Sialyl Transfer

It has so far only been possible to solve the complex problem associated with chemical sialylation in a few examples in a convincing way [44]. Due to the lack of a neighboring group at C-3 there is little possibility to influence the stereochemistry at the anomeric center except by solvents. Further, the marked reverse anomeric effect of the carboxyl group leads to the electronically preferred -configuration. Thus, this area would be ideally suited for the apphcation of stereo- and regioselective biocatalysts. 

In contrast to the diphosphosugars mentioned, sialic acids are found to be anomerically linked via a single phosphate unit to the nucleoside cytidine. The 

A purely chemical synthesis of CMP-Neu5Ac has also been reported by Schmidt et al. [49]. 

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The inhibitory influence of some glycosyl esters of nucleotides on sialyl transfer will be described later. 

Schmidt and co-workers have been investigating a series of potent inhibitors of a(2,6)-sialyl-transferase (O Fig. 9). They developed the transition analog 42 [153], based on the proposed mechanism of the sialyl transfer, that involves the partial dissociation of the CMP and the formation of the planer oxocarbenium ion structure in the transition state. They also found that the planer neuraminyl moiety in 42 can be replaced by the aromatic groups, leading to the readily accessible aromatic inhibitors of a(2,6)-sialyltransferase from rat liver. Further library... 

In Tn-syndrome the sialyl transfer must also be defective, but Dahr etaL could not measure it and have suggested that the enzyme in erythrocyte and leucocyte membranes differs from that in submaxillary salivary gland in requiring Gaipi,3GalNAxal-Ser as its substrate. Hence the structure produced by the sialyl transfer would be a trisaccharide, not a disaccharide, as in salivary mucins. In that case Tn-syndrome would require only one, and not two, enzyme defects. 

Scheme 17.19 Overcoming yield limitations from a less favorable aldol equilibrium by coupling the latter to a thermodynamically favorable in situ activation/sialyl transfer cascade without isolation of intermediates.

S. S. Basu, Ph.D. Thesis, University of Notre Dame, 1996, Cloning and expression of two glycolipid glycosyltransferases, pl,4-galactosyltransferase (GalT-4) and an alpha 2,3 sialyl-transferance (SAT-3), involved in the biosynthesis of sialyTLe epitope in embryonic chicken brain. 

Washiya, K., Furuike, T., Nakajima, F., Lee, Y. C. and Nishimura, S. I. (2000). Design of fluorogenic substrates for continuous assay of sialyl-transferase by resonance energy transfer. Anal. Biochem. 283, 39-48. 

This set of enzymes consists of those proteins that catalyze the transfer of sialyl moieties. (1) CMP-A-acetyl-neuraminate-j8-galactoside o -2,6-sialyltransferase [EC... 

A unique, membrane-associated, sialyltransferase system in E. coli has been described that catalyzes the synthesis of sialic acid polymers with the aid of sialyl-monophosphonoundecaprenol.293 In mammalian systems, no evidence was obtained lor the involvement of lipid intermediates in sialic acid transfer, or in the fonnation of sialyl-(2— -8) link-... 

Several fucosyltransferases have been isolated and used for in vitro synthesis. The Lewis A al,4-fucosyltransferase transfers unnatural fucose derivatives in from their GDP esters [23]. The enzyme al,3-fiicosyltransferase has been used to L-fucosylate the 3-position of the GlcNAc of A-acetyllactosamine and of sialyl a2,3-N-acetyllactosamine [24]. Several acceptor substrates with modifications in the GlcNAc residue could also be... 

Sialyl residues in oligosaccharides are introduced by the reaction of cyti-dine monophosphate-V-acetylneuraminic acid (49) as the sugar donor with the appropriate substrate, in the presence of specific transferases. Three of these have been utilized in syntheses which may be considered to be preparative. None are readily available. The most common, which we have called STA (see Table I), catalyzes the transfer of a 5-acetamido-3,5-di-deoxy-D-g/ycm>-a -D-ga/arfo-2-hexulopyranosonic acid unit (the a-D-pyra-nose form of JV-acetylneuraminic acid) to the primary position of D-galactose in a JV-acetyllactosamine residue.86 This enzyme also transfers vV-acetyl-9-O-acetylneuraminic acid (20) and V-glycolylneuraminic acid (12) from the corresponding cytidine monophosphate derivatives.16 The commercial enzyme is rather expensive, but pork liver from a butcher is a... 

So far, we have never observed, by n.m.r. spectroscopy, oligosaccharides or glycopeptides bearing Fuc and NeuAc both linked to the same N-acetyllactosamine branch this is in agreement with the biosynthetic principle of mutual exclusion existing in transferring (1— 3)-1 inked a-fucosyl and (2— 6)-linked a-sialyl groups to the same N-acetyl lactosamine branch, as formulated by Hill and coworkers.88... 

A number of functional sialyl Lewis mimetics have been synthesized. Their activities in vitro are equal or even better than those of the tetrasac-charide itself. To overcome synthetic problems, efficient stereoselective glycosylations as well as new chemoenzymatic methods for C-C bond formations had to be developed. The substitution of neuraminic acid by (5)-phenyl- and (5)-cyclohexyl lactic acid, as less flexible glycol acid residues, turned out to be very successful [10]. Also, a phosphate and a sulfate group, respectively, mimic neuraminic acid without loss of activity [11]. (5)-Cyclohexyl lactic acid-mimetic 2 shows a more than ten-fold efficacy compared with sialyl Lewis, whereas the corresponding (/ )-isomer 3 is almost inactive [10]. The deviating orientation of the carboxylic acid functionality compared to the bioactive sialyl Lewis conformation leads to the examined loss of activity. It was shown by transfer-NOE measurements of the corresponding E-selectin complexes that the coordinates of the bioactive conformation of sialyl Lewis and of compound 2 are similar. Con.se-quently structure 2 should bind to E-selectin in the same manner as that of sialyl Lewis [ 10a, b]. 

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