The Structure of a Family 110 Glycoside Hydrolase Provides Insight into the Hydrolysis of Α-1,3-galactosidic Linkages in Λ-Carrageenan and Blood Group Antigens

alpha-Linked galactose is a common carbohydrate motif in nature that is processed by a variety of glycoside hydrolases from different families. Terminal Gal alpha 1-3Gal motifs are found as a defining feature of different blood group and tissue antigens, as well as the building block of the marine algal galactan lambda-carrageenan. The blood group B antigen and linear alpha-Gal epitope can be processed by glycoside hydrolases in family GH110, whereas the presence of genes encoding GH110 enzymes in polysaccharide utilization loci from marine bacteria suggests a role in processing lambda-carrageenan. However, the structure-function relationships underpinning the alpha-1,3-galactosidase activity within family GH110 remain unknown. Here we focus on a GH110 enzyme (PdGH110B) from the carrageenolytic marine bacterium Pseudoalteromonas distincta U2A. We showed that the enzyme was active on Gal alpha 1-3Gal but not the blood group B antigen. X-ray crystal structures in complex with galactose and unhydrolyzed Gal alpha 1-3Gal revealed the parallel beta-helix fold of the enzyme and the structural basis of its inverting catalytic mechanism. Moreover, an examination of the active site reveals likely adaptations that allow accommodation of fucose in blood group B active GH110 enzymes or, in the case of PdGH110, accommodation of the sulfate groups found on lambda-carrageenan. Overall, this work provides insight into the first member of a predominantly marine clade of GH110 enzymes while also illuminating the structural basis of alpha-1,3-galactoside processing by the family as a whole.