The first glycosynthase was a retaining exoglycosidase that catalyzed the formation of β 1-4 linked glycosides of glucose and galactose.  Glycosynthase enzymes have since been expanded to include mutants of endoglycosidase, as well as mutants of inverting glycosidase.  Substrates of glycosynthase include glucose, galactose, mannose, xylose, and glucuronic acid.  
Modern methods to prepare glycosynthase use directed evolution to introduce modifications, which improve the enzymes function.  This process was made available due to the development of high throughput screens for glycosynthase activity.