title: Binding of N-acetylglucosamine (GlcNAc) β1-6-branched oligosaccharide acceptors to β4-galactosyltransferase I reveals a new ligand binding mode.
abstract: N-acetyllactosamine is the most prevalent disaccharide moiety in the glycans on the surface of mammalian cells and often found as repeat units in the linear and branched polylactosamines, known as i- and I-antigen, respectively. The β1-4-galactosyltransferase-I (β4Gal-T1) enzyme is responsible for the synthesis of the N-acetyllactosamine moiety. To understand its oligosaccharide acceptor specificity, we have previously investigated the binding of tri- and pentasaccharides of N-glycan with a GlcNAc at their nonreducing end and found that the extended sugar moiety in these acceptor substrates binds to the crevice present at the acceptor substrate binding site of the β4Gal-T1 molecule. Here we report seven crystal structures of β4Gal-T1 in complex with an oligosaccharide acceptor with a nonreducing end GlcNAc that has a β1-6-glycosidic link and that are analogous to either N-glycan or i/I-antigen. In the crystal structure of the complex of β4Gal-T1 with I-antigen analog pentasaccharide, the β1-6-branched GlcNAc moiety is bound to the sugar acceptor binding site of the β4Gal-T1 molecule in a way similar to the crystal structures described previously; however, the extended linear tetrasaccharide moiety does not interact with the previously found extended sugar binding site on the β4Gal-T1 molecule. Instead, it interacts with the different hydrophobic surface of the protein molecule formed by the residues Tyr-276, Trp-310, and Phe-356. Results from the present and previous studies suggest that β4Gal-T1 molecule has two different oligosaccharide binding regions for the binding of the extended oligosaccharide moiety of the acceptor substrate.
authors: Structural Glycobiology Section, SAIC-Frederick, Inc., Center for Cancer Research Nanobiology Program, Center for Cancer Research, Frederick National Laboratory for Cancer Research, National Institutes of Health, Frederick, Maryland 21702, USA.
authors: Boopathy
authors: B
authors: Ramakrishnan
authors: Elizabeth
authors: E
authors: Boeggeman
authors: Pradman K
authors: PK
authors: Qasba
mesh_terms: D000117:Acetylglucosamine / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D018360:Crystallography, X-Ray; D005700:Galactosyltransferases; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D025942:Oligosaccharides, Branched-Chain / Q000737:chemistry* / Q000378:metabolism; D017434:Protein Structure, Tertiary
chemical_list: D025942:Oligosaccharides, Branched-Chain; D005700:Galactosyltransferases; C119701:beta-1,4-galactosyltransferase I; D000117:Acetylglucosamine