Selective reduction of xylose to xylitol from a mixture of hemicellulosic sugars.

The biocatalytic reduction of D-xylose to xylitol requires separation of the substrate from L-arabinose, another major component of hemicellulosic hydrolysate. This step is necessitated by the innate promiscuity of xylose reductases, which can efficiently reduce L-arabinose to L-arabinitol, an unwanted byproduct. Unfortunately,due to the epimeric nature of D-xylose and L-arabinose, separation can be difficult, leading to high production costs. To overcome this issue, we engineered an E. coli strain to efficiently produce xylitol from D-xylose with minimal production of L-arabinitol byproduct. By combining this strain with a previously engineered xylose reductase mutant, we were able to eliminate L-arabinitol formation and produce xylitol to near 100% purity from an equiweight mixture of D-xylose, L-arabinose, and D-glucose.