...the new minicellulosome exhibited significantly enhanced glucose liberation and produced ethanol directly from phosphoric acid-swollen cellulose. The final ethanol concentration of 3.5 g/liter was 2.6-fold higher than that obtained by using the same amounts of added purified cellulases. The overall yield was 0.49 g of ethanol produced per g of carbohydrate consumed, which corresponds to 95% of the theoretical value. This result confirms that simultaneous and synergistic saccharification and fermentation of cellulose to ethanol can be efficiently accomplished with a yeast strain displaying a functional minicellulosome containing all three required cellulolytic enzymes.

—Tsai et al.

The use of multiple enzymes in the cellulosome greatly increases the efficiency of hydrolysis because heterogeneous forms of cellulose can be digested. The artificial cellulosome developed at UCR is highly modular and can be engineered to display ten or more different cellulases, the composition of which can be tuned to optimized hydrolysis of any feedstock.

Cellulosomes are self-assembled structures found on the exterior of certain bacteria that allow the organisms to efficiently break down cellulose. The cellulosome contains multiple types of cellulases (enzymes that break down cellulose), optimally spaced for maximum activity.

The experimental cellulosome contains three different cellulases. Yeast engineered with this triple cellulase cellulosome was able to multiply to high levels with cellulose as the only carbon source. Compared to controls engineered with one or two cellulases, the triple cellulase displaying yeast had higher rates of hydrolysis, demonstrating the benefit of using diverse cellulytic enzymes in a single organism.

The process is described in a paper published in the 1 October 2009 issue of the American Society of Microbiology’s journal Applied and Environmental Microbiology. The paper was co-authored by UCR students Shen-Long Tsai and Shailendra Singh, post-doctoral researcher Jeongseok Oh, and Ruizhen Chen, associate professor at the School of Chemical and Biomolecular Engineering at Georgia Institute of Technology.

Ongoing synthetic yeast cellulosome research being done at UCR is funded in part by grants from the National Science Foundation and the Department of Energy. University of California is looking for industry partners interested in evaluating this method of ethanol production.

Resources

• Shen-Long Tsai, Jeongseok Oh, Shailendra Singh, Ruizhen Chen, and Wilfred Chen (2009) Functional Assembly of Minicellulosomes on the Saccharomyces cerevisiae Cell Surface for Cellulose Hydrolysis and Ethanol Production. Appl. Environ. Microbiol. 75: 6087-6093 doi: 10.1128/AEM.01538-09