Polysaccharides monomeric mechanism

The term monomeric mechanism will be used for the mechanism depicted in the left-hand part of Scheme 2 (sequence a). In this case, the monosaccharide residues are transferred consecutively from the corresponding glycosyl donors (Z-A or Z -B) onto a membrane-bound glycosyl acceptor. The acceptor is generally a monosaccharide residue, which may be a fragment of an oligosaccharide chain linked to a hydrophobic molecule embedded in a cell membrane. In many instances, the acceptor that is used for assembly of the polymeric chain (Y) is not identical to the final acceptor (X) of the chain, and further transfer of the chain from Y to X, or liberation of the polysaccharide molecule in the case of exocellular polysaccharides, is a necessary step in the biosynthesis. 

The monomeric mechanism of chain assembly is characteristic for homopolysaccharides, which constitute the most difficult case for biosynthetic studies, as accumulation of intermediates could not be induced by removal of one of the glycosyl donors required for chain elongation. Among these polymers, the most extensive information has been obtained for polymers of JV-acetylneuraminic acid, namely, the capsular polysaccharides (36 and 37) of E. coli K1 (Ref. 354) and Neisseria meningitidis type c (Ref. 355), respectively. CMP-NeuAc serves as the glycosyl donor in the formation... 

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. 

Predominance of the monomeric mechanism of chain assembly is characteristic for polysaccharides of the G type (see structures 19 and 20) and the C type (see structures 36 and 37). Here again, the existence of a similar mechanism of chain assembly for other polymers of these types seems very probable. 

Unlike proteins, polysaccharides generally do not have definite molecular weights. This difference is a consequence of the mechanisms of assembly of the two types of polymers. As we shall see in Chapter 27, proteins are synthesized on a template (messenger RNA) of defined sequence and length, by enzymes that follow the template exactly. For polysaccharide synthesis there is no template rather, the program for polysaccharide synthesis is intrinsic to the enzymes that catalyze the polymerization of the monomeric units, and there is no specific stopping point in the synthetic process. 

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. 

The hydrolysis of polysaccharides into monomeric sugars is a well-studied process its mechanism is still believed to be accurately described by the work of Saeman in 1945.432 Its significance is linked to the production of fermentable sugars for ethanol manufacture. Many of these processes are based on acid catalysis and overlap closely the pretreatment methods described earlier.363,364,373 Treatments with both concentrated and dilute acids are well known as methods for the hydrolysis of cellulose and hemicellulose, as is the use of organic dicarboxylic acids as alternative catalysts to mineral acids.433... 

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