Prospecting for Microbial α--acetylgalactosaminidases Yields a New Class of GH31 -glycanase

α-Linked GalNAc (α-GalNAc) is most notably found at the nonreducing terminus of the blood type-determining A-antigen and as the initial point of attachment to the peptide backbone in mucin-type -glycans. However, despite their ubiquity in saccharolytic microbe-rich environments such as the human gut, relatively few α--acetylgalactosaminidases are known. Here, to discover and characterize novel microbial enzymes that hydrolyze α-GalNAc, we screened small-insert libraries containing metagenomic DNA from the human gut microbiome. Using a simple fluorogenic glycoside substrate, we identified and characterized a glycoside hydrolase 109 (GH109) that is active on blood type A-antigen, along with a new subfamily of glycoside hydrolase 31 (GH31) that specifically cleaves the initial α-GalNAc from mucin-type -glycans. This represents a new activity in this GH family and a potentially useful new enzyme class for analysis or modification of -glycans on protein or cell surfaces.