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.
Highlights
The enzyme 4Gal-T1 synthesizes the LacNAc moiety of glycans
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 4Gal-T1 molecule
In the present crystal structures, due to the disordered nature of the diphosphate groups, their coordination with the Mn2ϩ ion could not be clearly determined. This is similar to the crystal structure of the bovine W314A-4Gal-T1 complex with ␣-lactalbumin protein, where the 4Gal-T1 molecule was found in the closed conformation, with disordered phosphate groups and Mn2ϩ coordination [20]
Summary
The enzyme 4Gal-T1 synthesizes the LacNAc moiety of glycans. Results: The extended oligosaccharide moiety of 1– 6-branched GlcNAc acceptors binds to a different region on the enzyme. We present the crystal structure studies on the binding of oligosaccharide acceptors with 1– 6-linked GlcNAc at the nonreducing end that is analogous to i/I antigens and an N-glycan structure to the human 4Gal-T1 molecule (see Fig. 1 and supplemental Fig. S-F2).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
