Utilization of xylitol dehydrogenase in a combined microbial/enzymatic process for production of xylitol from d-glucose

Abstract

The production of xylitol from d-glucose occurs through a three-step process in which d-arabitol and d-xylulose are formed as the first and second intermediate product, respectively, and both are obtained via microbial bioconversion reactions. Catalytic hydrogenation of d-xylulose yields xylitol; however, it is contaminated 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 d-xylulose in an NADH-dependent reaction, and coenzyme regeneration was achieved by means 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 (45 g/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 600 could be increased significantly.