Polysaccharides block mechanism

In the case of branched polysaccharides, incorporation of side chains may take place through different mechanisms. In one of them, the assembly of the main chain is independent of the presence of side chains, and their incorporation into a polymeric molecule occurs as a modification of an initially formed, linear polysaccharide. Another situation is possible when incorporation of monosaccharide residues present in side chains is a necessary condition for elongation of the main chain, either through the monomeric or the block mechanism that is, intermediate formation of a linear, polysaccharide chain does not occur. Both mechanisms of incorporation of side chains were demonstrated to take place. 

These polysaccharides were the first examples where the block mechanism of polysaccharide-chain assembly was established, and the main features of the mechanism were first demonstrated in just this single case (for reviews, see Refs. 282-284). 

Another example of O-specific polysaccharides that are assembled through the block mechanism are the polymers from Salmonella serogroups C2 and C3. Their polysaccharide chains (18) are composed97 of linear, tetrasaccharide repeating-units having abequosyl and D-glucosyl branches,... 

It may be possible that the block mechanism of chain assembly also operates in the biosynthesis of colanic acid (33), the extracellular polysaccharide of E. coli.346 347... 

It seems possible that a block mechanism also operates in the assembly of the chain of polysaccharide 42, present in the cell wall of Bacillus cereus.124 In this case, formation of the polymer from a-GlcNAc-pp-Bpr, UDP-ManNAc, UDP-GalNAc, and UDP-Glc was observed.74... 

The block mechanism of chain assembly is characteristic for polymeric chains of the UGT type (see Salmonella O-specific polysaccharides 10-12 and 18) and the UG type (see capsular polysaccharides 25, 27, and 33), with UDP-activated sugars serving as initiators of chain growth. It seems rather safe to suggest that the biosynthesis of other polymers of these types occurs through a block mechanism as well. 

Formation of L-guluronic acid, a component of the alginic acid-like polysaccharide produced by P. aeruginosa and Azotobacter vinelandii, requires special comment. In this case, a polymer built from /3-(l- 4)-linked D-mannosyluronic acid residues serves as an intermediate in the biosynthesis.204,205 Part of the D-mannosyluronic acid residues in the polymer is subjected to an epimerization at C-5 catalyzed by an exocellular enzyme of the micro-organism,205-207 producing a polysaccharide composed of structural blocks that contain only D-mannosyluronic acid or only l-gulosyluronic acid residues, as well %s some having both. The mechanism of the epimerization remains unclear. 

Carbohydrate chains of teichuronic acids and neutral polysaccharides linked to the carbohydrate chains of peptidoglycans are fragments of macromolecules of Gram-positive cell-wall. Only two examples of the biosynthesis of these polymers have been studied in detail. Evidence for both block and monomeric mechanisms of the chain assembly was obtained. 

Coumarin reduced labeled glucose incorporation and carbohydrate and protein content of the cytoplasm of Pythium, but enzymes related to the metabolism of cell wall polysaccharides were not affected [296]. It has been reported that moulds are more sensitive to coumarin when they are cultured on the simple synthetic nutrient medium than on the yeast water [140]. This suggests that coumarin blocks the synthesis of an unknown metabolite of ndamental importance. Coumarin was found to affect the level of a variety of free amino acids present in a cell. In addition, some nucleic acid precursors partially reverse the coumarin-induced inhibition of maize mesocotyle [131]. The stimulation of germination and growth caused by coumarin in low concentrations may be assumed as a supercompensation mechanism. 

Fig. 9.7. Various mechanisms by which polysaccharides protect colloidal particles from flocculation (a) block polymers, (b) grafted polymers (covalent bonds), (c) linear polymers...

Figure 5.1 Branching Mechanisms in Polysaccharides, (a) Single glycosyl transfer from sugar nucleotides, (b) Block transfer by a branching enzyme (either within or between molecules).

The conformation of acidic polysaccharides and their interactions with calcium ions was examined by molecular simulation, and the authors demonstrated the existence of specific calcium binding with poly-a-L-guluronate [56, 57], The mechanism of complex formation involves calcium interactions with different oxygen atoms of two adjacent guluronic acid units and with two inter-chain units, as visualized in the egg-box model (Fig. 24.7). The mechanism of gelation is a two-step process first step is a dimer formation, followed by precipitation for small chains, or gelation for long ones formed with different types of blocks. 

In regenerative medicine, there are various different materials suitable as implantable scaffolds. These can be fabricated from natural or synthetic materials. Common examples are polysaccharides (eg, chitosan), or polyesters (eg, poly e-caprolactone), for natural and synthetic polymers, respectively. However, they are both capable of degradation (either enzyme mediated, or hydrolysis) in vivo (Bassi et al., 2011 Cunha-Reis et al., 2007). Often, polyesters are used as implantable biodegradable biomaterials, as they have controllable degradation and mechanical properties through formation of block copolymers. Where degradation occurs, the scaffolds... 

---