Structural basis for the flexible recognition of α-glucan substrates by Bacteroides thetaiotaomicron SusG

Bacteria that reside in the mammalian intestinal tract efficiently hydrolyze dietary carbohydrates, including starch, that escape digestion in the small intestine. Starch is an abundant dietary carbohydrate comprised of α1,4 and α1,6 linked glucose, yet mammalian intestinal glucoamylases cannot effectively hydrolyze starch that has frequent α1,6 branching as these structures hinder recognition and processing by α1,4-specific amylases. Here we present the structure of the cell surface amylase SusG from Bacteroides thetaiotaomicron complexed with a mixed linkage amylosaccharide generated from transglycosylation during crystallization. Although SusG is specific for α1,4 glucosidic bonds, binding of this new oligosaccharide at the active site demonstrates that SusG can accommodate α1,6 branch points at subsite -3 to -2, and also at subsite+1 adjacent to the site of hydrolysis, explaining how this enzyme may be able to process a wide range of limit dextrins in the intestinal environment. These data suggest that B. thetaiotaomicron and related organisms may have a selective advantage for amylosaccharide scavenging in the gut.