A-sialylation

In this technique, the enzyme solution is put inside a dialysis bag which is then immersed in a solution of substrate, or cofactors. Small molecules can diffuse through the wall of the bag and react in the presence of the enzyme, while products, if also small molecules, diffuse into the outside solution, where they may be recovered. This technique has been used in syntheses with sialyl aldolase, Kdo-synthetase, the common aldolase, a mixture of hexokinase and pyruvate kinase, a-(2— 6) sialyl transferase,26 a mixture of pyruvate kinase and adenylate kinase,27 and CMP-Neu5Ac synthetase.28... 

The spectral features of the a-(2— 6)-sialylated N-acetyllactosamine branches are identical to those of the corresponding parts of 29 and 30, whereas the spectral characteristics of the third branch, bearing a NeuAc in a-(2- 3) linkage to Gal-8, are identical to those of the same branch in 40 (see Table XII). The H-3a signals, and also the H-3e signals, of the a-(2-+6)-linked NeuAc groups are well separated furthermore, they are clearly distinguishable from those of NeuAc a-(2- 3)-linked to Gal-8. 

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... 

Introduction of a fluoride at the 3-position of neuraminic acid yielded a compound (57) that was a competitive inhibitor of the a-(2,6)-sialyl-trans-ferase. Base and sugar-modified analogues of CMP-Neu5Ac have also been prepared to investigate the tolerance of oc-(2,6)-sialyl-transferase to base exchange (58) and modification of the 5-, 8- or 9-position of neuraminic acid (59), (60). While base-exchange was not tolerated, modifications of the acid moiety yielded compounds that were substrates for the enzyme. 

Table 14. a(2-6)Sialylation of non-natural type I disaccharides compare Fig. 17... 

Scheme 10.9 Optimization of a-(2-6)-sialylation between donor and acceptor using microreactor [28]. Source Copyright 2007 Taylor and Francis.

Paulson, J. C., Weinstein, J., and de Souza-E-Silva, U., 1984, Biosynthesis of a disialated sequence in N-linked oligosaccharides Identification of an N-acetylglucosaminide a(2-6) sialyl transferase in Golgi apparatus from rat liver, Eur. J. Biochem. 140 523-530. 

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