Intracellular fluxes in a recombinant xylose-utilizing Saccharomyces cerevisiae cultivated anaerobically at different dilution rates and feed concentrations

A metabolic flux model was constructed for the yeast Saccharomyces cerevisi ae comprising the most important reactions during anaerobic metabolism of x ylose and glucose. The model was used to calculate the intracellular fluxes in a recombinant, xylose-utilizing strain of S. cerevisiae (TMB 3001) grow n anaerobically in a defined medium at dilution rates of 0.03, 0.06, and 0. 18 h(-1). The feed concentration was varied from 0 g/L xylose and 20 g/L gl ucose to a mixture of 15 g/L xylose and 5 g/L glucose, so that the total co ncentration of carbon source was kept at 20 g/L. The specific uptake of xyl ose increased with the xylose concentration in the feed and with increasing dilution rate. The excreted xylitol was less than half of the xylose consu med. With increasing xylose concentration in the feed, the fluxes in the pe ntose phosphate pathway increased, whereas the flux through glycolysis decr eased. Under all cultivation conditions, nicotinamide adenine dinucleotide (NADH) was the preferred cofactor for xylose reductase. The model showed th at the flux through the reaction from ribulose 5-phosphate to xylulose 5-ph osphate was very low under all cultivation conditions. (C) 2001 John Wiley & Sons, Inc.