The term glycosynthase refers to a class of proteins that have been engineered to catalyze the formation of a glycosidic bond.  Glycosynthase are derived from glycosidase enzymes, which catalyze the hydrolysis of glycosidic bonds. They were traditionally formed from retaining glycosidase by mutating the active site nucleophilic amino acid (usually an aspartate or glutamate) to a small non-nucleophilic amino acid (usually alanine or glycine).  More modern approaches use directed evolution to screen for amino acid substitutions that enhance glycosynthase activity.