Exopolysaccharide structure

W. Sutherland Microbial exopolysaccharides—structural subtleties and their consequences... 

Sutherland IW (1997) Microbial exopolysaccharides- structural subtleties and their consequences. Pure Appl Chem 69 1911—1917 Sutherland IW (1998) Novel and established applications of microbial polysaccharides. Trends Biotechnol 16 41-46... 

Commercial applications for polysaccharides include their use as food additives, medicines and industrial products. Although plant polysaccharides (such as starch, agar and alginate) have been exploited commercially for many years, microbial exopolysaccharides have only become widely used over the past few decades. The diversity of polysaccharide structure is far greater in micro-organisms compared to plants and around 20 microbial polysaccharides with market potential have been described. However, microorganisms are still considered to be a rich and as yet underexploited source of exopolysaccharides. 

Although exopolysaccharides do not normally have a structural role, they do form structures that can be detected by either light or electron microscopy. Exopolysaccharides may form part of a capsule closely attached to the microbial cell surface, or appear as loose slime secreted by the cell but not directly attached to it mucoid Exopolysaccharide producing cells usually form mucoid colonies on solid media and colonies liquid cultures of these cells may become very viscous. However, growth conditions can... 

Figure 7.1 Structures of some of the components of microbial exopolysaccharides.

Some microbial exopolysaccharides contain the inorganic substituents phosphate and sulphate. Phosphate has been found in exopolysaccharide from bacteria of medical importance, including Escherichia coli. Sulphate is far less common than phosphate and has only been found in spedes of cyanobaderia. In addition to these inorganic components, which form part of the structure of some exopolysaccharides, all polyanionic polymers will bind a mixture of cations. Exopolysaccharides are, therefore, purified in the salt form. The strength of binding of the various cations depend on the exopolysaccharide some bind the divalent cations calrium, barium and strontium very strongly, whereas others prefer certain monovalent cations, eg Na ... 

The unique physical properties of microbial exopolysaccharides (considered in Section 7.7), which determine their commercial importance, arises from their molecular conformation. This, in turn, is determined by the primary structure and from associations between molecules in solution. 

The intermolecular interactions stabilise the helices and greatly influence the properties of exopolysaccharides in solution, ie solubility, viscosity and gel-formation. A strong interaction or good-fit between molecules will lead to insolubility, whereas poor interaction will lead to solubility of exopolysaccharides. The interactions between molecules is influenced by the presence of side-chains. For example, cellulose is insoluble but introduction of a three monosaccharide side-chain into the cellulose chain gives the soluble xanthan. Small changes in the structure of the side-chains can alter the molecular interactions and thus properties of the exopolysaccharide. 

Another feature of this particular exopolysaccharide is that gel strength depends upon the temperature used. It is constant between 60-80°C, increasing in strength from 80-100°C and finally changing structure from a single to a triple stranded helix at temperatures over 120°C. This makes it particularly well suited for use as a molecular sieve, immobilised enzyme support and a binding agent. 

Many polysaccharides of eukaryotic origin show non-specific anti-viral activity and this property may be shared by some of the exopolysaccharides. The structural requirements for activity are not immediately evident as the polysaccharides exhibiting this activity are very diverse. 

Sutherland, I. W. (1999). Biofilm exopolysaccharides. In Microbial Extracellular Polymeric Substances. Characterization, Structure and Function, eds. Wingender, J., Neu, T. R. and Flemming, H.-C., Springer-Verlag, Berlin, pp. 73-92... 

The first report of the occurrence of KDO in a structure other than LPS was that of Taylor47 on the capsular polysaccharide (K-antigen) of a clinical isolate, Escherichia coli LP1092. Almost simultaneously, Bhattachaijee and coworkers83 described an exopolysaccharide from Neisseria meningitidis serogroup 29e. This material contains KDO... 

Selbmann L, Onofri S, Fenice M, Federici F, PetruccioH M, Production and structural characterization of the exopolysaccharide of the Antarctic fungus Phoma herbarum CCFEE 5080, Res Microbiol 153 585-592, 2002. 

Plant structural material is the polysaccharide cellulose, which is a linear p (1 —> 4) linked polymer. Some structural polysaccharides incorporate nitrogen into their molecular structure an example is chitin, the material which comprises the hard exoskeletons of insects and crustaceans. Chitin is a cellulose derivative wherein the OH at C-2 is replaced by an acetylated amino group (—NHCOCH3). Microbial polysaccharides, of which the capsular or extracellular (exopolysaccharides) are probably the most important class, show more diversity both in monomer units and the nature of their linkages. 

Finally, details of the synthesis of heteropolysaccharides in plants are as yet completely unknown. The structural similarities among some plant gums and such bacterial exopolysaccharides as xanthan gum suggest that similar mechanisms may be operative in bacteria and in plants. Lipid intermediates could be suggested as potential glycosyl donors in the formation of plant gums and mucilages. 

Biofilms are sessile microbial communities, the formation of which is initiated by surface attachment of individual bacteria, followed by cell-cell interactions and development in a three-dimensional structure of the colonies (O Toole et al., 2000). Biofilm formation is a multi-step development process over a period of several hours (Costerton et al., 1995). The initial surface interaction is mediated by flagella and pili functioning, then the exopolysaccharides stabilize the biofilm and, finally, intercellular communication occurs through signaling molecules (Watnic and Kolter, 1999). 

Rhizobial exopolysaccharides show a highly diverse pattern of differently pyru-vated structures (see Table 1), and are therefore attractive targets for chemical syntheses. Some examples using pyruvated glycosyl donors are presented here in order to further demonstrate the special features of these donors for the synthesis of complex rhizobial oligosaccharides. 

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