Why Is Breaking Down Plant Material for Biofuels So Slow?

by Gail McCormick (Pennsylvania State University) Penn State researchers have revealed how molecular roadblocks slow the breakdown of cellulose, an abundant renewable resource in plants -- Cellulose, which helps give plant cell walls their rigid structure, holds promise as a renewable raw material for biofuels — if researchers can accelerate the production process. Compared to the breakdown of other biofuel materials like corn, breaking down cellulose is slow and inefficient but could avoid concerns around using a food source while taking advantage of abundant plant materials that might otherwise go to waste. New research led by Penn State investigators has revealed how several molecular roadblocks slow this process.

The team’s most recent study, published in the Proceedings of the National Academy of Sciences, describes the molecular process by which cellobiose — a two-sugar fragment of cellulose that is made during cellulose deconstruction — can clog up the pipeline and interfere with subsequent cellulose breakdown.

Biofuel production relies on the breakdown of compounds like starch or cellulose into glucose, which can then be efficiently fermented into ethanol for use as a fuel or converted into other useful materials. The predominant biofuel option on the market today is generated from corn, in part because, the researchers said, their starches break down easily.

“There are several concerns about using corn as a biofuel source, including competing with the global food supply and the large quantity of greenhouse gasses produced when generating corn-based ethanol,” said Charles Anderson, professor of biology in the Penn State Eberly College of Science and an author of the paper. “A promising alternative is to break down cellulose from the non-edible parts of plants like corn stalks, other plant waste like forestry residue, and potentially dedicated crops that could be grown on marginal land. But one of the major things holding back so-called second-generation biofuels from being economically competitive is that the current process to break down cellulose is slow and inefficient.”

We have been using a relatively new imaging technique to explore the molecular mechanisms that slow down this process.”

Cellulose is composed of chains of glucose, held together by hydrogen bonds into crystalline structures. Scientists use enzymes called cellulases, derived from fungi or bacteria, to break down plant material and extract the glucose from the cellulose. But, the researchers said, cellulose’s crystalline structure paired with other compounds called xylan and lignin — also present in cell walls — provide additional challenges to the cellulose breakdown. Traditional techniques, however, were unable to reveal the specific molecular mechanisms of these slowdowns.

To explore these unclear mechanisms, the researchers chemically tagged individual cellulases with fluorescent markers. They then used Penn State’s SCATTIRSTORM microscope, which the team designed and built for this very purpose, to trace the molecules through each step of the breakdown process and interpreted the resulting videos using computational processing and biochemical modeling.

“Now that we have a better understanding of how exactly cellobiose is mucking things up, we can explore new ways to fine tune this process. For example, we could alter the front or the back door of the tunnel or change aspects of the Cel7A enzyme to be more efficient at preventing this inhibition. There has been a lot of work to engineer more efficient cellulase enzymes over the last two decades, and it’s an incredibly powerful approach. Having a better understanding of the molecular mechanisms that limit cellulose degradation will help us direct this effort.”

“We found that xylan and lignin operate in different ways to interfere with the breakdown of cellulose,” said Nerya Zexer, postdoctoral researcher in biology in the Penn State Eberly College of Science and lead author of the RSC Sustainability paper. “Xylan coats the cellulose, reducing the proportion of the enzymes that can bind to and move cellulose. Lignin inhibits the enzyme’s ability to bind to cellulose as well as its movement, reducing the velocity and distance of the enzyme.” READ MORE