Microbial system for polysaccharide depolymerization: enzymatic route for xanthan depolymerization by Bacillus sp strain GL1

An enzymatic route for the depolymerization of a heteropolysaccharide (xant han) in Bacillus sp. strain GL1, which aas closely related to Brevibacillus thermoruber, was determined by analyzing the structures of xanthan depolym erization products. The bacterium produces extracellular xanthan lyase cata lyzing the cleavage of the glycosidic bond between pyruvylated mannosyl and glucuronyl residues in xanthan side chains (W. Hashimoto et al., Appl. Env iron. Microbiol. 64:3765-3768, 1998). The modified xanthan after the lyase reaction was then depolymerized by extracellular beta-D-glucanase to a tetr asaccharide, without the terminal mannosyl residue of the side chain in a p entasaccharide, a repeating unit of xanthan. The tetrasaccharide was taken into cells and converted to a trisaccharide (unsaturated glucuronyl-acetyla ted mannosyl-glucose) by beta-D-glucosidase. The trisaccharide was then con verted to the unsaturated glucuronic acid and a disaccharide (mannosyl-gluc ose) by unsaturated glucuronyl hydrolase. Finally, the disaccharide was hyd rolyzed to mannose and glucose by alpha-D-mannosidase. This is the first co mplete report on xanthan depolymerization by bacteria. Novel beta-D-glucana se, one of the five enzymes involved in the depolymerization route, was pur ified om the culture fluid. This enzyme was a homodimer with a subunit mole cular mass of 173 kDa and was most active at pH 6.0 and 45 degrees C. The e nzyme specifically acted on xanthan after treatment with xanthan lyase and released the tetrasaccharide.