Engineering industrial yeast strains for lignocellulosic ethanol production

Saccharomyces cerevisiae;Industrial strain;Xylose;Lignocellulosic fermentation;Acetate;Polyploid

Kong, In Iok

关键词: Saccharomyces cerevisiae;    Industrial strain;    Xylose;    Lignocellulosic fermentation;    Acetate;    Polyploid;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/95596/KONG-DISSERTATION-2016.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

As human population is expanding, the demand for food, crop feeds, and energy have also increased. The majority of energy source is from petroleum and the massive production carbon dioxide from high-energy consumption links to global climate changes. Moreover, there are rising concerns toward the food versus fuels issue which intensified the necessity of searching sustainable alternative biofuels. With the recent development of metabolic engineering toolbox for strain improvement, microbial fermentation by the conventional yeast, Saccharomyces cerevisiae, has provided significant potential for producing advanced biofuels and value-added products. Engineered yeast can be employed to convert fermentable sugars, which consist of agricultural and forest residues, into numerous chemical products. However, the generation of the fermentable sugars from the agricultural and forest residue requires harsh chemical and physicochemical pretreatments which would generate various toxic compounds that inhibit the growth of microorganisms and cellular metabolism as well as affect the yield of target products. Other challenges from the fermenting microorganisms are their inability to ferment all available sugars from lignocellulosic biomass, lack of other desirable traits such as higher ethanol productivities, and low tolerances toward the environment stresses. Therefore, it is necessary to develop more robust and efficient fermenting strains with the ability to tolerate the environmental stresses in lignocellulosic hydrolysates. The industrial yeasts are one of the potential hosts for superior fermentation of lignocellulosic hydrolysates containing toxic fermentation inhibitors due to their adapted evolution adaptation under industrial environments. They can withstand extreme environment conditions, and exhibit a better and distinctive fermentation characteristic than laboratory yeast strains. Even though industrial yeast strains possess increased tolerances towards fermentation inhibitors and have a wider range of optimal growth temperature, they are not well characterized, and it is challenging to introduce designed genetic perturbations into industrial yeast strains due to their complex genetic structures and high degree of heterozygosity. Therefore, the overall goals of my thesis are 1) to increase the industrial fitness of the industrial S. cerevisiae against toxic inhibitors in lignocellulosic hydrolysates through a metabolic engineering approach, 2) to develop an optimal yeast strain which is capable of efficiently fermenting xylose, second abundant sugar in the lignocellulosic hydrolysates which S. cerevisiae could not metabolize naturally, 3) to allow in situ detoxification of acetic acid which is an inevitable byproduct in lignocellulosic hydrolysates by introducing an acetic acid reduction pathway into engineered strain, 4) to achieve an efficient sugar fermentation in minimal media under low pH conditions through laboratory evolution of the engineered strain, and 5) to investigate the role of an uncharacterized protein that is a beneficial deletion target for an efficient xylose fermentation under low pH conditions. The ultimate motivation is to obtain a robust industrial strain with efficient fermentation that tolerates fermentation inhibitors under low pH conditions. The strains and methodologies developed in my dissertation will be broadly applicable for developing robust and advanced yeast strains to produce biofuels and chemicals from renewable biomass.