Glycoproteins sialic acid transfer

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). 

The synthesis of a CMP-NeuAc derivative that was bound to a solid support through the 9-position of the sialic acid has been reported by the Kajihara group [47]. This derivative is quite useful in that it can be utilized to immobilize glycoproteins onto a solid support by transferring the sialic acid to the terminus of the carbohydrate chain of the glycoprotein. 

The sialyltransferases investigated were found to accept many variations at C5 and C9 of the sialic acid moiety. Halcomb and Chappell confirmed this fact with studies of sialyltransferases from different sources [45]. The C9- and C5-modified substrates in Scheme 37 were good substrates. However no activity was detected when compounds 111 and 117 were assayed. In addition to these examples, other CMP-NeuAc analogs that have been prepared and successfully transferred include C9 fluorescent compounds, C9 thioacetyl, C5 thioacetyl, and C4 dexoy [37]. A particularly useful modification introduced by the Wong lab incorporates a mercury atom at C9 (Scheme 38) [48]. Transfer of this sialic acid to glycoproteins could gready aid in X-ray crystallographic analysis of these biomolecules. 

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