Enzymatic polymerization hybrid polysaccharides

The examples reviewed in this chapter prove that enzymatic polymerizations using glycosyltransferases and glycosidases are powerful techniques for synthesizing various well-defined polysaccharides ranging from natural saccharides such as cellulose, amylose, amylopectin etc. to non-natural hybrid polysaccharides. 

Scheme 8 Enzymatic polymerizations to a cellulose-chitin hybrid polysaccharide.

Kobayashi et have also investigated other modes of enzymatic polymerization to produce a novel cellulose-chitin hybrid polysaccharide. A sugar fluoride monomer of GlcNAc/3(l 4)Glc (17) was designed as a TSAS monomer for polymerization catalyzed by cellulase from Trichoderma viride. In the polymerization, monomer 17 was recognized by cellulase from T. viride, leading to a cellulose-chitin hybrid polysaccharide 18 with perfect regioselectivity and stereochemistry (Scheme 17). 

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... 

Figure 1. Synthes of hybrid polysaccharides via enzymatic polymerization.

Kobyashi S., Makino A., Matsumoto H., Kunii S., Ohmae M., Kiyosada T., Makiguchi K., Matsumoto A., Horie M., Shoda S.I., Enzymatic polymerization to novel polysaccharides having a glucose-JV-acetylglucosamine repeating unit, a ceUulose-chitin hybrid polysaccharide. Biomacromolecules, 7, 2006,1644-1656. 

Amylose can be prepared by phosphorylase-catalyzed enzymatic polymerization using a-D-glucose 1-phosphate (G-l-P) as a monomer from a meltooligosaccharide as a primer, which is hereinafter described in detail [2], Because of the natural polysaccharides such as amylose are recycled carbon resources and considered to be eco-friendly substances [3], it is expected that use of the polysaccharides as one component of hybrid polymers will lead to the production of environmentally benign materials. 

As shown above, hybrid structures bearing amylose blocks can be synthesized by covalent attachment of primer recognition units for phosphorylase and subsequent enzymatic grafting from polymerization. Following the same route we are currently synthesizing hybrid materials bearing (hyper)branched polysaccharide structures as shown in figure 6 with the described tandem reaction of two enzymes. 

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