UDP-Gal: BetaGlcNAc Beta 1,4-Galactosyltransferase, Polypeptide 1 (B4GALT1)

  • Boopathy Ramakrishnan
  • Pradman K. Qasba
Reference work entry


The enzyme β4-galactosyltransferase-I (β4GalT1), also called β-N-acetylglucosaminyl-glycopeptide β-1,4-galactosyltransferase (UDP-galactose: N-acetyl-β-d-glucosaminyl-glycopeptide β-1,4-galactosyltransferase; EC, is a type II, trans-Golgi resident enzyme that, in the presence of a manganese (Mn2+) ion, transfers galactose from UDP-galactose to the N-acetylglucosamine (β-GlcNAc) molecule present in the nonreducing end of acceptor glycan molecules, synthesizing the disaccharide moiety LacNAc (Galβ1-4Glc) that has a β1-4-glycosidic linkage. β4GalT1 is found in all vertebrates, and the protein sequence of their catalytic domain shows very high similarity. The LacNAc moiety is the most prevalent carbohydrate epitope in vertebrate cells. The β4GalT1 has also played an important role in the evolution of mammals. During lactation, a mammary gland-specific 14 kD protein α-lactalbumin is made in large quantities to interact with β4GalT1, forming a lactose synthase (LS) complex (UDP-galactose:d-glucose β-1,4-galactosyltransferase; EC The LS complex catalyzes the transfer of galactose from UDP-galactose to a glucose molecule, synthesizing the milk sugar lactose, a hallmark of mammals (Brodbeck and Ebner 1966). In the absence of α-lactalbumin, glucose is a poor acceptor substrate for β4GalT1. In the presence of a sugar-donor substrate, the β4GalT1 interacts with α-lactalbumin, lowering the Km for glucose by a 1,000-fold, thus making it a preferred acceptor substrate over GlcNAc. The levels of both β4GalT1 and α-lactalbumin are elevated late during pregnancy, and both proteins are secreted in milk during lactation. α-Lactalbumin is also known to appear very early during pregnancy, disappear, and then reappear during late pregnancy and lactation (Nakhasi and Qasba 1979). It is interesting to note that in the presence of α-lactalbumin, the β4GalT1 protein from nonmammalian vertebrates can also make in vitro lactose, suggesting that the α-lactalbumin binding site exists in the β4GalT1 protein from nonmammalian vertebrates (Shaper et al. 1997). The β4GalT1 enzyme in invertebrates is a β4GalNAc-T enzyme that has a single mutated residue in the protein that relates it to the vertebrate β4GalT1 enzyme (Ramakrishnan and Qasba 2007).


Acceptor Substrate Oligosaccharide Moiety Nonmammalian Vertebrate Galactose Moiety Acceptor Sugar 
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Copyright information

© Springer Japan 2014

Authors and Affiliations

  1. 1.Structural Glycobiology Section, Nanobiology Program, and Basic Research ProgramFrederick National Laboratory for Cancer Research, SAIC-Frederick, Inc., Center for Cancer ResearchFrederickUSA
  2. 2.CCR Nanobiology Program, National Cancer Institute-Frederick, NIHFrederickUSA

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