Polysaccharides with lectins

Further evidence has supported the theory that the interaction between lectins of host plants and Rhizobium is the key to the mechanism of host specificity in Rhizobium-legame symbiosis and that the lectin receptor of Rhizobial cells exists in the cell surface lipopolysaccharide. No evidence was obtained to show interaction of extracellular polysaccharides with the lectins. It is possible that a specific recognition system may involve the specific binding of, for instance, pea (Pisum sativum) root hairs to an antigen that is situated on the lipopolysaccharide of infective strains of R. leguminosarum... 

The property of conformational restriction in polysaccharides make them candidates for being the initial self-ordering molecules of prebiotic evolution. This same property in the complex carbohydrate moiety of glycoproteins is the basis of carbohydrate-mediated information transfer through cell surface oligosaccharides interacting with each other or with lectin-like proteins in cell-cell recognition processes. 

If the polysaccharide is composed of uronic acid residues, it can be specifically precipitated by lowering the pH. Different pH values between 5 and 1 can be used to precipitate polysaccharides with different amounts and kinds of uronic acids [6]. Likewise, mixtures of neutral polysaccharides can. sometimes be precipitated by differential alcohol precipitation in which the polysaccharides have different water solubilities due to differences in their degrees of branching [7]. Saccharides with certain specific structures can be selectively separated by using lectins (see section 12.10). 

The ability of PO to interact with the acetyl residues of chitin allows us to compare them with monovalent lectins (i.e. extensins) which when binding with hemicellulose are only affected in a medium with a high ionic strength (Brownleader et al., 2006). As a rule, POs are bound with the plant cell wall and act as its modifiers. Some POs can form complexes with an extensin of cell walls (Brownleader et al., 2006). Consequently, chitin-specific sites that are capable of interacting with polysaccharides exist in the molecules of PO, and these sites can resemble the membrane receptor binding sites or else be similar to the domains of heparinbinding proteins (Kim et al., 2001). 

The content by weight of carbohydrate in glycoproteins may vary from only a few percent to over 50 percent in some proteins in mucous secretions. Although the function of the polysaccharide in most glycoproteins is unknown, in some cases it may provide hydrophilicity, recognition, and points of noncovalent interaction with other proteins through lectin-like affinity binding. 

Retention of native carbohydrate structure also is important in applications that utilize the conjugated polysaccharide in binding studies with receptors or lectins. In these cases, the carbohydrate should be modified at limited sites, preferentially only at its reducing end. Section 4.6 of this chapter discusses glycan conjugation techniques in greater detail. 

Some plant glycosides and polysaccharides cross-react195,197 with reagents for blood-group antigens, and interesting cross-reactions were also shown by methylated-fucose haptens and L-fucose-specific lectins for example, it was found that 2-O-methyl-L-fucose was immunodominant in the Japanese yew (Taxus cuspidata), and that the 3-0-... 

A striking new discussion of the "sugar code" looks at polysaccharides as informational molecules, with detailed discussions of lectins, selectins, and oligosaccharide-bearing hormones. 

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