Characterization and Structural Analysis of a Novel exo-Type Enzyme Acting on beta-1,2-Glucooligosaccharides from Parabacteroides distasonis

beta-l,2-Glucan is a polysaccharide produced mainly by some Gram-negative bacteria as a symbiosis and infectious factor. We recently identified endo-beta-1,2-glucanase from Chitinophaga pinensis (CpSGL) as an enzyme comprising a new family. Here, we report the characteristics and crystal structure of a CpSGL homologue from Parabacteroides distasonis, an intestinal bacterium (BDI_3064 protein), which exhibits distinctive properties of known beta- ] ,2-glucan-degrading enzymes. BDI_3064 hydrolyzed linear beta-l,2-glucan and beta-l,2-glucoohgosaccharides with degrees of polymerization (DPs) of >= 4 to produce sophorose specifically but did not hydrolyze cyclic beta-1,2-glucan. This result indicates that BDI_3064 is a new exo-type enzyme. BDI_3064 also produced sophorose from beta-l,2-glucooligosaccharide analogues that have a modified reducing end, indicating that BDI_3064 acts on its substrates from the nonreducing end. The crystal structure showed that BDI_3064 possesses additional Nterminal domains 1 and 2, unlike CpSGL. Superimposition of BDI_3064 and CpSGL complexed with ligands showed that R93 in domain 1 overlapped subsite -3 in CpSGL. Docking analysis involving a beta-1,2-glucooligosaccharide with DP4 showed that R93 completely blocks the nonreducing end of the docked beta-1,2-glucoohgosaccharide. This indicates that BDI_3064 employs a distinct mechanism of recognition at the nonreducing end of substrates to act as an exo-type enzyme. Thus, we propose 2-beta-D-glucooligosaccharide sophorohydrolase (nonreducing end) as a systematic name for BDI_3064.