Sorbitol production using recombinant Zymomonas mobilis strain

A recombinant Zymomonas mobilis strain harboring the plasmid pHW20a-gfo for over-expression of glucose–fructose oxidoreductase (GFOR) was constructed. The specific activity of GFOR enzyme in the new recombinant strain was at least two folds greater than that in the wild strain. The maximum GFOR activity achieved in terms of the volumetric, and the cellular were 2.59 U ml−1, and 0.70 U mg−1, respectively, in the batch cultures. A significant improvement of the bioconversion process for the production of sorbitol and gluconic acid from glucose and fructose was made using divalent metal ions which drastically reduced the ethanol yield and significantly increased the yield of target product. Among several divalent metal ions evaluated, Zn2+ was found to be most effective by inhibiting the Entner–Doudoroff pathway enzymes. The yield of the byproduct ethanol was reduced from 16.7 to 1.8 g l−1 and the sorbitol yield was increased to almost 100% from 89%. The Ca2+ enhanced the sorbitol yield and the formation of calcium gluconate salt made the separation of gluconate from the reaction system easier.