Engineered Saccharomyces cerevisiae for lignocellulosic valorization: a review and perspectives on bioethanol production

Abstract

The biorefinery concept, consisting in using renewable biomass with economical and energy goals, appeared in response to the ongoing exhaustion of fossil reserves. Bioethanol is the most prominent biofuel and has been considered one of the top chemicals to be obtained from biomass. Saccharomyces cerevisiae, the preferred microorganism for ethanol production, has been the target of extensive genetic modifications to improve the production of this alcohol from renewable biomasses. Additionally, S. cerevisiae strains from harsh industrial environments have been exploited due to their robust traits and improved fermentative capacity. Nevertheless, there is still not an optimized strain capable of turning second generation bioprocesses economically viable. Considering this, and aiming to facilitate and guide the future development of effective S. cerevisiae strains, this work reviews genetic engineering strategies envisioning improvements in 2 nd generation bioethanol production, with special focus in process-related traits, xylose consumption, and consolidated bioprocessing. Altogether, the genetic toolbox described proves S. cerevisiae to be a key microorganism for the establishment of a bioeconomy, not only for the production of lignocellulosic bioethanol, but also having potential as a cell factory platform for overall valorization of renewable biomasses.

The Pentose Phosphate Pathway in Yeasts–More Than a Poor Cousin of Glycolysis

“…Based on the long-term experience in these fermentation processes, and the ease of its genetic manipulation, S. cerevisiae has been applied to the conversion of alternative carbon sources for second-generation biofuels, especially from pentoses [ 99 ]. The latter are not only found abundantly in the waste-streams of the paper industry, but also form a major part of the biomass in plant-derived lignocellulosic material [ 100 ]. Two alternative approaches of heterologous gene expression have been used to convert xylose from such sources into xylulose-5-phosphate as a substrate that can be metabolized by the PPP, and, through its connection with glycolysis, ultimately be fermented to ethanol ( Figure 3 ).…”

“…Further details on the importance of pentoses and lignocellulosic sources for the production of next-generation biofuels, and the important role of the PPP for detoxification of oxidants like furfural produced in these processes, would go far beyond the scope of this overview, but have been extensively reviewed, e.g., in [ 99 , 100 , 109 , 111 , 113 , 114 ].…”

“…Efficient fermentation of the two pentoses has also been considerably improved by the engineering of their transport into S. cerevisiae, as reviewed in [111]. Further details on the importance of pentoses and lignocellulosic sources for the production of next-generation biofuels, and the important role of the PPP for detoxification of oxidants like furfural produced in these processes, would go far beyond the scope of this overview, but have been extensively reviewed, e.g., in [99,100,109,111,113,114].…”

Strategies towards Reduction of Cellulases Consumption: Debottlenecking the Economics of Lignocellulosics Valorization Processes

“…The yeast S. cerevisiae, being the preferred microorganism for the production of bioethanol, has been the most explored microorganism for application in CBP processes. In addition, this yeast plays a central role in lignocellulosic valorization processes not only to bioethanol [104] but also to top chemicals [105] due to its tolerance to adverse lignocellulose-based process conditions [106]. Taking advantage of the extensive genetic toolbox available, a wide variety of modifications has been applied to this yeast in order to provide it with efficient cellulolytic activity, following three distinct approaches: enzyme secretion, cellulosomes and cell surface display [104].…”

Industrialization progress of lignocellulosic ethanol

“…However, traditionally Saccharomyces cerevisiae which is used for ethanol production is an efficient glucose consumer but unable to utilize xylose. Therefore, scientists try to introduce the xylose metabolic pathway into Saccharomyces cerevisiae through genetic engineering, so that Saccharomyces cerevisiae has the ability to ferment xylose to improve ethanol yield, but there is still no strain that is efficient and perfect [83][84][85][86], and the efficient strain should be able to tolerate high concentration inhibitors, make full use of the difficult fermentable sugar in the enzymatic hydrolysate, and have high ethanol yield.…”