The present invention provides for recombinant Endo S mutants that exhibit reduced hydrolysis activity and increased transglycosylation activity for the synthesis of glycoproteins wherein a desired sialylated oxazoline or synthetic oligosaccharide oxazoline is added to a core fucosylated or nonfucosylated GlcNAc protein acceptor. Such recombinant Endo S mutants are useful for efficient glycosylation remodeling of IgGl Fc domain to provide different antibody glycoforms carrying structurally well defined Fc N glycans.