A study promises to develop a new source of renewable energy

A new artificial enzyme has been shown to chew up lignin, the tough polymer that helps woody plants retain their shape. Lignin also stores a huge potential of renewable energy and materials.

Report in the newspaper Nature Communicationa team of researchers from Washington State University and the Department of Energy’s Pacific Northwest National Laboratory showed that their artificial enzyme successfully digested lignin, which stubbornly resisted previous attempts to develop it into an economically useful energy source.

Lignin, which is the second most abundant renewable carbon source on Earth, is wasted primarily as a fuel source. When wood is burned for cooking, lignin byproducts help to impart that smoky flavor to foods. But combustion releases all that carbon into the atmosphere instead of capturing it for other uses.

“Our biomimicking enzyme has shown promise in degrading true lignin, which is considered a breakthrough,” said Xiao Zhang, corresponding author of the paper and associate professor at WSU’s Gene and Linda Voiland School of Chemical. Engineering and Bioengineering. Zhang also holds a cross-appointment to PNNL. “We believe there is an opportunity to develop a new class of catalysts and to really challenge the limits of biological and chemical catalysts.”

Lignin is present in all vascular plants, where it forms cell walls and gives plants rigidity. Lignin helps trees stand upright, gives vegetables their firmness, and makes up about 20-35% of the weight of wood. Because lignin yellows when exposed to air, the wood products industry removes it as part of the fine papermaking process. Once removed, it is often inefficiently burned to produce fuel and electricity.

Chemists have tried and failed for over a century to make valuable products from lignin. This record of frustration may be about to change.

A better than nature

“This is nature’s first mimetic enzyme that we know can efficiently digest lignin to produce compounds that can be used as biofuels and for chemical production,” added corresponding author Chun-Long Chen. , Pacific Northwest National Laboratory researcher and affiliate. professor of chemical engineering and chemistry at the University of Washington.

In nature, fungi and bacteria are able to break down lignin with their enzymes, which is how a fungus-covered log breaks down in the forest. Enzymes offer a much more environmentally benign process than chemical degradation, which requires high heat and consumes more energy than it produces.

However, natural enzymes degrade over time, which makes them difficult to use in an industrial process. They are expensive too.

“It’s really difficult to produce these enzymes from microorganisms in significant quantities for practical use,” Zhang said. “Then, once you’ve isolated them, they’re very fragile and unstable. But these enzymes offer a great opportunity to inspire models that copy their basic design.”

While researchers haven’t been able to harness natural enzymes to work for them, over the decades they have learned a great deal about how they work. A recent review article by Zhang’s research team outlines the challenges and barriers to the application of lignin-degrading enzymes. “Understanding these barriers provides new insights into the design of biomimetic enzymes,” Zhang added.

Peptoid scaffolding is the key

In the current study, the researchers replaced the peptides that surround the active site of natural enzymes with protein-like molecules called peptoids. These peptoids then self-assembled into nanoscale crystalline tubes and sheets. Peptoids were first developed in the 1990s to mimic protein function. They have several unique characteristics, including high stability, that allow scientists to address deficiencies in natural enzymes. In this case, they offer a high density of active sites, impossible to obtain with a natural enzyme.

“We can precisely organize these active sites and tune their local environments for catalytic activity,” Chen said, “and we have a much higher density of active sites, instead of a single active site.”

As expected, these artificial enzymes are also much more stable and robust than the natural versions, so they can operate at temperatures up to 60 degrees Celsius, a temperature that would destroy a natural enzyme.

“This work really opens up new opportunities,” Chen said. “This is a significant step forward in the ability to convert lignin into valuable products using an environmentally friendly approach.”

If the new bio-mimetic enzyme can be further improved to increase conversion efficiency, to generate more selective products, it has potential for industrial scale-up. The technology offers new pathways to renewable materials for aviation biofuels and bio-based materials, among other applications.

The research collaboration was facilitated by the WSU-PNNL Bioproducts Institute. Tengyue Jian, Wenchao Yang, Peng Mu, Xin Zhang of PNNL and Yicheng Zhou and Peipei Wang of WSU also contributed to the research.