Synthesizes the galactose-alpha(1,3)-galactose group on the glycosphingolipid isoglobotrihexosylceramide or isogloboside 3 (iGb3) by catalyzing the transfer of galactose from UDP-Galactose to its acceptor molecule Gal-beta-1,4-Glc-ceramide. Can also catalyze the addition of galactose to iGb3 itself to form polygalactose structures. a beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + UDP-alpha-D-galactose = an alpha-D-galactosyl-(1->3)-beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + H(+) + UDP a beta-D-Gal-(1->4)-beta-D-Glc-(1<->1)-Cer(d18:1(4E)) + UDP-alpha-D-galactose = H(+) + isogloboside iGb3Cer (d18:1(4E)) + UDP globoside Gb3Cer + UDP-alpha-D-galactose = GalGb3Cer + H(+) + UDP Also found in numerous large vesicles throughout the cytoplasm of the soma. Expressed in thymus and monocyte derived dendritic cells. The conserved DXD motif is involved in cofactor binding. The manganese ion interacts with the beta-phosphate group of UDP and may also have a role in catalysis (By similarity). Belongs to the glycosyltransferase 6 family. According to a report, the spliced A3GALT2 mRNA is not detected in human tissues (PubMed:18630988). The functional activity of human A3GALT2 tested by expressing a chimeric protein containing the catalytic domain of human A3GALT2 is unable to synthesize isogloboside 3 (iGb3). Futhermore mutagenesis experiments in rat, show that the mutant 'Asn-253' of human A3GALT2 completely eliminates alpha-1,3-galactosyltransferase activity (PubMed:18630988).