Structural comparison of Streptococcus mutans dextran glucosidase with glucoside hydrolases in GH13

In glycoside hydrorase family (GH) 13, alpha-glucosidase, oligo-1,6-glucosidase and dextran glucosidase, which hydrolyze the non-reducing end glucosidic linkages of maltooligo- and/or isomaltoolligosaccharides, are categorized as alpha-glucoside hydrolase. Despite a high similarity in the sequence and overall structure of those family enzymes, GH 13 alpha-glucoside hydrolases show a wide range of substrate specificity. Until now, three crystal structures of alpha-glucoside hydrolase, dextran glucosidase from Streptococcus mutans (DGase), oligo-1,6-glucosidase from Bacillus cereus (O16G), and alpha-glucosidase from Geobacillus sp. HTA-462 (GSJ) have been determined. In this study, we have performed the structural comparison of these alpha-glucoside hydrolases. Their overall structures are generally similar, and consist of three major domains A, B and C as found in many alpha-amylase family enzymes. The significant structural differences in these enzymes are mainly found in loop regions. GSJ has a shorter beta-alpha loop 6 in a different orientation in addition to the disordered regions, whereas DGase and O16G show high similarity in their tertiary structures. Though these enzymes have the different substrate preference, they all possess the completely conserved configuration at subsite -1. Therefore, the substrate preference will be originated from the structure at subsite for the reducing end side of substrate. The substrate binding modes of these glucoside hydrolases were predicted by superimposing the substrate molecule of substrate-complex structures of DGase and alpha-amylase. DGase and O16G are thought to have a smilar manner in substrate binding with conserved amino acid residues. The substrate recognition of GSJ at subsite -1 and +1 would be similar to that of alpha-amylases since the key residues in substrate binding are conserved in both primary and tertiary structures.