Enzymatic sialylations

SCHEME 1 S.26 Enzymatic sialylations of the mucin-type Tn and T antigen glycopeptides 199 and 201 gave glycopeptides 200, 202 and 203 which contained the sialyl-Tn and the sialyl-T antigens. 

Using MUNeuSAc as donor, the enzymatic sialylation of N-linked oligosaccharides released from the glycoproteins and derivatized with 2-aminopyridine has been reported, but no details on yields were provided.167... 

N. Takahashi, K. B. Lee, H. Nakagawa, Y. Tsukamoto, Y. Kawamura, and Y. C. Lee, Enzymatic sialylation of N-linked oligosaccharides using an alpha-(2,3)-specific trans-sialidase from Trypanosoma cruzi. structural identification using a three-dimensional elution mapping technique, Anal. Biochem., 230 (1995) 333-342. 

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). 

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... 

Cell-surface sialic acid residues play important roles in diverse biological processes [86]. Sialic acids are usually found in terminal positions linked through an a-glyco-sidic linkage. The stereoselective synthesis of a-sialosides remains a challenge because the glycosides have the thermodynamically unfavorable equatorial orientation and the anomeric carbon is a very hindered quaternary center [4]. Enzymatic sialylation is, therefore, an attractive alternative means of preparation of a-sialosides in an efficient and stereocontrolled manner. 

Flgure10.23 Sialyl Lewis -related selectin inhibitorandfluorogenicscreening compound for transketolase prepared using enzymatic aldolization, and multienzymatic oxidation-aldolization strategy for the synthesis of bicyclic higher carbon sugars. 

T. Furiuke, R. Sadamoto, K. Niikura, K. Monde, N. Sakairi, and S.-I. Nishimura, Chemical and enzymatic synthesis of glycocluster having seven sialyl lewis X arrays using /i-cyclodextrin as a key scaffold material, Tetrahedron, 61 (2005) 1737-1742. 

Scheme 9.11 Enzymatic transsialylation of four MPEG-DOXyl glycosides with sialidases from Vibrio cholerae and Salmonella typhimurium. V. cholerae sialylated in position 6 of the terminal monosaccharide units with yields of 12-17%, whereas S. typhimurium sialylated position 3 of the same monosaccharide units (yields 14-24%).

Complex alkaloid glycosides bearing, e.g., lactosyl (Lac), lactosaminidyl (Lac-NAc), or sialyl (Neu5Ac) moieties were required for immunomodulation tests (see below). Because of the paucity of starting material (alkaloid monoglycosides) and because of the need of regioselective glycosylation, only enzymatic reactions were practicable. 

Zanini D, Roy R (1996) Chemo-enzymatic synthesis of multivalent N-acetyllactosamine compounds as dendritic sialyl LewisX precursors. Proceedings of the XVIlIth International Carbohydrate Symposium, Milan, Italy, Juli 21 -26, p 236 Pag6 D, Zanini, D, Roy R (1996) Bioorg Med Chem 4 1949 Roy R, Baek MG, Zanini D, (unpublished data)... 

A chemoenzymatic synthesis of di-, tetra-, and octavalent sialyl Lewis X ligands scaffolded on dendrimers has been reported (Scheme 67). 508 Hypervalent L-lysine cores with covalently attached 2-acetamido-2-deoxy-D-glucose residues were chemically prepared and enzymatically transformed into sialyl Lewis X-containing dendrimers using (3-(l— 4) ga-... 

In order to try to overcome some of the problems associated with chemical synthesis of oligosaccharides containing N-acetylneuraminic acid, Sabesan and Paulson [277] have used a combination of chemical and enzymatic methods using purified sialyl-transferases in the presence of CMP-iV-acetylneuraminic acid and synthetic acceptor molecules to give sialyl derivatives of oligosaccharides which were characterised by NMR. Thus, methyl P-D-galactopyranoside, methyl P-D-lactoside and iV-acetyl-... 

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