UDP-Glc, the central sugar nucleotide in cells, can be prepared from UTP and Glc-1-P in the presence of glucose-1-phosphate uridylyltransferase (GalU, EC 188.8.131.52) (66,67) or from sucrose and UDP with sucrose synthase (SusA, EC 184.108.40.206) (Scheme 3) (59,68-70). UDP-Gal can be prepared from UDP-Glc by C4 epimeriza-tion using GalE (71), from galactose-1-phosphate (Gal-1-P) and UTP, or from UDP-Glc and Gal-1-P using GalPUT (54,72,73). Besides the pioneering work of Whitesides, Wong, and their collaborators, Wang et al. have also applied five recombinant enzymes to regenerate UDP-Gal (Scheme 4) (74,75). Finally, UDP-GlcA can be readily prepared by the nicotinamide adenine dinucleotide-dependent oxidation of the C6 hydroxyl group of UDP-Glc catalyzed by UDP-glucose 6-dehydrogenase (UGD, EC 220.127.116.11) (76) (Scheme 5).
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