Abstract
The efficient conversion of xylose-containing biomass hydrolysate by the ethanologenic yeast Saccharomyces cerevisiae to useful chemicals such as ethanol still remains elusive, despite significant efforts in both strain and process development. This study focused on the recovery and characterization of xylose chemostat isolates of a S. cerevisiae strain that overexpresses xylose reductase- and xylitol dehydrogenase-encoding genes from Pichia stipitis and the gene encoding the endogenous xylulokinase. The isolates were recovered from aerobic chemostat cultivations on xylose as the sole or main carbon source. Under aerobic conditions, on minimal medium with 30 g l-1 xylose, the growth rate of the chemostat isolates was 3-fold higher than that of the original strain (0.15 h-1 vs 0.05 h-1). In a detailed characterization comparing the metabolism of the isolates with the metabolism of xylose, glucose, and ethanol in the original strain, the isolates showed improved properties in the assumed bottlenecks of xylose metabolism. The xylose uptake rate was increased almost 2-fold. Activities of the key enzymes in the pentose phosphate pathway (transketolase, transaldolase) increased 2-fold while the concentrations of their substrates (pentose 5-phosphates, sedoheptulose 7-phosphate) decreased correspondingly. Under anaerobic conditions, on minimal medium with 45 g l-1 xylose, the ethanol productivity (in terms of cell dry weight; CDW) of one of the isolates increased from 0.012 g g -1 CDW h-1 to 0.017 g g-1 CDW h-1 and the yield from 0.09 g g-1 xylose to 0.14 g g-1 xylose, respectively.
Original language | English |
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Pages (from-to) | 827-837 |
Journal | Applied Microbiology and Biotechnology |
Volume | 67 |
Issue number | 6 |
DOIs | |
Publication status | Published - 1 Jun 2005 |
MoE publication type | A1 Journal article-refereed |
Funding
Acknowledgements The financial support of the Academy of Finland (Graduate School in Chemical Engineering, for J.-P.P.) and the National Technology Agency (Tekes, project no. 40416/01) is gratefully acknowledged. This work is part of the research program “VTT Industrial Biotechnology” (Academy of Finland, Finnish Center of Excellence program 2000–2005, project no. 64330). The authors wish to thank Eila Leino, Seija Rissanen, Satu Bruun, and Tarja Hakkarainen for their indispensable help in the laboratory, and M.Sci. Laura Salusjärvi, Dr. Hannu Maaheimo, Dr. John Londesbor-ough, Dr. Peter Richard, and M.Sci Mervi Toivari for fruitful and instructive discussions and important comments on the manuscript. The experiments comply with the current legislation of Finland