A 23Sialyltransferaseand 26Sialyltransferase

a2,3-SialT (a2,3-SialT, EC 2.4.99.6, ST3Gal III) from rat liver transfers a sialic acid unit from CMP-sialic acid to 3-OH of the terminal Gal residue in Gal01 ^ 4GlcNAc or Gal01 ^ 3GlcNAc sequences (Fig. 13) while a2,6-SialT (a2,6-SialT, EC 2.4.99.1) transfers the sialic acid to 6-OH of the terminal Gal residue in Gal01 ^ 4GlcNAc (Fig. 14) [115,116].

The finding that CMP-sialic acid synthetase can accept many sialic acid analogs has facilitated the production of donor analogs for sialyltransferase specificity studies [119-121]. Both a2,3-SialT and a2,6-SialT tolerate substitutions at C9 of CMP-sialic acid (Figs. 15 and 16) [120,122,123]. The 9-OH group can be replaced with fluoro, azido, amino, acetamido, hexanoylamido, benzamido, and fluorescent labels. 9-O-Acetylsialic acid is also transferred. Donors with the 5-NHAc of sialic acid replaced with OH, NHC(O)CH2OH, or NHCbz (Cbz = benzyloxycarbonyl) are also utilized by a2,3-SialT [124]. CMP-4-deoxysialic acid was found to be a donor substrate for a2,6-SialT [125].

Figure 12 Unnatural donors and acceptors for FucT VI.

Figure 12 Unnatural donors and acceptors for FucT VI.

CMP-Sialic acid (CMP-Neu5Ac)
Figure 13 Reactions catalyzed by a2,3-SialT.

Chemical mapping studies of acceptor binding have indicated that 6-OH of Gal and 2-NHAc are required for recognition by rat liver a2,6-SialT, while rat liver a2,3-SialT requires an intact 3,4,6-triol system on the Gal residue [126]. Analogs of Gal^1 ^ 4GlcNAc/81 ^ 2Mana-OOct, where the 3-OH or 4-OH group of the Gal residue is deoxygenated or substituted with a fluoro atom, were found to be active as substrates for a2,6-SialT (Fig. 16) [127]. The 4"-O-methyl derivative of this trisaccharide is an acceptor for a2,3-SialT [127]. A variety of substitutions on the N-acetyl group of either type I or type II acceptors are tolerated by a2,3-SialT [128,129]. a2,3-SialT also transfers sialic acid to lactal, lactose, and 2-O-pivaloyl lactose acceptors (Fig. 15) [44]. a2,3-SialTs from N. meningitidis [17,130] and myxoma virus [18] and an

Figure 14 Reaction catalyzed by a2,6-SialT.
Figure 15 Unnatural donors and acceptors for a2,3-SialT.

a2,6-SialT from Photobacterium damsela [16] have acceptor specificities somewhat different from those of their mammalian counterparts, allowing for broader synthetic applications. Large-scale production of CMP-NeuAc and sialylated oligosaccharides has been achieved by bacterial coupling [131].

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