Quantitative control of sialylation

The present disclosure is directed to the use of certain glycosyltransferase variants having N-terminal truncation deletions. Contrary to previous findings certain truncations were found to exhibit sialidase enzymatic activity, particularly a variant of human sialyltransferase (hST6Gal-I) with a truncation deletion involving the first 89 N-terminal amino acids of the respective wild-type polypeptide. A fundamental finding documented in the present disclosure is that there exists a variant of this enzyme which is capable of catalyzing transfer of a glycosyl moiety as well as hydrolysis thereof. Thus, disclosed is a specific exemplary variant of mammalian glycosyltransferase, nucleic acids encoding the same, methods and means for recombinantly producing the variant of mammalian glycosyltransferase and use thereof, particularly for sialylating in a quantitatively controlled manner terminal acceptor groups of glycan moieties being part of glycoproteins such as immunoglobulins.