Optimizing ethanol yield in Saccharomyces cerevisiae fermentations by engineering redox metabolism

Abstract

Mankind’s energy requirements, which are currently mainly covered by the combustion of fossil fuels, have been steadily increasing in the past half century. While fossil fuels have a high energy content, their use results in significant emissions of greenhouse gases (mainly CO2, methane and nitrous oxide). As the industrialization of developing nations continues, the requirement for a paradigm shift is becoming increasingly evident. Microbial fermentation can provide an alternative by enabling the sustainable production of transport fuels that combine a lower carbon footprint with compatibility with current internal combustion engine technology. Bioethanol is, by volume, the biofuel with the highest annual production (ca. 100 billion liters in 2016). Current ‘first generation’ industrial bioethanol production processes are mainly based on fermentation of hydrolysed corn starch or sugar-cane sucrose by the budding yeast Saccharomyces cerevisiae and capitalize on the naturally high sugar-uptake rates and ethanol yield of this microorganism. The first full-scale ‘second generation’ ethanol production plants that are now coming on line use lignocellulosic hydrolysates, derived from agricultural ‘’waste’’ such as corn stover or wheat straw, as feedstocks. Second-generation bioethanol production can have a smaller carbon footprint than first-generation processes. Moreover, it uses feedstocks that are not a part of the human food chain. However, yeast-based second-generation bioethanol production poses multiple challenges for scientists. Lignocellulosic hydrolysates contain significant amounts of pentose sugars (mainly Dxylose and L-arabinose) which are not naturally fermentable by S. cerevisiae. Further, during biomass pretreatment, inhibitors of yeast performance (phenolics, aldehydes and organic acids) are released into the hydrolysates. To mitigate the negative effects of these inhibitors, yeast strains used in second-generation bioethanol production processes need to maintain high rates of sugar fermentation, both for hexoses and for pentoses. In both first- and second-generation bioethanol production, the price of the hydrolysed feedstock represents the single largest factor in production 2 costs. Therefore, in an industry that generally operates at low profit margins, maximization of the ethanol yield on fermentable sugars is of paramount importance…