Identification and Characterization of Dextran α-1,2-Debranching Enzyme from Microbacterium dextranolyticum.

Dextran α-1,2-debranching enzyme (DDE) releases glucose with hydrolyzing α-(1→2)-glucosidic linkages in α-glucans, which are made up of dextran with α-(1→2)-branches and are generated by Leuconostoc bacteria. DDE was isolated from Microbacterium dextranolyticum (formerly known as Flavobacterium sp. M-73) 40 years ago, although the amino acid sequence of the enzyme has not been determined. Herein, we found a gene for this enzyme based on the partial amino acid sequences from native DDE and characterized the recombinant enzyme. DDE had a signal peptide, a glycoside hydrolase family 65 domain, a carbohydrate-binding module family 35 domain, a domain (D-domain) similar to the C-terminal domain of Arthrobacter globiformis glucodextranase, and a transmembrane region at the C-terminus. Recombinant DDE released glucose from α-(1→2)-branched α-glucans produced by Leuconostoc citreum strains B-1299, S-32, and S-64 and showed weak hydrolytic activity with kojibiose and kojitriose. No activity was detected for commercial dextran and Leuconostoc citreum B-1355 α-glucan, which do not contain α-(1→2)-linkages. The removal of the D-domain decreased the affinity for α-(1→2)-branched α-glucans but not for kojioligosaccharides, suggesting that D-domain plays a role in α-glucan binding. Genes for putative dextranases, oligo-1,6-glucosidase, sugar-binding protein, and permease were present in the vicinity of the DDE gene, and as a result these gene products may be necessary for the use of α-(1→2)-branched glucans. Our findings shed new light on how actinobacteria utilize polysaccharides produced by lactic acid bacteria.