Galactosylation in Polylactosamine Backbones

Distal a3-sialylation of branched polylactosamines was catalyzed successfully by the activity (ST3Gal IV ) present in human placental microsomes [40, 52] both 3-and 6-linkeci short polylactosamine branches reacted well. More reeently, large, tetravalent sialopolylactosamines have been synthesized in low micromolar amounts (L. Penttila et al., unpublished experiments). Even long polylactosamine chains of blood group i-type are effectively sialylated by placental microsomes [54]. 

Several a3-sialyltransferases have been cloned, and ST3Gal III is commercially available in recombinant form. A particularly interesting report of Gilbert et al. [55] describe a fusion protein consisting of CMP-Neu5Ac synthetase and o2,3-sialyl-transferase from Neisseria meningitidis. This polypeptide was able to catalyze the reactions shown in eqs (2) and (3). 

In small-scale reactions the fusion protein worked well with glycosides of Lac-NAc and lactose, as well as the bi-antennary N-linked glycan, and used as donors V-acetylneuraminic acid, A-propionylneuraminic acid, and iV-glycolylneuraminic acid. 

The two tethered enzymes were recovered by a simple procedure in functionally pure form, free from contaminating enzyme activity that can hydrolyze sugar nu- 

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