Structural and functional analyses of a glycoside hydrolase family 5 enzyme with an unexpected β-fucosidase activity.

We present characterization of PbFucA, a family 5 glycoside hydrolase (GH5) from Prevotella bryantii B(1)4. While GH5 members typically are xylanases, PbFucA shows no activity toward xylan polysaccharides. A screen against a panel of p-nitrophenol coupled sugars identifies PbFucA as a β-D-fucosidase. We also present the 2.2 Å resolution structure of PbFucA and use structure-based mutational analysis to confirm the role of catalytically essential residues. A comparison of the active sites of PbFucA with those of family 5 and 51 glycosidases reveals that while the essential catalytic framework is identical between these enzymes, the steric contours of the respective active site clefts are distinct and likely account for substrate discrimination. Our results show that members of this cluster of orthologous group (COG) 5520 have β-D-fucosidase activities, despite showing an overall sequence and structural similarity to GH-5 xylanases.