Abstract
Synthetic ways towards uridine 5'-diphosphate (UDP)-xylose are scarce and not well established, although this compound plays an important role in the glycobiology of various organisms and cell types. We show here how UDP-glucose 6-dehydrogenase (hUGDH) and UDP-xylose synthase 1 (hUXS) from Homo sapiens can be used for the efficient production of pure UDP-α-xylose from UDP-glucose. In a mimic of the natural biosynthetic route, UDP-glucose is converted to UDP-glucuronic acid by hUGDH, followed by subsequent formation of UDP-xylose by hUXS. The nicotinamide adenine dinucleotide (NAD(+)) required in the hUGDH reaction is continuously regenerated in a three-step chemo-enzymatic cascade. In the first step, reduced NAD(+) (NADH) is recycled by xylose reductase from Candida tenuis via reduction of 9,10-phenanthrenequinone (PQ). Radical chemical re-oxidation of this mediator in the second step reduces molecular oxygen to hydrogen peroxide (H(2)O(2)) that is cleaved by bovine liver catalase in the last step. A comprehensive analysis of the coupled chemo-enzymatic reactions revealed pronounced inhibition of hUGDH by NADH and UDP-xylose as well as an adequate oxygen supply for PQ re-oxidation as major bottlenecks of effective performance of the overall multi-step reaction system. Net oxidation of UDP-glucose to UDP-xylose by hydrogen peroxide (H(2)O(2)) could thus be achieved when using an in situ oxygen supply through periodic external feed of H(2)O(2) during the reaction. Engineering of the interrelated reaction parameters finally enabled production of 19.5 mM (10.5 g l(-1)) UDP-α-xylose. After two-step chromatographic purification the compound was obtained in high purity (>98%) and good overall yield (46%). The results provide a strong case for application of multi-step redox cascades in the synthesis of nucleotide sugar products.