ORNL team converts cellulose into isobutanol

By Erin Voegele | March 15, 2011

A team of researchers affiliated with Oak Ridge National Laboratory has successfully converted cellulose into isobutanol using a genetically engineered microbe developed at the U.S. DOE’s BioEnergy Science Center. The Oak Ridge team, led by James Liao, chancellor’s professor and vice chair of chemical and biomolecular engineering at the University of California, Los Angeles’ Henry Samueli School of Engineering and Applied Science, used consolidated bioprocessing to directly convert cellulose into isobutanol in a single step.

The genetically engineered microorganism employed in the process was developed by Liao, UCLA postdoctoral researcher Wendy Higashide, and ORNL researchers Yongchao Li and Yunfeng Yang. The microbe is a strain of Clostridium cellulolyticum, a native cellulose-degrading microorganism that has been altered to synthesize isobutanol directly from cellulose. While there were many possible microbial candidates, the research term chose Clostridium cellulolyticum, which was isolated from decayed grass. According to the team, their strategy allowed them to exploit the microbe’s natural cellulolytic activity and its amino acid biosynthetic pathway.

“This work is based on our earlier work at UCLA in building a synthetic pathway for isobutanol production,” Liao said. “In nature, no microorganisms have been identified that posses all of the characteristics necessary for the ideal consolidated bioprocessing strain, so we knew we had to genetically engineer a strain for this purpose.”

According to Liao, the ability to use a consolidated bioprocessing technique to produce isobutanol offers significant economic advantages. “Previously, biomass first had to be pretreated and then degraded using additional cellulases,” he continued. Cellulases are enzymes that degrade cellulose and release trapped sugars. The released sugars are then transferred to a fermentation tank for conversion. The process developed by Liao and his team is able to combine these two steps, as the microorganism they developed secretes cellulase and ferments sugars into isobutanol.

While a great deal of research has focused on the conversion of cellulose into ethanol, Liao said that isobutanol is a better candidate for gasoline replacement due to its relatively high energy density and octane value. Its Reid vapor pressure is also more similar to that of gasoline. Liao says his team targeted the production of isobutanol rather than other butanol molecules because isobutanol is a higher octane chemical that is easier to process into 6-, 8- and 12-carbon compounds.

To date, Liao and his team have demonstrated the process on the lab scale. The team is currently working to develop larger-scale evaluations. An academic research paper on the team’s work, titled “Metabolic Engineering of Clostridium Cellulolyticum for Isobutanol Production from Cellulose,” was recently published online in Applied and Environmental Microbiology