Dual α-1,4- and β-1,4-Glycosidase Activities by the Novel Carbohydrate-Binding Module in α-l-Fucosidase from sp. Strain EJY3.

Carbohydrates are structurally and functionally diverse materials including polysaccharides, and marine organisms are known to have many enzymes for the breakdown of complex polysaccharides. Here, we identified an α-l-fucosidase enzyme from the marine bacterium sp. strain EJY3 (FCD) that has dual α-1,4-glucosidic and β-1,4-galactosidic specificities. We determined the crystal structure of FCD and provided the structural basis underlying the dual α- and β-glycosidase activities of the enzyme. Unlike other three-domain FCDs, in FCD, carbohydrate-binding module-B (CBM-B) with a novel β-sandwich fold tightly contacts with the CatD/CBM-B main body and provides key residues for the β-1,4-glycosidase activity of the enzyme. The phylogenetic tree analysis suggests that only a few FCDs from marine microorganisms have the key structural features for dual α-1,4- and β-1,4-glycosidase activities. This study provides the structural insights into the mechanism underlying the novel glycoside hydrolase activities and could be applied for more efficient utilization in the hydrolysis of complex carbohydrates in biotechnological applications.