283638 The Impacts of Glucose Repression on Invertase Expression in Various Yeast Strains During the Ethanol Fermentation of Sucrose-Containing Substrates

Tuesday, October 30, 2012: 9:20 AM
334 (Convention Center )
Lei Zhang, Institute of New Energy Technology, Tsinghua University, Beijing, China and Shi-zhong Li, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, China

Recently, the sweet sorghum ethanol using advanced solid-state fermentation (SSF) developed by Tsinghua University in China has been applied in the largest solid-state fermenter (550 m3) of the world, which makes sugar-based fuel ethanol production more cost-competitive, first demonstrating the SSF for fuel ethanol production is feasible in industrial scale. Although the relatively high ethanol productivity have been achieved, seeking the yeast strains with high fermentation rate is still an important means to further strengthen the cost competitiveness of the process, due to the inherently insufficient mass and heat transfer of SSF. Due to the high contents of sucrose in sweet sorghum and sugarcane, the hydrolysis rate of sucrose by yeast into fermentable sugars, including glucose and fructose, significantly determines the ethanol fermentation rate of sucrose-containing substrates. Invertase is the key enzyme for yeast involved in the cleavage of sucrose, which is subjected to the glucose repression that poses significantly adverse effects on the yeasts’ fermentation of mix-sugar substrates by extension of fermentation time. The dynamic changes of glucose levels during the utilization of sucrose-containing substrates make the regulation of invertase expression in yeast more complicated. Thereby, in this study we investigated the impacts of invertase expression on the final fermentation performances of the various Saccharomyces cerevisiae strains through monitoring the dynamic variation of specific invertase acivities of yeasts during the fermentation of sucrose (120 g l-1 of sucrose) or mixed sugars (70 g l-1 of sucrose, 30 g l-1 of glucose, and 23 g l-1 of fructose). During the cell growth in YPD media with glucose as sole carbon source, the both commercial strains CA and AF showed much higher induction levels of invertase than both CL and AL did when the glucose levels in the media decreased to the threshold value of triggering the invertase induction. In both sucrose and mix sugars media, surprisingly, the both originally invertase-repressed CA and AF showed an obviously increasing trends of invertase activities after 7 h of fermentations even under the glucose repression conditions at the time. Moreover, the both CA and AF showed highest ethanol productivities in sucrose and mix sugars media. Meanwhile, AL showed a stable and relatively high background invertase activity during the whole process of ethanol fermentation even under the impact of glucose repression, which led the high sucrose hydrolysis rate of AL at the early stage of fermentation. On the other hand, ethanol inhibits the induction of invertase to a great extent, and the invertase induction of both CA and AF showed much stronger tolerance to ethanol. All these results suggested that the glucose repression might be not the only mechanism for the regulation of the invertase involved in the process of ethanol fermentation in S. cerevisiae but a net effect of multi-factors, such as glucose repression, stress response. Meanwhile, these results can provide us some useful indicators for isolating or developing the yeast strains with high ethanol productivities with sucrose-containing feedstocks as sustrates.

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