Enzymatic glycosidation sialyltransferases

The total synthesis of sialosides by using the chemoenzymatic approach is as follows [74]. Sialic acid itself can be synthesized from ManNAc, mannose, or their derivatives by sialic acid aldolase enzyme through aldol condensation reaction. If ManNAc is chemically or enzymatically modified at C2, C4—C6 positions, sialic acid has structural modifications at C5, C7-C9 positions, respectively. The sialic acids are subsequently activated by a CMP-siahc acid synthetase to form a CMP-sialic acid, which is the donor used by sialyltransferases. Because CMP-sialic acid is tmstable, the CMP-Neu5Ac synthetase is valuable for the preparative enzymatic synthesis of sialosides. In the last steps, the CMP-sialic acid is transferred to galactose or GalNAc terminated glycosides by sialyltransferases to form structurally defined sialosides. Examples are that Chen and co-workers have recently developed a one-pot multienzyme system for the efficient synthesis of a-sialosides (Table 2) [12,76,79]. In this system, recombinant E. coli K-12 sialic acid aldolase catalyzed the synthesis of sialic acid precursors for... 

In the chemo-enzymatic synthesis of LLG-3 performed by Withers et al. [34], the glycoside of 8-0-Me-Neu5Ac with glycolic acid 59 was synthesized from NeuSAc derivative 58 by a chemical method because of the absence of a sialyltransferase capable of sialyltransfer to the hydroxyl group of the glycosyl amide in the penultimate sialic acid residue and an 8-O-methyltransferase for NeuAc (Scheme 11.10). 

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