Xylose isomerase improves growth and ethanol production rates from biomass sugars for both Saccharomyces pastorianus and Saccharomyces cerevisiae

Abstract

The demand for biofuel ethanol made from clean, renewable nonfood sources is growing. Cellulosic biomass, such as switch grass (Panicum virgatum L.), is an alternative feedstock for ethanol production; however, cellulosic feedstock hydrolysates contain high levels of xylose, which needs to be converted to ethanol to meet economic feasibility. In this study, the effects of xylose isomerase on cell growth and ethanol production from biomass sugars representative of switch grass were investigated using low cell density cultures. The lager yeast species Saccharomyces pastorianus was grown with immobilized xylose isomerase in the fermentation step to determine the impact of the glucose and xylose concentrations on the ethanol production rates. Ethanol production rates were improved due to xylose isomerase; however, the positive effect was not due solely to the conversion of xylose to xylulose. Xylose isomerase also has glucose isomerase activity, so to better understand the impact of the xylose isomerase on S. pastorianus, growth and ethanol production were examined in cultures provided fructose as the sole carbon. It was observed that growth and ethanol production rates were higher for the fructose cultures with xylose isomerase even in the absence of xylose. To determine whether the positive effects of xylose isomerase extended to other yeast species, a side-by-side comparison of S. pastorianus and Saccharomyces cerevisiae was conducted. These comparisons demonstrated that the xylose isomerase increased ethanol productivity for both the yeast species by increasing the glucose consumption rate. These results suggest that xylose isomerase can contribute to improved ethanol productivity, even without significant xylose conversion.

Novel two‐stage fermentation process for bioethanol production using Saccharomyces pastorianus

“…S. pastorianus was purchased from Winemakeri, located in Nova Scotia, Canada, under the commercial name of Liquor QuikSuperYeast X‐press . S. cerevisiae was purchased from Lesaffre Yeast Corporation in Milwaukee, WI, under the commercial name of RED STAR Active Dry Yeast.…”

“…The minimal medium used for yeast (adjusted to pH 5.8) was described in Miller et al (2012) and is based on the medium first described by Korz et al (1995) and modified by Sharma et al (2007) for E. coli (pH 7.0) . Briefly, the phosphate/citrate buffer, glucose, MgSO 4 , CaCl 2 stock solutions were sterilized separately at 121°C for 30 min in an autoclave.…”

“…Samples were centrifuged at 14,500 g for 6 min, and the supernatant was stored at −20°C until analyzed. Ethanol concentrations were measured using an Agilent 7890A Gas Chromatography system as previously described . Samples from S. pastorianus and S. cerevisiae comparison fermentations were diluted and filtered through a 0.22 µm filter prior to high pressure liquid chromatography analysis for glucose and xylose as described previously .…”

“…Ethanol concentrations were measured using an Agilent 7890A Gas Chromatography system as previously described . Samples from S. pastorianus and S. cerevisiae comparison fermentations were diluted and filtered through a 0.22 µm filter prior to high pressure liquid chromatography analysis for glucose and xylose as described previously . The corn stover hydrolysate and pure sugar fermentation samples were analyzed for glucose and xylose concentrations using a Yellow Springs Instruments (Yellow Springs, OH) 2900 Biochemistry Analyzer.…”

Combinatorial Design of a Highly Efficient Xylose-Utilizing Pathway in Saccharomyces cerevisiae for the Production of Cellulosic Biofuels

“…1). Alternatively, heterologous expression of the bacterial xylose isomerase (XI) can be used to covert xylose into xylulose (7)(8)(9)(10). This pathway allows circumventing of the cofactor imbalance problem caused by the different cofactor preferences of XR and XDH.…”