H. Nankai et al., Microbial system for polysaccharide depolymerization: enzymatic route for xanthan depolymerization by Bacillus sp strain GL1, APPL ENVIR, 65(6), 1999, pp. 2520-2526
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.