Sialyltransferases with Oligosaccharide Substrates

In agreement with the occurrence of Neu5Gc in colostrum and milk oligosaccharides from bovine and porcine sources, transfer from CMP-Neu5Gc to lactose could be demonstrated (Bartholomew et al. 1973, Carlson et al. 1973 a). 

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Sialyltransferases with N-Glycosidically Linked Oligosaccharide Substrates... 

Substrate-specificity studies on microsomal, frog-liver sialyltrans-ferase revealed the presence of (2—>3) and (2—>6) activities.277 This enzyme system readily sialylates oligosaccharides, but is almost inactive with asialofetuin, which is in contrast to the sialylation of oligosaccharides, as well as asialofetuin, by rat-liver sialyltransferase.278 The conclusion from this observation is that acceptor specificity of sialyl-transferases isolated from liver of evolutionary distant animals is similar for substrates of low molecular weight, but differs for compounds of high molecular weight.279... 

The synthetic utilities of sialyltransferases as alternative methods to sialylate oligosaccharide and glycoconjugates have been further explored by Halcomb who described the synthesis of a novel anomeric sulfur analogue of CMP-sialic acid (103). The key step in the synthesis of this novel CMP-sialic acid was a tetrazole-promoted coupling of a cytidine-5 -phosphoramidite with a glycosyl thiol of a protected sialic acid. The rate of solvolysis of (103) in aqueous buffer was reported to be 50-fold slower than that of CMP-sialic acid. Thioester (103) was a substrate for the transferase with a three-fold higher than the of the... 

The enzymes catalyzing reactions (1) and (2) both add sialic acid to j8-linked galactosyl residues however, they are distinct and separate enzymes. Again, as with the glycoprotein and mucin sialytransferases, the enzyme discerns the fine structure of the acceptor molecule. Further, these sialyltransferases will not transfer sialic acid to )8-linked galactosyl residues of glycoproteins, mucins, or oligosaccharides. The third enzyme catalyzes the addition of sialic acid to another sialic acid residue. Little is known about this enzyme since its activity is very low. It is not clear at this time whether this enzyme catalyzes the addition as shown in reaction (3) or utilizes the product of reaction (1) as its substrate. 

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