Characterization of α-l-Rhamnosidase of Bacillus sp. GL1 Responsible for the Complete Depolymerization of Gellan

A bacterium, Bacillus sp. GL1, depolymerizes a heteropolysaccharide (gellan) to a tetrasaccharide (unsaturated glucuronyl-glucosyl-rhamnosyl-glucose) by extracellular gellan lyase. The resultant tetrasaccharide was degraded to the constituent monosaccharides by subsequent reactions of unsaturated glucuronyl hydrolase, β-d-glucosidase, and α-l-rhamnosidase. α-l-Rhamnosidase was substantially induced in the bacterial cells when grown in a medium containing gellan as a carbon source. The purified enzyme from the cells was a monomer with a molecular mass of about 100 kDa and was most active at pH 7.0 and 50°C. The enzyme acted on the gellan-degrading product (rhamnosyl-glucose) formed after successive reactions catalyzed by gellan lyase, unsaturated-glucuronyl hydrolase and β-d-glucosidase, and released rhamnose from the disaccharide. Therefore, the α-l-rhamnosidase is found to be responsible as the final enzyme for the complete depolymerization of gellan.