Sorbitol Production Using Recombinant Zymomonas Mobilis Strain

The primary goal of this research was to develop a competitive bioprocess technology enabling production of high value added intermediate building block chemicals such as sorbitol and gluconic acid from fructose and glucose. A recombinant Zymomonas mobilis ZM4 (pHW20a-gfo) strain harboring the conjugation plasmid pHW20a-gfo was constructed and glucose-fructose oxidoreductase (GFOR) was over-expressed. The productivity of GFOR enzyme in the new recombinant strain constructed is at least two folds greater than that in the wild strain at both flasks and 3-liter fermenters. The maximum GFOR yields achieved in terms of volumetric yield, cellular yield, and the yield based on glucose substrate under the experimental conditions used are 2.59 units per ml,702.7 units per gram of cells, and 24.4 units per gram of glucose, respectively, in the batch cultures. A significant improvement of the bioconversion process for production of sorbitol and gluconic acid as target products from glucose and fructose was made by using divalent metal ions which reduced the ethanol yield drastically and significantly increased the yields of target products. Among several metal ions evaluated Zn2+and Ca2+are found to be the most effective ions in the yield enhancement of target products and the reduction of ethanol yield. The Zn2+ion inhibits the Entner-Doudoroff pathway enzymes and it reduces the ethanol yield from 16.7 to 1.78 gram per liter and at the same time it increased the sorbitol yield to almost 100% from 89%. The Ca2+ ion shows dual effects. It not only enhanced the sorbitol yield to almost 100% but also formed calcium gluconate salt in the reaction system resulting in an increased rate of gluconate production and easy separation of gluconate from the mixture of sorbitol and gluconate. Besides, the separation method of sorbitol from the reaction mixture was tested.