Exploring the Promiscuity of Polysaccharide Synthase Biosynthetic Pathways to Incorporate Unusual Sugars

There has been intensive studies to explore incorporation of non-native repeat units in bacterial proteins and polyesters. However, prior to the work in our laboratory between 1997 and 2001, the idea of varying polysaccharides via physiological mechanisms had not been explored. Polysaccharides perform an incredibly diverse set of critical functions in the biosphere including structural, adhesion and barrier functions. Polysaccharides are more problematic for studies of structure-function relationships because, unlike proteins, the direct link between genetic blueprints and polymer structure has largely not been established. Thus, more complex control pathways to regulate synthesis and processing are involved. Our group began to explore the extent that the polysaccharide synthesis biosynthetic pathways are flexibile for entry of unusual or non-natural repeat units along exopolysaccharide chains. Through this work we hoped to develop new insights into the rational synthesis of polysaccharides with controlled compositions and chemistries.

Figure 26. ESEM micrograph of microbial cellulose formed by Acetobacter xylinum that consists of both glucose and 2-acetimido-2-deoxy-glucose repeat units.

The first goal was to establish synthetic methods by co-opting the biosynthetic machinery. To this end, we studied bioengineering of polysaccharides, including changes in the composition of glucose and galactose in zoogloea gum from Zoogloea ramigera (Lee et al., 1997) and changes in the content of glucose and mannose in pullulan from the fungus Aureobasidium pullulans (Lee et al., 1999; Hyung-Pil et al., 2004). As an alternative approach to modify pendant groups on polysaccharide main chains, an enzymatic route was used to regioselectively form fatty acid esters on the backbone of amylose and cellulose. Curdlan, an unbranched homo-(1,3)-glucan produced by Agrobacterium sp., was modified in vivo by the direct incorporation of the carbon source 3-O-methyl-D-glucose (Lee et al, 1997). Up to 12 mol% of this modified polyglucan was 3-O-methyl-D-glucose. Studies completed on the biosynthesis of cellulose and modified cellulose by Acetobacter xylinum support the concept that the non-glucose analogs 2-amino-2-deoxy-glucose and 2-acetimido-2-deoxy-glucose can be directly incorporated into microbial cellulose formed by A. xylinum. This strategy resulted in the formation of novel copolymers, glucose/2-amino-2-deoxy-glucose (cellulose-chitosan) and glucose/2-acetimido-2-deoxy-glucose (cellulose-chitin). These types of cellulose-chitin and cellulose-chitosan copolymers had not been previously been reported (Lee et al., 2001). Thus, this work established a new group of polysaccharides based on solubility, functional properties and structure. These types of ‘tailorable’ polysaccharides will hopefully lead to important new biomaterials.

References

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