Nucleotide sugar donors membrane

Nucleotide sugar donors, such as UDP-GalNAc, UDP-GlcNAc, GDP-fucose, and CMP-sialic acid, serve as substrates for the glycosyltransferases and are the source of the sugars that are added to substrate proteins (Table 1). Nucleotide sugar donors are synthesized in the cytoplasm and imported into the secretory pathway by membrane-resident transporters (1). 

In this context, the use of enzymes has emerged as a practical alternative to chemical synthesis [6-13]. Several examples have been reported that are based on the use of Leloir type-glycosyltransferases, which are mostly membrane-associated and act on nucleotide-activated sugars as donor substrates. However, the use of these enzymes in vitro still remains limited by the difficulty of enzyme purification and by their need for expensive sugar-nucleotides [6]. Re-engineering of microbial cells producing these proteins appears to be promising for the synthesis of specific carbohydrate stmctures [14]. 

In S, typhimurium this transfer is from UDPGal to form undecaprenyl galactosyl pyrophosphate. Each later transfer from a sugar nucleotide in this organism involves a different class of nucleotide donor. While this is not universal, it is quite common in other species. The galactose-1-phosphate transferase is membrane-bound, but can be partially solubilised with nonionic detergents. 

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