So far, twenty members of the mouse sialyltransferase family have been identified. Among them, the cDNA cloning of a second type of β-galactoside α2, 6-sialyltransferase (ST6Gal II) and a sixth type of α2, 8-sialyltransferase (ST8Sia VI) has been performed most recently. ST6Gal II is a counterpart of ST6Gal I, and the ST6Gal II gene has a similar genomic structure to the ST6Gal II gene. But unlike ST6Gal I, which exhibits broad substrate specificity toward oligosaccharides, glycoproteins, and glycolipids, ST6Gal II exhibited limited substrate specificity toward some oligosaccharides and glycoproteins, all of which have the Galβ1, 4GlcNAc sequence at the nonreducing end of their carbohydrate groups. The expression pattern of the ST6Gal II gene was also different from that of the ST6Gal I gene. Another enzyme, ST8Sia VI, exhibited broad substrate specificity toward glycoproteins, glycolipids, and sialyloligosaccharides, all of which have the NeuAcα2, 3(6)Gal sequence at the nonreducing end of their carbohydrate groups. For glycoproteins, ST8Sia VI prefered
O-glycans to
N-glycans as acceptor substrates. In addition, ST8Sia VI also exhibited higher activity toward
O-glycans than glycolipids. It has been shown that ST8Sia VI is the first mammalian α2, 8-sialyltransferase that sialylates
O-glycans preferentially.
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