The structure of human α-2,6-sialyltransferase reveals the binding mode of complex glycans

Acta Crystallogr D Biol Crystallogr. 2013 Sep;69(Pt 9):1826-38. doi: 10.1107/S0907444913015412. Epub 2013 Aug 17.

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.

Keywords: glycosylation; sialyltransferases; β-galactoside α-2,6-sialyltransferase I.

MeSH terms

  • Antigens, CD / chemistry*
  • Antigens, CD / metabolism
  • Catalytic Domain
  • Crystallography, X-Ray
  • Humans
  • Polysaccharides / chemistry*
  • Polysaccharides / metabolism*
  • Protein Binding
  • Sialyltransferases / chemistry*
  • Sialyltransferases / metabolism
  • beta-D-Galactoside alpha 2-6-Sialyltransferase

Substances

  • Antigens, CD
  • Polysaccharides
  • Sialyltransferases
  • ST6GAL1 protein, human
  • beta-D-Galactoside alpha 2-6-Sialyltransferase

Associated data

  • PDB/4JS1