Construction of recombinant Escherichia coli expressing xylitol-4-dehydrogenase and optimization for enhanced L-xylulose biotransformation from xylitol.

L-Xylulose is a rare ketopentose which inhibits α-glucosidase and is an indicator of hepatitis or liver cirrhosis. This pentose is also a precursor of other rare sugars such as L-xylose, L-ribose or L-lyxose. Recombinant E. coli expressing xylitol-4-dehydrogenase gene of Pantoea ananatis was constructed. A cost-effective culture media were used for L-xylulose production using the recombinant E. coli strain constructed. Response surface methodology was used to optimize these media components for L-xylulose production. A high conversion rate of 96.5% was achieved under an optimized pH and temperature using 20 g/L xylitol, which is the highest among the reports. The recombinant E. coli cells expressing the xdh gene were immobilized in calcium alginate to improve recycling of cells. Effective immobilization was achieved with 2% (w/v) sodium alginate and 3% (w/v) calcium chloride. The immobilized E. coli cells retained good stability and enzyme activity for 9 batches with conversion between 53 and 92% which would be beneficial for economical production of L-xylulose.