Biofuels Production projects
Simultaneous Bioconversion of Glucose and Xylose into Fuels and Chemicals
This project builds on a past study that developed an efficient xylose-fermenting yeast strain. However, the engineered strain could not ferment cellulosic sugars (glucose and xylose) simultaneously due to glucose repression of xylose. In this project, several yeast metabolic engineering approaches are used in an effort to overcome the inhibition mechanism, allowing co-fermentation of glucose and xylose. Industrial applications of the co-fermentation system are also demonstrated.
The primary goal of this project is to engineer S. cerevisiae that is optimized for cellulosic sugar fermentation. Although our current engineered strain (SR8) ferments xylose very efficiently, several aspects of the strain suggest room for further improvement for cellulosic processes. Our focus is to discover a cellular regulatory mechanism underlying glucose repression of xylose, and to engineer a strain that overcomes it, resulting in simultaneous fermentation of glucose and xylose. The finding from this project can improve the efficiency of any cellulosic bioconversion systems, especially under continuous fermentation processes.
Although numerous chemical/biological technologies have been developed for utilizing cellulosic materials, there has been little discussion about the feasibility of cellulosic fuel production in a continuous process, which would provide various economic benefits over a batch process. In this project, continuous fermentation of cellulosic sugars (glucose and xylose) was demonstrated using a current biocatalyst (xylose-fermenting yeast), and three key biological limitations were identified: 1) inefficient xylose uptake, 2) glucose repression of xylose, and 3) xylose-induced inactivation of hexokinase. To solve the problems, we performed various metabolic engineering approaches integrating genomics, transcriptomics, and metabolomics. First, the sugar transport system of Saccharomyces cerevisiae was engineered to be favorable to xylose and mixed sugars. We found several heterologous xylose transporters whose overexpression accelerated the xylose uptake under continuous fermentation conditions. Moreover, based on transcript profiles for cells grown on xylose, we identified putative knockout/overexpression targets to improve the native sugar transport system for cellulosic sugars. Second, through an evolutionary engineering approach, mutants capable of the simultaneous fermentation of glucose and xylose were isolated. Genome sequencing of the mutants led to the tailored inverse engineering of the sugar preference of the yeast. Lastly, a search is being conducted for new heterologous hexokinase genes to overcome the xylose-induced inactivation of hexokinase in a mutant uptaking both sugars simultaneously.
Published in 2014
Leveraging Transcription Factors to Speed Cellobiose Fermentation by Saccharomyces cerevisiae, Yuping Lin, Kulika Chomvong, Ligia Acosta-Sampson, Raíssa Estrela Curado, Jonathan M. Galazka, Soo Rin Kim, Yong-Su Jin, Jamie H. D. Cate, Biotechnology and Biofuels, V. 7, pp. 126, doi:10.1186/s13068-014-0126-6, August 27, 2014.