Sialyltransferases acceptor specificity

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... 

Photobacterium damsela (x2,6-siaiyltransferase (Pd2,6ST) was the first bacterial sialyltransferase which has been cloned and purified by the Yamamoto group (33, 34). This enzyme has a relaxed acceptor specificity (35, 36). For example, it has been applied for the enzymatic sialylation of Tn glycopeptides with GalNAc a-linked to either serine or threonine residue) (37). It was also shown to be able to transfer sialic acid to both N- and 0-linkM glycoproteins (38). 

In contrast to the acceptor specificity, the donor specificity of sialyltransferases is less pronounced, although differences exist. They can transfer not only Neu5Ac but... 

So far, the acceptor specificity of nearly 20 eu- and prokaryotic sialyltransferases has been elucidated and 15 different cDNA clones of these enzymes have been obtained (Table 14 and references therein). This list demonstrates the rapid progress in this field and also includes the first cloning of mammalian polysialyltransferase (polysialyltransferase-1 from hamster ovary cells) [620]. The same group has recently published the molecular analysis of the biosynthetic pathway of the a-2,8-polysialic acid capsule by Neisseria meningitidis serogroup B [658]. 

Notes and discussion. A major drawback of enzymatic sialylation with sialyl-transferases is the strict acceptor specificity of these enzymes. This synthesis addresses this limitation by showing a widely applicable transfer of sialic acid (NeuAc) from a donor substrate of the sequence NeuAca-(2 —3)Gal-ORi to virtually any galactose acceptor (Gal-OR2). This method uses the less substrate specific glycosidase from Trypanosoma cruzi. The problem with this method is that the product is made at the expense of another sialoside, used as the donor substrate, and as NeuAc transfer is a reversible process, it is difficult to drive the equilibrium in favour of the desired sialoside. For this reason the sialoside donor substrate is regenerated in situ by an a-(2 3)-sialytransferase enzyme, thus enhancing the production of the desired product. The specificity of the sialyltransferase ensures that only the galactose byproduct, formed from the sialyl donor, is re-sialyated as the Gal-OR2 acceptor substrate is a poor substrate. Due to the broad specificity of the trans-sialidase, many a-NeuAc-(2 3)-Gal-OR sequences can be synthesised by... 

Jourdian et al. (1%3) and Carlson et al. (1973a,h) reported the properties of a particulate sialyltransferase prepared from rat mammary gland. The acceptor specificity is shown in Table I. 

TABLE II. Acceptor Specificity of Goat Colostrum Sialyltransferase... 

It is interesting that all four reactions studied gave the same apparent Km values for CMP-NeuNAc. This observation suggests that the CMP-NeuNAc binding sites of the different species of sialyltransferases are similar in structure and affinity properties, although the binding sites for the acceptors and the catalytic sites may be very different, providing a basis for substrate specificities. 

Another multifunctional bacterial enzyme that has been reported recently is Pasteurella multocida sialyltransferase (PmSTl or tPm0188Ph) (67, 68). It has four different activities, including an a2,3SiaT, a2,6SiaT, a2,3-sialidase, and trans-sialidase activities. PmSTl is a highly active sialyltransferase with broad donor and acceptor substrate specificity therefore it is a powerful tool... 

Wlasichuk KB, Kashem MA, Nikrad PV, Bird P, Jiang C, Venot AP. Determination of the specificities of rat liver Gal(beta 1—>4)GlcNAc alpha 2,6-sialyltransferase and Gal(beta 1—>3/4) GlcNAc alpha 2,3-sialyltransferase using synthetic modified acceptors. J. Biol. Chem. 1993 268 13971-13977. 

---