Polysaccharides chain linkage

Many bacterial polysaccharides contain phosphoric ester groups. There is a limited number of examples of monoesters. More common are phosphoric diesters, connecting an amino alcohol or an alditol to the polysaccharide chain. Another possibility is that oligosaccharide or oligosaccharide-alditol repeating units are connected to a polymer by phosphoric diester linkages. In addition to the intracellular teichoic acids, several bacteria, for example, different types of Streptococcus pneumoniae, elaborate extracellular polymers of this type. These polymers are generally discussed in connection with the bacterial polysaccharides. 

Fig. 2.—Schematic Representation of the Heparin Proteoglycan. [The polysaccharide chains, bound to the polypeptide matrix through a linkage region," are cleaved by tissue endoglycosidases (arrows) after the transformations illustrated in Scheme 1.]...

In some instances, reducing sugars are present that can be reductively aminated without prior periodate treatment. A reducing end of a monosaccharide, a disaccharide, or a polysaccharide chain may be coupled to a diamine by reductive amination to yield an aminoalkyl derivative bound by a secondary amine linkage (Figure 1.96). Also see Section 4.6, this chapter, for an extensive discussion on carbohydrate modification techniques. 

A bacterial phosphatidylinositol specific phospholipase C (PI-PLC) had been available for many years before it was demonstrated to strip a number of membrane-bound proteins from eukaryotic cell surfaces [1], Such proteins are anchored by a PI moiety in which the 6 position of inositol is glycosidically linked to glucosamine, which in turn is bonded to a polymannan backbone (Fig. 3-10). The polysaccharide chain is joined to the carboxyl terminal of the anchored protein via amide linkage to ethanolamine phosphate. The presence of a free NH2 group in the glucosamine residue makes the structure labile to nitrous acid. Bacterial PI-PLC hydrolyzes the bond between DAG and phosphati-dylinositols, releasing the water-soluble protein polysac charide-inositol phosphate moiety. These proteins are tethered by glycosylphosphatidylinositol (GPI) anchors. 

Such reactions may be of considerable significance. This is because, if two pendant p-coumarate linkages (or related molecules) are attached to two adjacent polysaccharide chains, an effective means of cross-linking via photochemical coupling could be achieved. However, there is no evidence at present to indicate that these dimers function as either intermolecular or intramolecular cross-linking reagents. 

Phosphorylase catalyzes the polymerization of glucose-1-phosphate in order to obtain linear polysaccharide chains with a-(1 4) glycosidic linkages the glycogen... 

Most potato starches are composed of a mixture of two polysaccharides, a linear fraction, amylose, and a highly branched fraction, amylopectin. The content of amylose is between 15 and 25% for most starches. The ratio of amylose to amylopectin varies from one starch to another. The two polysaccharides are homoglucans with only two types of chain linkage, a-(l 4) in the main chain and a-(l 6)-linked branch chains. Physicochemical properties of potato and its starch are believed to be influenced by amylose and amylopectin content, molecular weight, and molecular weight distribution, chain length and its distribution, and phosphorus content (Jane and Chen, 1992). 

In the polymers of groups (1) and (2), polysaccharide chains composed of oligosaccharide repeating-units (sometimes, partially modified) are usually linked to a unique oligosaccharide unit present near the point of attachment of the chain to another polymeric chain, or to a lipid anchor. This unit is called the linkage region in the polymers of bacterial cell-wall, and the core region in lipopolysaccharides. 

For glycoproteins, the formation of anhydro rings from hexosamine residues is important in chromogen formation, especially in relation to the Ehrlich reaction and its modifications (3). / -Elimination is involved in the removal of oligosaccharide or polysaccharide chains from the protein core of glycoproteins when they are attached by O-glycosidic linkages to serine or threonine residues. A subsequent -elimination may also... 

On the other end of the cross-linker, the hydrazide functional group can react with periodate-oxidized carbohydrate molecules to form hydrazone linkages (Chapter 1, Sections 2 and 4.5). Thus, glycoproteins can be targeted specifically at their polysaccharide chains, avoiding cross-linking at active sites which can lead to activity losses (Fig. 167). 

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