Glucose-fructose oxidoreductase, a new enzyme isolated from Zymomonas mobilis that is responsible for sorbitol production

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Abstract
The enzymes responsible for sorbitol formation in Zymomonas mobilis were investigated. A previously undescribed enzyme catalyzes the intermolecular oxidation-reduction of glucose and fructose to form gluconolactone and sorbitol. This enzyme has been purified; it had a subunit size of 40,000 daltons and is probably tetrameric at low pH. It contained tightly bound NADP as the hydrogen carrier and did not require any added cofactor for activity. In addition, a gluconolactonase has been isolated, although not completely purified. Together these two enzymes were capable of completely converting a 54% (wt/vol) equimolar mixture of glucose and fructose to sorbitol and sodium gluconate at the optimum pH of close to 6.2. The oxidoreductase had low affinities for its substrates, but natural environmental conditions would expose it to high concentrations of sugars. The amount of the enzyme in Z. mobilis cells was sufficient to account for the rate of sorbitol formation in vivo. However, the enzyme was present in the highest amounts when the cells were grown on glucose alone, and it was repressed by the presence of fructose; this was not the case with the gluconolactonase.