Modulating the regioselectivity of a Pasteurella multocida sialyltransferase for biocatalytic production of 3'- and 6'-sialyllactose

•Sialyltransferase mutations change regioselectivity in trans-sialidase reaction.•P34H changes an α-2,3- to an α-2,6-sialyltransferase.•P34H preferentially yields 6′-sialyllactose in trans-sialidase reactions.•E271F and R313Y preferentially yield 3′-sialyllactose in trans-sialidase reactions.