[Effects of nitrogen-limitation on xylose metabolism and key enzymes activity in Rhizopus oryzae].

We studied the effects of nitrogen concentration on xylose metabolism and organic acid production in Rhizopus oryzae. We studied the effect of different nitrogen concentration in media on biomass, organic acids accumulation, activity of key enzymes (xylose reductase, XR, glucose-6-phosphate dehydrogenase, G6PD) involved in the metabolism and intracellular redox ratios of R. oryzae. Both intracellular redox ratios and key enzymes activities stayed low, resulting in 4.02 g/L biomass, 6.55 g/L fumaric acid production and high concentration of residual xylose under nitrogen-limitation conditions (0.15 g/L urea). Intracellular redox ratios and activity of key enzymes increased under rich nitrogen culture (2.4 g/L urea), leading to high biomass 18.01 g/L, high xylose uptake speed 2.03 g/(L x h) but low fumaric acid production (0.27 g/L). When concentration of urea was 0.6 g/L, biomass 9.11 g/L and fumaric acid 12.28 g/L, NADPH/NADP(+), XR and G6PD activities were medium whereas NADH/NAD(+) reached the highest compared to those in rich nitrogen or nitrogen limitation. Sufficient nitrogen source strengthened the xylose metabolic activity by promoting the key enzymes activity and intracellular redox ratios in R. oryzae. Optimized nitrogen concentration will enhance fumaric acid production with R. oryzae.