The knowledge of enzyme mechanism is not only fundamental to understand the basic principles of biocatalysis, but it can also lead to the design and the synthesis of potent inhibitors of biologically relevant enzymes. The development of the mechanism-based glycosidase inhibitor Relenza and its validation as a drug to treat influenza virus infections is probably one of the most significant achievement of this research field,¹  still inspiring new prototypes.²  Together with the crystal structure analysis of glycosyl-processing enzymes/inhibitor/substrate complexes and the measurements of kinetic isotopic effects (KIE) of isotopically labelled sugars and analogues, the biological evaluation and spectroscopic...