Researchers add cellulosome to yeast

By Erin Voegele | December 09, 2009
Researchers at the University of California, Riverside (UCR) have successfully constructed a synthetic cellulosome in yeast. This development may have important implications for the cellulosic ethanol industry by potentially allowing simultaneous hydrolysis and fermentation of cellulosic material, reducing the cost of production.

Cellulosomes are self-assembled structures naturally found on the exterior surfaces of certain bacteria that allow these organisms to efficiently break down cellulosic material. The cellulosomes contain multiple types of cellulases, which are enzymes that break down cellulose, optimally spaced for maximum activity.

Essentially, cellulosomes are the structures certain bacteria use to degrade cellulose, said Wilfred Chen, a professor of chemical engineering at UCR who is leading the project. Rather than secreting cellulases such as fungus to, these bacteria actually anchor a whole bunch of different cellulases on their surface. This allows them to very efficiently complete hydrolysis of the cellulose without using a lot of enzymes, Chen continued. "It's a very clever way to increase the efficiency," he said.

While cellulosomes occur naturally on some bacteria, these bacteria are not as ethanol-tolerant as yeast. By adding synthetic cellulosomes to yeast, it becomes possible to combine the hydrolysis and fermentation processes of cellulosic ethanol production.

The cellulosomes found in nature are much more complex than the ones Chen's team has produced in the lab, at least to date. "[Those bacteria] typically have 19 to 20 cellulases that display on the surface in organized cellulosome structure," Chen said. These cellulases complete hydrolysis of cellulose, allowing the bacteria to utilize the resulting sugar as a substrate for growth, he continued.

The experimental cellulosome created by Chen and his team contains three different cellulases. The yeast engineered with this 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 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 by Chen and his team is modular, and can be engineered to display ten or more different cellulases. This composition of cellulases can be tuned to optimize hydrolysis for any feedstock.

While it would be possible to tailor yeast for specific feedstocks, Chen said the ideal solution is to create a system that is so intelligent it can adapt to any feedstock employed in the cellulosic ethanol production process. Although the synthetic cellulosome created by the team has only three enzymes right now, Chen said future research will seek to create more complex structures.

"[This yeast] is the initial proof of concept," Chen said. "What we want to do is try to build on what we know already and make much more complex structures." He said the long term goal is to increase the number of cellulases to 10, 15 or more.

Ultimately, the yeast cellulosome could enable an efficient one-step consolidated bioprocessing method by maximizing the catalytic efficiency of cellulosic hydrolysis with simultaneous fermentation. A process employing this kind of engineered yeast could potentially make the production of cellulosic ethanol more efficient and economical.

While the researchers have not been working with yeast that has been genetically enhanced to optimize ethanol production, Chen said that could be part of his team's future research. "Certainly we want to combine [the results] with some of the other genetically engineered yeast that have been proven to produce a lot of ethanol in the future," he said.

According to Chen, the research project has been ongoing for about a year and half. In late 2009, the team received a three-year grant from the U.S. DOE that will support additional research on synthetic cellulosomes. The research has also been funded in part through a grant awarded by the National Science Foundation.