|
Poster Presentation 2-15
Xylose Utilization by Recombinant Saccharomyces cerevisiae
Eija Rintala, Laura Salusjärvi, Anu Saloheimo, Kari Koivuranta, Mervi Toivari, Peter Richard, Jaana Uusitalo, John Londesborough, Laura Ruohonen and Merja Penttilä
VTT Biotechnology, P.O. Box 1500 02044 VTT, ESPOO 02044, Finland
Telephone: (358) 9-456 5827; Fax: (358) 9-455 2103; E-mail: Eija.Rintala@vtt.fi
Lignocellulosic biomass is an attractive substrate to be used for the production of ethanol as a renewable energy source to replace fossil fuels. Hexose sugars of lignocellulosics are readily fermented by many microorganisms; whereas pentoses, the major constituent of hemicellulose, are a challenge in the process to make it economically feasible.
S. cerevisiae, a well known and well established fermentation process organism, cannot naturally utilise xylose although it is able to ferment xylulose, an isomer of xylose. Introduction of Pichia stipitis genes encoding xylose reductase (XR) and xylitol dehydrogenase (XDH) gives S. cerevisiae the ability to utilize xylose. Over-expression of its own xylulokinase encoding gene (XKS1) further enhances xylose consumption (1).
The XR and XDH reactions generate a cofactor imbalance because XR has a preference for NADPH, whereas XDH is specific for NAD+. One attempt to solve this imbalance has been the introduction of a transhydrogenase cycle into the cells, to convert NADP and NADH, the products of XR and XDH, to NADPH and NAD, the substrates for XR and XDH (2). One of the transhydrogenase cycles has malic enzyme as one of its partners. We have further developed the cycle by trying to optimize the cellular localization and cofactor specificity of the malic enzyme.
In S. cerevisiae xylose is taken into the cells by its hexose transporters, which have a much higher affinity for hexoses, hampering efficient simultaneous fermentation of these sugars. Directing the endogenous hexose transporters, or other suitable candidates, more towards xylose specificity is one approach for enhancing the utilization of xylose.
Genome wide approaches have become an attractive way for studies of overall cellular metabolism under different physiological conditions. We have studied both on proteomic and genomic level the recombinant Saccharomyces to reveal novel and unpredictable changes in the metabolism of xylose fermenting yeast.
1Toivari, M.H., Aristidou A., Ruohonen, L. & Penttilä, M. (2001). Conversion of xylose to ethanol by recombinant Saccharomyces cerevisiae: Importance of xylulokinase (XKS1) and oxygen availability. Metab. Eng.3:236-249. 2Aristidou, A., Londesborough, J., Penttilä , M., Richard, P., Ruohonen, L., Söderlund, H., Teleman, A. & Toivari, M. (1999). Patent Application. Transformed microorganisms with improved properties. PCT/FI99/00185.
|