist of the thermostable lipopolysaccharide (LPS-) fraction of the cell membrane anchored on the outer membranes of Gram-negative bacteria. The LPS consists of lipid A, the chain polysaccharide, and the O-specific chain lipid A is responsible for the toxic action of LPS. E. are found in all Enterobacteriaceae, e.g.. Salmonella (typhus), Shigella (dysentery), and many other Gramnegative pathogens. In the host organism E. stimulate mediators (cytokines) of the immune system. One of... 

Olsthoom et al. to obtain a PSD spectrum from the LPS from a rough strain (lacking the O-specific chain) of Klebsiella pneumoniae. Many other examples can be found in the literature. 

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