Efficient Alcoholic Conversion of Glycerol by Engineered Saccharomyces cerevisiae

Abstract

Glycerol is an eco-friendly solvent that enhances plant biomass decomposition via glycerolysis in many pretreatment methods. Nonetheless, the lack of efficient conversion of glycerol by natural Saccharomyces cerevisiae hinders its use in these methods. Here, we have aimed to develop a complete strategy for the generation of efficient glycerol-converting yeast by modifying the oxidation of cytosolic nicotinamide adenine dinucleotide (NADH) by an O2-dependent dynamic shuttle, while abolishing both glycerol phosphorylation and biosynthesis. By following a vigorous glycerol oxidation pathway, the engineered strain increased the conversion efficiency (CE) to up to 0.49 g ethanol/g glycerol (98% of theoretical CE), with production rate > 1 g{middle dot}L{middle dot}h, when glycerol was supplemented in a single fed-batch fermentation in a rich medium. Furthermore, the engineered strain fermented a mixture of glycerol and glucose, producing > 86 g/L bioethanol with 92.8% CE. To our knowledge, this is the highest ever reported titer in this field. Notably, this strategy changed conventional yeast from a non-grower on minimal medium containing glycerol to a fermenting strain with productivity of 0.25-0.5 g{middle dot}L{middle dot}h and 84-78% CE, which converted 90% of the substrate to products. Our findings may improve the utilization of glycerol in several eco-friendly biorefinery approaches. IMPORTANCEWith the development of efficient lignocellulosic biorefineries, glycerol has attracted attention as an eco-friendly biomass-derived solvent that can enhance the dissociation of lignin and cell wall polysaccharides during the pretreatment process. Co-conversion of glycerol with the sugars released from biomass after glycerolysis increases the resources for ethanol production and lowers the burden of component separation. However, titer productivity hinders the industrial application of this process. Therefore, generation of efficient glycerol-fermenting yeast will promote the applicability of integrated biorefineries. Furthermore, glycerol is an important carbon source for the production of various chemicals. Hence, control of the metabolic flux of yeast grown on glycerol will contribute to the generation of cell factories that produce chemicals such as ethanol from glycerol, which will boost biodiesel and bioethanol industries. Additionally, the use of glycerol-fermenting yeast will reduce global warming and generation of agricultural waste, leading to the establishment of a sustainable society.