Scheme of the chemoenzymatic synthesis of galacto-chitotetrose, Galβ1,4-GlcNAcβ1,4-(GlcNAcβ1,4)3) (2) and the transfer of modified sugars by the α3Gal–T mutants, SGG or AGG. Galactose was transferred to chitotetrose (1) by the wild-type (WT) β1,4Gal-T1 to give galacto-chitotetrose (2) that has a terminal LacNAc disaccharide moiety. Modified sugars, GalNAz (A) or 2-keto-Gal (B), were transferred from their UDP derivatives to galacto-chitotetrose, synthesizing the products (3) or (4), respectively, by the mutants SGG or AGG.