Utilization of Xylitol Dehydrogenase in a Combined Microbial/Enzymatic Process for Production of Xylitol from D-Glucose

摘要

The production of xylitol from D0glucose occurs through a three-step process in which D-arabitol and D-xylulose are fromed as the first and second intermediate product respectively, and both are obtined via microbial bioconversion reactions. Catalytic hydrogenation of D-xylulose yields xylitol; however, it is containated with D-arabitol. The aim of this study was to increase the stereoselectivity of the D-xylulose reduction step by using enzymatic catalysis. Recombinant xylitol dehydrogenase from the yeast Galactocandida mastotermitis was employed to catalyze xylitol formation from D0xylulose in an NADH-dependent reaction, and coenzyme regeneration was achieved by means of formate of formate dehydrogenase-catalyzed oxidation of formate into carbon dioxide. The xylitol yield from D-xylulose was close to 100%. Optimal productivity was found for initial coenzyme concentrations of between 0.5 and 0.75 mM. In the presence of 0.30 M (45g/L) D-xylulose and 2000 U/L of both dehydrogenases, exhaustive substrate turnover was achieved typically in a 4-h reaction time. The enzymes were recovered after the reaction in yields of approx 90% by means of ultrafiltration and could be reused for up to six cycles of D-xylulose reduction. The advantages of incorporating the enzyme-catalyzed step in a process for producing xylitol from D-glucose are discussed, and strategies for downstream processing are proposed by which the observed coenzyme turnover number of approx 60 could be increased significantly.