Enhancing the flux of D-glucose to the pentose phosphate pathway in Saccharomyces cerevisiae for the production of D-ribose and ribitol
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Phosphoglucose isomerase-deficient (pgi1) strains of Saccharomyces cerevisiae were studied for the production of D-ribose and ribitol from D-glucose via the intermediates of the pentose phosphate pathway. Overexpression of the genes coding for NAD+-specific glutamate dehydrogenase (GDH2) of S. cerevisiae or NADPH-utilising glyceraldehyde-3-phosphate dehydrogenase (gapB) of Bacillus subtilis enabled growth of the pgi1 mutant strains on D-glucose. Overexpression of the gene encoding sugar phosphate phosphatase (DOG1) of S. cerevisiae was needed for the production of D-ribose and ribitol; however, it reduced the growth of the pgi1 strains expressing GDH2 or gapB in the presence of higher D-glucose concentrations. The CEN.PK2-1D laboratory strain expressing both gapB and DOG1 produced approximately 0.4 g l−1 of D-ribose and ribitol when grown on 20 g l−1 (w/v) D-fructose with 4 g l−1 (w/v) D-glucose. Nuclear magnetic resonance measurements of the cells grown with 13C-labelled D-glucose showed that about 60% of the D-ribose produced was derived from D-glucose. Strains deficient in both phosphoglucose isomerase and transketolase activities, and expressing DOG1 and GDH2 tolerated only low D-glucose concentrations (≤2 g l−1 (w/v)), but produced 1 g l−1 (w/v) D-ribose and ribitol when grown on 20 g l−1 (w/v) D-fructose with 2 g l−1 (w/v) D-glucose.