Effective Production Of SAM And Co-production Of GSH By Yeast Fed-batch Fermentation

Both S-adenosyl-L-methionine (SAM) and giutathione (GSH) are intracellular molecules with important physiological activities. The metabolic pathways of SAM and GSH in yeast cell are tightly connected, which leads the original attempt to interview the possibility to produce them jointly in our laboratory.This dissertation aimed to maximize the production of SAM and co-production of GSH simultaneously by high cell density culture of baker's yeast. A control system for fed-batch fermentation based on the remained weight of feeding stock was developed to precisely control the glucose feeding rate. To optimize the cell growth and product accumulation, a two-stage fed-batch process was 'developed. The first exponential feeding stage was aimed to eliminate Crabtree effect and to enhance cell concentration. For this purpose, the initial culture media was modified and the final concentration of SAM, GSH and cells obtained in experiment were increased to 8.77g/L, 0.81g/L and 105g/L respectively. In this case, the yields of SAM and GSH on glucose were calculated as 0.039 and 0.0035 respectively, which are still relatively low due to the fermentative consumption of some glucose. To enhance the yields, a constant feeding stage during the transformation phase was taken to avoid the formation of ethanol coursed by oxygen limitation. Due to the elimination of ethanol production, the yields SAM and GSH on glucose achieved in experiment were enhanced to 0.048 and 0.0038 respectively.To improve the productivity of the reactor further, a repeated fed-batch strategy was investigated. At the end of a single fed-batch operation, the 3.2 L medium in the reactor was drained to 2.0 L, then a constant feeding with a rate of 24.7g/h glucose was conducted and the productivity of biomass, SAM and GSH were enhanced by three times, five times and five times approximately.In addition, a compartment model was proposed to describe the time courses of cell growth and the synthesis of SAM and GSH in fed-batch fermentation. The model simulation is in good agreement with experimental data.