Unnatural polysaccharides

When medicinal chemists are looking for ways to attack bacteria, one approach is to interfere with chemistry carried out by prokaryotes but not by us. The most famous of these attacks is aimed at the construction of the cell walls of some bacteria that contain unnatural (R) (or D-) amino acids. Bacterial cell walls are made from glycopeptides of an unusual kind. Polysaccharide chains are cross-linked with short peptides containing (J )-alanine (D-Ala). Before they are linked up, one chain ends with a glycine molecule and the other with D-Ala-D-AJa, In the final step in the cell wall synthesis, the glycine attacks the D-Ala-D-Ala sequence to form a new peptide bond by displacing one D-Ala residue. 

Glycosidase-catalyzed in vitro polymerizations enabled the synthesis of not only various natural polysaccharides, but also some unnatural polysaccharides, when the substrate monomer is appropriately designed in combination with a selected enzyme. 

Unnatural primers bearing functional groups can also be used to prepare tailor-made polysaccharides for further manipulation, e. g. attachment to protein or other compounds (Fig. 11.3-23). 

Biocatalysis is a key route to both natural and non-natural polysaccharide structures. Research in this area is particularly rich and generally involves at least one of the following three synthetic approaches 1) isolated enzyme, 2) whole-cell, and 3) some combination of chemical and enzymatic catalysts (i.e. chemoenzymatic methods) (87-90). Two elegant examples that used cell-fi-ee enzymatic catalysts were described by Makino and Kobayashi (25) and van der Vlist and Loos (27). Indeed, for many years, Kobayashi has pioneered the use of glycosidic hydrolases as catalysts for polymerizations to prepare polysaccharides (88,91). In their paper, Makino and Kobayashi (25) made new monomers and synthesized unnatural hybrid polysaccharides with regio- and stereochemical-control. Van der Vlist and Loos (27) made use of tandem reactions catalyzed by two different enzymes in order to prepare branched amylose. One enzyme catalyzed the synthesis of linear structures (amylose) where the second enzyme introduced branches. In this way, artificial starch can be prepared with controlled quantities of branched regions. 

Synthesis of Unnatural Hybrid Polysaccharides via Enzymatic Polymerization... 

While this approach afforded gram quantities of a heparin-like polysaccharide with anticoagulant activity, unnatural saccharide units, such as 3-0-sulfo-D-glucuronic acid, were present in their product, suggesting a limitation in the selectivity of chemical sulfonation/desulfonation in HS synthesis. 

We have achieved the synthesis of structurally well-defined natural and unnatural oligo- and polysaccharides via enzymatic polymerization utilizing natural glycosyl hydrolases as catalysts (26-57) cellulose and xylan prepared by cellulase (52-54), an amylose oligomer by amylase (55), chitin by chitinase (56, 57), altematingly 6-(9-methylated cellulose by cellulase (55), and a cellulose-... 

For the polysaccharide synthesis, enzymatic polymerization has been developed as a new in vitro synthesis method of natural and unnatural polysaccharides having complicated structures.The method utilizes a hydrolysis enzyme to catalyze the bond formation for the polymer construction, a reverse direction of the hydrolysis to cleave the bond. This catalysis is due to the enzymatic characteristics, where enzymes catalyze the reverse reaction involving a common intermediate in both forward and backward reactions. In nature, there are many polysaccharides having N-acetyl groups called mucopolysaccharides such as chitin, hyaluronic acid (HA), and chondroitin (Ch). 

Variously substituted oxazoline monomers, 2-ethyl, 2-n-pro-pyl, and 2-vinyl oxazoline monomers for HA, were newly prepared and polymerized with HAase catalysis. The reactions proceeded with total control of regioselectivity and stereochemistry, to afford the corresponding HA and Ch derivatives (unnatural polysaccharides) possessing N-propionyl, N-butyryl, and N-acryloyl group in every hexosamine unit (Scheme 40). Similarly, Ch derivatives were also achieved. The resulting N-acryloyl HA and Ch are functional polymers having a reactive vinyl group. 

Bacterial cell walls consist of parallel polysaccharide chains linked by short oligopeptide sequences of 3-5 amino acids. These include the unnatural R enantiomers of alanine, glutamine, and glutamic acid. Because these amino acids do not occur in hydrolytic enzymes, they are not recognized by them as substrates (Real Life 5-5), affording ingenious protection. 

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