Redox balancing in recombinant xylose utilising Saccharomyces cerevisiae

Lignocellulosic biomass is an attractive substrate to be used in 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 hemicellulose are a challenge in the process to make it economically feasible. S. cerevisiae, a well-known 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 utilise xylose. Overexpression of its own xylulokinase-encoding gene (XKS1) further enhances xylose consumption [1]. The XR and XDH reactions generate a cofactor imbalance into the cell 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]. We have studied a of transhydrogenase cycle, which relies on overexpression of malic enzyme. We have also used genome wide approaches to study the physiology of xylose utilisation in recombinant S. cerevisiae. We have made studies both on proteomic and genomic level to reveal novel and unpredictable changes in the metabolism of xylose fermenting yeast [3,4]. [1] Toivari et al. (2001). Metab.Eng.3, 236-249. [2] Aristidou et al.(1999). Patent Application. PCT/FI99/00185. [3] Salusjarvi et al.(2002). Yeast 20,295-314. [4] Salusjarvi et at. Manuscript in preparation.