Linkages in Bacterial Polysaccharides

As already mentioned, formation of glycosidic linkages between monomeric units of the carbohydrate-containing polymers of the bacterial cell-surface is catalyzed by membrane-bound glycosyltransferases, and glycosyl nucleotides are the usual glycosyl donors in the reaction. 

This concept, known as273 275 the one enzyme-one linkage concept, has been widely used in discussions of the biosynthesis of carbohydrate chains in animal glycoproteins and proteoglycans. It implies that the structure of a carbohydrate chain is dictated by the specificity of the enzymes that participate in glycosidic-bond formation. Application of the same concept for discussion of the biosynthesis of bacterial polysaccharides seems worth while. 

Approximately 400 different glycosyltransferases are necessary in order to ensure the synthesis of those bacterial polysaccharides whose structures have thus far been elucidated. This estimate is based on the results of an analysis of the structures, made in order to ascertain how many different disaccharide fragments are present. An example of such an analysis is shown in Table V for the disaccharide sequences L-rhamnopyranosyl-D-galacto-pyranose, D-mannopyranosyl-L-rhamnopyranose, and D-galactopyranosyl-D-mannopyranose that are characteristic for the O-specific polysaccharides of Salmonella serogroups A, B, D, and E, the objects of many biosynthetic studies. Full details of similar analyses for other disaccharide sequences will be published elsewhere, as the resulting Tables are too voluminous for inclusion in this Chapter, but the most interesting results are summarized in Tables VI and VII. 

The list of the most common monosaccharide components of bacterial polysaccharides, shown in Table VI, includes the sugars that were found at the nonreducing end of nine or more different disaccharide fragments of the polymers. The right-hand column of the Table, which shows the total number of such fragments, gives an estimate of the number of different glycosyltransferases that use the activated form of the monosaccharides as 

Number of Different Disacchnride Fragments of the Polysaccharides Having the Most Common Monosaccharides at the Nonreducing End° 

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Information concerning glycosidie linkages, also, may be obtained from 13C-n.m.r. spectra. In this way, the type of glycosidie linkage in bacterial polysaccharides has been studied by Bhattachaijee and co-... 

Table VII illustrates the diversity of inter-monomeric linkages in bacterial polysaccharides. It shows how many different isomeric disaccharide frag-...

Strictly speaking, the glycosidic linkages in bacterial polysaccharides are not always formed with participation of glycosyltransferases (see Section V,l). 

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