Polysaccharide structure patterns

How can the many complex polysaccharides found in nature be synthesized Are there genetically determined patterns How are these controlled The answer can be found in the specificities of the hundreds of known glycosyltransferases171 172 and in the patterns of expression of the genes for transferases and other proteins. As a consequence, a great variety of structurally varied polysaccharide structures arise, especially on cell surfaces. The structures are not random but depend upon the assortment of glycosyltransferases available at the particular stage of development in a tissue. 

Figure 1. Variety of structural patterns found among Klebsiella bacterial polysaccharides...

Enzymes are used in the determination of polysaccharide structures. Because enzymes have specificity, i.e. they produce specific products of low molecular weight and can cleave specific kinds of bonds they can be used to determine the fine structure of starch. A first premise, however, is that the action patterns and specificity must need to be thoroughly investigated and elucidated before reliable information can be obtained about the structure of the polysaccharide or oligosaccharide being studied. 

Related materials can be prepared in which the polysaccharides are linked to a silica support by covalently bound tether groups. For example, silica derivatized by 3-aminopropyl groups can be linked to polysaccharides using diisocyanates. These materials seem to adopt organized structural patterns on the surface, and this factor is believed to contribute to their resolving power. The precise structural basis of the chiral recognition and discrimination of derivatized polysaccharides has not been elucidated, but it appears that in addition to polar interactions, tt-tt stacking is important for aromatic compounds. ... 

Numerous examples further illustrate the great value of the Smith degradation in determinations of the fine structure of polysaccharides. They include studies on arabinoxylans, mesquite gum, an exocellular yeast mannan, and a type-specific bacterial polysaccharide. Branching patterns in complex types of glycoproteins from several different origins have been elucidated, and detailed structures of gum exudates, seed polysaccharides, and pectic sub-... 

A similar position exists with most other polysaccharides. In general, the same chemical structures, produced by prokaryotes and eukaryotes are unlikely to be related in terms of their biosynthesis. Within the eukaryotes, evolutionary relationships may well exist where similar polymers are found in species that are known, from other evidence, to be closely related. Likewise, the present-day pattern of occurrence of a polymer such as chitin may well suggest that a single t3q>e of mechanism exists for its synthesis and that this has persisted in some phyla, but has been lost during the evolution of others. Where a polysaccharide seems to reappear after an evolutionary gap, there is always a question as to whether this is a re-emergence of the old synthetic pathway, or the acquisition of a new one. It should not be forgotten that the relative simplicity of polysaccharide structure does make the latter quite possible, in principle at least. 

The side chains in the latter are flexible disaccharides on account of poor-quality diffraction patterns, their tentative molecular structures are known only from computer modeling.1" On the other hand, well-defined crystal structures are available for gellan and welan, and they can be correlated with the physical properties of the polysaccharides the details are presented here. Their conformation angles are listed in Table VI. 

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