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Background  Xylitol is a five-carbon sugar alcohol that has numerous beneficial health properties. It has almost the same sweetness as sucrose but has lower energy value compared to the sucrose. Metabolism of xylitol is insulin independent and thus it is an ideal sweetener for diabetics. It is widely used in food products, oral and personal care, and animal nutrition as well. Here we present a two-stage strategy to produce bio-xylitol from  d -xylose using a recombinant  Pichia pastoris  expressing a heterologous xylose reductase gene. The recombinant  P. pastoris  cells were first generated by a low-cost, standard procedure. The cells were then used as a catalyst to make the bio-xylitol from  d -xylose.    Results   Pichia pastoris  expressing  XYL1  from  P. stipitis  and  gdh  from  B. subtilis  demonstrated that the biotransformation was very efficient with as high as 80% (w/w) conversion within two hours. The whole cells could be re-used for multiple rounds of catalysis without loss of activity. Also, the cells could directly transform  d -xylose in a non-detoxified hemicelluloses hydrolysate to xylitol at 70% (w/w) yield.    Conclusions  We demonstrated here that the recombinant  P. pastoris  expressing xylose reductase could transform  d -xylose, either in pure form or in crude hemicelluloses hydrolysate, to bio-xylitol very efficiently. This biocatalytic reaction happened without the external addition of any NAD(P)H, NAD(P) + , and auxiliary substrate as an electron donor. Our experimental design & findings reported here are not limited to the conversion of  d -xylose to xylitol only but can be used with other many oxidoreductase reactions also, such as ketone reductases/alcohol dehydrogenases and amino acid dehydrogenases, which are widely used for the synthesis of high-value chemicals and pharmaceutical intermediates.

microbial cell factories

Production of bio-xylitol from d-xylose by an engineered Pichia pastoris expressing a recombinant xylose reductase did not require any auxiliary substrate as electron donor