Abstract
Genetically engineered Saccharomyces cerevisiae strains are able to ferment xylose present in lignocellulosic biomass. However, better xylose fermenting strains are required to reach complete xylose uptake in simultaneous saccharification and co-fermentation (SSCF) of lignocellulosic hydrolyzates. In the current study, haploid Saccharomyces cerevisiae strains expressing a heterologous xylose pathway including either the native xylose reductase (XR) from P. stipitis, a mutated variant of XR (mXR) with altered co-factor preference, a glucose/xylose facilitator (Gxf1) from Candida intermedia or both mXR and Gxf1 were assessed in SSCF of acid-pretreated non-detoxified wheat straw. The xylose conversion in SSCF was doubled with the S. cerevisiae strain expressing mXR compared to the isogenic strain expressing the native XR, converting 76% and 38%, respectively. The xylitol yield was less than half using mXR in comparison with the native variant. As a result of this, the ethanol yield increased from 0.33 to 0.39 g g-1 when the native XR was replaced by mXR. In contrast, the expression of Gxf1 only slightly increased the xylose uptake, and did not increase the ethanol production. The results suggest that ethanolic xylose fermentation under SSCF conditions is controlled primarily by the XR activity and to a much lesser extent by xylose transport.
Highlights
The yeast Saccharomyces cerevisiae has been extensively engineered for ethanolic fermentation of the pentose sugar xylose either by introducing genes encoding xylose reductase (XR) and xylitol dehydrogenase (XDH), or by introducing the gene encoding xylose isomerase (XI) (Hahn-Hägerdal et al 2007; Van Vleet and Jeffries 2009; Matsushika et al 2009)
Gxf1 either alone or together with mutated variant of XR (mXR), at most increased xylose uptake with about 10% leaving the ethanol formation unchanged
The current study aimed to evaluate the relative contribution of a mutated xylose reductase (Runquist et al 2010a) and a glucose/xylose facilitator (Gxf1) (Runquist et al 2009) (Fonseca et al submitted) to the fermentation of xylose in a simultaneous saccharification and co-fermentation (SSCF) set-up (Olofsson et al 2008a) of pretreated wheat straw
Summary
The yeast Saccharomyces cerevisiae has been extensively engineered for ethanolic fermentation of the pentose sugar xylose either by introducing genes encoding xylose reductase (XR) and xylitol dehydrogenase (XDH), or by introducing the gene encoding xylose isomerase (XI) (Hahn-Hägerdal et al 2007; Van Vleet and Jeffries 2009; Matsushika et al 2009). Still xylose fermentation with recombinant S. cerevisiae is significantly less efficient than hexose fermentation. Among others this has been ascribed to the. Slow xylose fermentation has been ascribed to be the less efficient xylose transport. In S. cerevisiae xylose and glucose compete for the same transport systems (Kilian and Uden 1988; Meinander and Hahn-Hägerdal 1997) and the affinity for xylose is orders of magnitude lower than for glucose (Kötter and Ciriacy 1993; Saloheimo et al 2007; Gárdonyi et al 2003). Several homologous and heterologous xylose transporters have been expressed in S. cerevisiae (Hamacher et al 2002; Saloheimo et al 2007; Runquist et al 2009; Katahira et al 2008; Hector et al 2008). Gxf has been expressed in the industrial xylose fermenting S. cerevisiae strain TMB3400 (Fonseca et al submitted). Its presence increased xylose consumption in simultaneous saccharification and cofermentation (SSCF) of acid-pretreated wheat straw, without increasing the ethanol yield
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