A heterologous reductase affects the redox balance of recombinant Saccharomyces cerevisiae

Recombinant Saccharomyces cerevisiae harbouring the xylose reductase (XR) gene XYL1 from Pichia stipitis was grown in anoxic chemostat culture at two different dilution rates. At each dilution rate a transient experiment, encompassing a shift in the sugar content of the medium from glucose to glucose plus xylose was performed. The steady states at the beginning and the end of the transients were compared in terms of specific product fluxes from glucose metabolism. At both dilution rates, the specific glycerol flux decreased and the specific acetate and CO2 fluxes increased. The specific ethanol flux was not affected. At the lower dilution rate, the production of biomass decreased during the transient, but at the higher dilution rate it increased. The changes in product pattern can be explained as being due to the redox perturbation caused by the consumption of reduced cofactors in the XR-catalysed reaction. Regeneration of NAD partly through xylose reduction instead of glycerol production decreased the formation of glycerol. Additionally, xylose reduction activated those pathways which produce reduced cofactors, such as acetate formation and the pentose phosphate pathway, indicated by increased acetate and CO, production. The dual cofactor specificity of XR, with a preference for NADPH over NADH, was evident from the effects of xylose reduction on product fluxes. Comparison of the xylose reduction rates at low and high glucose flux indicated that the supply of reduced cofactors partly controlled the reaction rate. At the higher dilution rate, control by some other factor such as xylose transport or XR activity increased. Calculation of carbon balances at the steady states showed that all substrate carbon was recovered in biomass or products. Based on the specific product fluxes, calculations of quantitative cofactor balances at the steady states was attempted. However, sensitivity calculations showed that analysis errors in the range of 5% caused substantial errors in the cofactor balance, without affecting the carbon balance.