title: The structure of human α-2,6-sialyltransferase reveals the binding mode of complex glycans.
abstract: Human β-galactoside α-2,6-sialyltransferase I (ST6Gal-I) establishes the final glycosylation pattern of many glycoproteins by transferring a sialyl moiety to a terminal galactose. Complete sialylation of therapeutic immunoglobulins is essential for their anti-inflammatory activity and protein stability, but is difficult to achieve in vitro owing to the limited activity of ST6Gal-I towards some galactose acceptors. No structural information on ST6Gal-I that could help to improve the enzymatic properties of ST6Gal-I for biotechnological purposes is currently available. Here, the crystal structures of human ST6Gal-I in complex with the product cytidine 5'-monophosphate and in complex with cytidine and phosphate are described. These complexes allow the rationalization of the inhibitory activity of cytosine-based nucleotides. ST6Gal-I adopts a variant of the canonical glycosyltransferase A fold and differs from related sialyltransferases by several large insertions and deletions that determine its regiospecificity and substrate specificity. A large glycan from a symmetry mate localizes to the active site of ST6Gal-I in an orientation compatible with catalysis. The glycan binding mode can be generalized to any glycoprotein that is a substrate of ST6Gal-I. Comparison with a bacterial sialyltransferase in complex with a modified sialyl donor lends insight into the Michaelis complex. The results support an SN2 mechanism with inversion of configuration at the sialyl residue and suggest substrate-assisted catalysis with a charge-relay mechanism that bears a conceptual similarity to serine proteases.
authors.affiliation: pRED Pharma Research and Early Development, Discovery Technologies, F. Hoffmann-La Roche AG, Grenzacher Strasse 124, 4070 Basel, Switzerland.
authors.forename: Bernd
authors.initials: B
authors.lastname: Kuhn
authors.forename: Jörg
authors.initials: J
authors.lastname: Benz
authors.forename: Michael
authors.initials: M
authors.lastname: Greif
authors.forename: Alfred M
authors.initials: AM
authors.lastname: Engel
authors.forename: Harald
authors.initials: H
authors.lastname: Sobek
authors.forename: Markus G
authors.initials: MG
authors.lastname: Rudolph
mesh_terms: D015703:Antigens, CD / Q000737:chemistry* / Q000378:metabolism; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D006801:Humans; D011134:Polysaccharides / Q000737:chemistry* / Q000378:metabolism*; D011485:Protein Binding; D012799:Sialyltransferases / Q000737:chemistry* / Q000378:metabolism; D000094802:beta-D-Galactoside alpha 2-6-Sialyltransferase
chemical_list: D015703:Antigens, CD; D011134:Polysaccharides; D012799:Sialyltransferases; C093493:ST6GAL1 protein, human; D000094802:beta-D-Galactoside alpha 2-6-Sialyltransferase
keywords: glycosylation; sialyltransferases; β-galactoside α-2,6-sialyltransferase I