Engineering enzymes: The glutamic acid nucleophile of a retaining β-mannosidase has been replaced with a serine residue to form a β-mannosynthase. When the new enzyme is provided with an α-mannosyl fluoride donor and an appropriate acceptor, β-mannoside linkages are synthesized. Remarkably, α-mannosyl fluoride can be generated in situ by providing the mannosynthase with excess fluoride ion.