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

An enzymatic route for the depolymerization of a heteropolysaccharide (xanthan) in Bacillus sp. strain GL1, which was closely related to Brevibacillus thermoruber, was determined by analyzing the structures of xanthan depolymerization products. The bacterium produces extracellular xanthan lyase catalyzing the cleavage of the glycosidic bond between pyruvylated mannosyl and glucuronyl residues in xanthan side chains (W. Hashimoto et al., Appl. Environ. Microbiol. 64:3765-3768, 1998). The modified xanthan after the lyase reaction was then depolymerized by extracellular beta-D-glucanase to a tetrasaccharide, without the terminal mannosyl residue of the side chain in a pentasaccharide, a repeating unit of xanthan. The tetrasaccharide was taken into cells and converted to a trisaccharide (unsaturated glucuronyl-acetylated mannosyl-glucose) by beta-D-glucosidase. The trisaccharide was then converted to the unsaturated glucuronic acid and a disaccharide (mannosyl-glucose) by unsaturated glucuronyl hydrolase. Finally, the disaccharide was hydrolyzed to mannose and glucose by alpha-D-mannosidase. This is the first complete report on xanthan depolymerization by bacteria. Novel beta-D-glucanase, one of the five enzymes involved in the depolymerization route, was purified from the culture fluid. This enzyme was a homodimer with a subunit molecular mass of 173 kDa and was most active at pH 6.0 and 45 degrees C. The enzyme specifically acted on xanthan after treatment with xanthan lyase and released the tetrasaccharide.