Nucleotide sugar donors

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). 

The commercially available enzymes have been employed in the synthesis of a large variety of natural and imnatural oligosaccharides [63-65]. The cost of nucleotide-sugar donors is also very high. UDP-Gal, used in the synthesis of the well-known a-gal (Galal-3Gal-R) epitopes is listed at 4000/g, making its stoichiometric use prohibitively expensive. 

The formation of pectin and its assembly for final export across the plasmamembrane will be considered here as a production line to indicate the various limiting steps which are controlled to monitor its production. The various channels for the movement and synthesis will be described separately so that the control points can be assessed. The channels which operate for production and movement of the polymers within the cytoplasm and endomembrane system of the cells are - Channel 1. Production and movement of nucleotide sugar donors. Channel 2. Synthesis and compartmentalization of the pectin polymers within the endomembrane system. Channel 3. Movement of vesicles and fusion with the plasmamembrane for assembly and deposition within the wall. 

Channel 1. Production and Movement of Nucleotide Sugar Donors... 

Figure 1. Epimerase reactions for the formation of nucleotide sugar donors.

Figure 17 Some examples of alternative nucleotide sugar donors that have been produced chemoenzymatically. Differences from a natural nucleotide sugar donor structure, UDP-glucose, are highlighted in red.

Figure 18 An aglycon exchange reaction can be used to produce unnatural nucleotide sugar donors. Shown here is an example using vancomycin.

T0 biosynthetically or chemoenzymatically prepare unnatural sialosides, the unnatural Sias must first be converted to nucleotide sugar donors. CMP-Sia synthetases catalyze the reaction of Sia with CTP to yield CMP-Sia and pyrophosphate (Figure 22). The E. coli synthetase is commonly used in chemoenzymatic... 

Sialyltransferases. In eukaryotic cells, addition of terminal sialic acid to glycoconjugates is carried out by specific enzymes called sialyltransferases (STs). STs catalyze the transfer of a sialic acid from the activated nucleotide sugar donor CMP-N-acelylneuraminic acid (CMP-NeuSAc) to the terminal nonreducing position of oligosaccharide chains either of glycoproteins or glycolipids. 

Before the days of molecular cloning of glycosyltransferases the classification of these enzymes was based on common nucleotide-sugar donor substrate usage with sub-classification based on the linkage formed and the acceptor sugar used. 

Blood groups A and B are synthesized by blood group A-dependent a3-GalNAc and blood group B-dependent a3-Gal-transferases, respectively. These two enzymes have similar acceptor substrate specificities and require the FI determinant as a substrate, while another a3-Gal-transferase, that is absent from humans and old world monkeys, acts on non-fucosylated terminal P-Gal residues and makes the linear B determinant, Galal-3Gaip- [89], The A and B transferases differ only by a few amino acids, which appear to determine the nucleotide-sugar donor specificities [90,91]. 

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