Protein-carbohydrate Interactions Leading to Hydrolysis and Transglycosylation in Plant Glycoside Hydrolase Family 1 Enzymes

Abstract

Glycoside hydrolase family 1 (GH1) includes enzymes with a wide range of specificities in terms of reactions, substrates and products, with plant GH1 enzymes covering a particularly wide range of hydrolases and transglycosylases. In plants, in addition to β-D-glucosidases, β-D-mannosidases, disaccharidases, thioglucosidases and hydroxyisourate hydrolase, GH1 has recently been found to include galactosyl and glucosyl transferases that utilize galactolipid and acyl glucose donors, respectively. The amino acids binding to the nonreducing monosaccharide residue of glycosides and oligosaccharides in subsite -1 are largely conserved in GH1 glycoside hydrolases, despite their different glycon specificities, and residues outside this subsite contribute to sugar specificity. The conserved subsite -1 residues form extensive hydrogen bonding and aromatic stacking interactions to the glycon to distort it toward the transition state, so they must make different interactions with different sugars. Aglycon specificity is largely determined by interactions with the cleft leading into the active site, but different enzymes appear to interact with their substrates via different residues. The most extended aglycon binding interactions that have been studied extensively are those for cellooligosaccharides. Rice Os3BGlu7 (BGlu1) β-D-glucosidase, which binds cellooligosaccharides residues in subsites +1 to +4 primarily by water-mediated hydrogen bonds and a few aromatic-sugar stacking interactions, appears to show remarkable plasticity in this binding. Although mutations that change the mechanism of the hydrolases, such as glycosynthases and thioglycoligases create transglycosylases, the structural basis for natural transglycosylase vs. glycoside hydrolase activities in GH1 enzymes remains to be determined.