α2,3-Sialylation of terminal GalNAcβ1-3Gal determinants by ST3Gal II reveals the multifunctionality of the enzyme -: The resulting Neu5Aece2-3GalNAc linkage is resistant to sialidases from Newcastle disease virus and Streptococcus pneumoniae

Enzymatic alpha2,3-sialylation of GalNAc has not been described previously, although some glycoconjugates containing alpha2,3-sialylated GalNAc residues have been reported. In the present experiments, recombinant soluble alpha2,3-sialyltransferase ST3Gal II efficiently sialylated the X-2 pentasaccharide GalNAc beta1-3Gal beta1-4GlcNAc beta1-3Gal beta1-4Glc, globo-N-tetraose GalNAc beta1-3Gal alpha1-4Gal beta1-4Glc, and the disaccharide GalNAc beta1-3Gal in vitro. The purified products were identified as Neu5Ac alpha2-3GalNAc beta1-3Gal beta1-4GlcNAc beta1-3Gal beta1-4Glc, Neu5Ac alpha2-3GalNAc beta1-3Gal alpha1-4Gal beta1-4Glc, and Neu5Ac alpha2-3GalNAc beta1-3Gal, respectively, by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, enzymatic degradations, and one- and two-dimensional NMR-spectroscopy. In particular, the presence of the Neu5Ac alpha2-3GalNAc linkage was firmly established in all three products by a long range correlation between Neu5Ac C2 and GalNAc H3 in heteronuclear multiple bond correlation spectra. Collectively, the data describe the first successful sialyltransfer reactions to the 3-position of GalNAc in any acceptor. Previously, ST3Gal II has been shown to transfer to the Gal beta1-3GalNAc determinant. Consequently, the present data show that the enzyme is multifunctional, and could be renamed ST3Gal(NAc) II. In contrast to ST3Gal II, ST3Gal III did not transfer to the X2 pentasaccharide. The Neu5Ac alpha2-3GalNAc linkage of sialyl X2 was cleaved by sialidases from Arthrobacter ureafaciens and Clostridium perfringens, but resisted the action of sialidases from Newcastle disease virus and Streptococcus pneumoniae. Therefore, the latter two enzymes cannot be used to differentiate between Neu5Ac alpha2-3GalNAc and Neu5Ac alpha2-6GalNAc linkages, as has been assumed previously.