Crystal Structure of Human α-2,6 Sialyltransferase

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 for protein stability. However, a complete glycan tree is difficult to achieve in vitro due to limited activity of ST6Gal-I for some galactose acceptors. No structural information on ST6Gal-I that could help to improve the enzymatic properties of ST6Gal-I for biotechnological purposes was previously available. We describe the crystal structure of human ST6Gal-I, which allows rationalizing the inhibitory activity of cytosine-based nucleotides. ST6Gal-I differs from related sialyltransferases by several large insertions and deletions that determine its regio- and substrate specificity. Excitingly, a large glycan binds to the active site in a catalytically competent orientation, representing the general binding mode of any substrate glycoprotein. This binding mode also rationalizes why some galactose acceptors are incompletely sialylated. Comparison with a bacterial sialyltransferase lends first 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 conceptual similarity to serine proteases.