Bioconversion of glucose and fructose to sorbitol and gluconic acid by untreated cells of Zymomonas mobilis

Abstract

The bioconversion of glucose and fructose to gluconic acid and sorbitol, respectively, by the enzymes glucose–fructose oxidoreductase (GFOR) and glucono-δ-lactonase (GL), contained in untreated cells of Zymomonas mobilis ATCC 29191, was investigated in batch runs with glucose plus fructose concentrations ( S 0) varying from 100 to 750 g l −1 in equimolar ratio. When S 0 was increased to 650 g l −1, the yields were improved, reaching a maximum of 91% for both products, with productivities of 1.6 and 1.5 g g −1 cell h −1 for gluconic acid and sorbitol, respectively. Above this level ( S 0=750 g l −1), no further improvement in yields was observed and productivities decreased due to the longer process time. The high yields of bioconversion runs with S 0≥650 g l −1 are a consequence of the sequential inhibition of the normal metabolism of Z. mobilis by substrates and products, resulting in preferential utilization of substrates via the GFOR/GL system.