Dual modulation of glucose 6-phosphate metabolism to increase NADPH-dependent xylitol production in recombinant Saccharomyces cerevisiae

To increase the metabolic flux toward the NADPH-generating pentose phosphate pathway in a recombinant Saccharomyces cerevisiae strain that harbors the xylose reductase gene from Pichia stipitis, expression of phosphoglucose isomerase (PGI) encoded by the PGI1 gene was modulated by a promoter replacement using the ADH1 promoter. Although the ADH1 promoter down-regulated PGI expression in glucose-limited condition, the decline of PGI activity did not exert a profound influence on xylitol production in a series of glucose-limited fed-batch cultivations. However, simultaneous enforcement of glucose 6-phosphate dehydrogenase (G6PDH) activity and attenuation of phosphoglucose isomerase activity worked in a cooperative manner to increase xylitol production and to reduce utilization of cosubstrate required for xylitol production in a glucose-limited fed-batch cultivation of the PGI mutant strain with an enhanced G6PDH activity. An 1.9-fold increase in specific xylitol productivity of 0.34 ± 0.03 g/g cells h was achieved compared with the control strain containing xylose reductase only.