Glycosyltransferase family 6 comprises enzymes responsible for the production of the human ABO blood group antigens.
Glycosyltransferase family 6, GT_6, comprises enzymes with three known activities: alpha-1,3-galactosyltransferase, alpha-1,3 N-acetylgalactosaminyltransferase, and alpha-galactosyltransferase. UDP-galactose:beta-galactosyl alpha-1,3-galactosyltransferase (alpha3GT) catalyzes the transfer of galactose from UDP-alpha-d-galactose into an alpha-1,3 linkage with beta-galactosyl groups in glycoconjugates. The enzyme exists in most mammalian species but is absent from humans, apes, and old world monkeys as a result of the mutational inactivation of the gene. The alpha-1,3 N-acetylgalactosaminyltransferase and alpha-galactosyltransferase are responsible for the production of the human ABO blood group antigens. A N-acetylgalactosaminyltransferases use a UDP-GalNAc donor to convert the H-antigen acceptor to the A antigen, whereas a galactosyltransferase uses a UDP-galactose donor to convert the H-antigen acceptor to the B antigen. Alpha-1,3 N-acetylgalactosaminyltransferase and alpha-galactosyltransferase differ only in the identity of four critical amino acid residues.