A new artificial enzyme has shown that it can chew lignin, the resistant polymer that helps woody plants maintain their shape. Lignin also stores tremendous potential for energy and renewable materials.

Reporting in the journal Nature Communications, a team of researchers from Washington State University and the Pacific Northwest National Laboratory of the Department of Energy showed that its artificial enzyme was able to digest 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 mainly wasted as a fuel source. When wood is burned for cooking, lignin byproducts help give that smoked flavor to food. But the burning releases all this carbon into the atmosphere instead of capturing it for other uses.

"Our biomimetic enzyme has shown promise in the degradation of real lignin, which is considered a breakthrough," said Xiao Zhang, corresponding author of the paper and associate professor at The School of Chemical Engineering and Bioengineering of Gene and Linda Voiland at WSU. Zhang also has a joint appointment on the PNNL. "We think there is an opportunity to develop a new class of catalysts and really address the limitations of biological and chemical catalysts."

Lignin is in all vascular plants, where it forms cell walls and gives stiffness to plants. Lignin allows trees to stand, gives firmness to vegetables and makes up about 20-35% of the wood weight. Because lignin turns yellow when exposed to air, the wood products industry removes it as part of the fine paper making process. Once removed, it is often burned inefficiently to produce fuel and electricity.

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

A better than nature

"This is the first mimetic enzyme in nature that we know can efficiently digest lignin to produce compounds that can be used as biofuels and for chemical production," added Chun-Long Chen, corresponding author, researcher at pacific northwest national laboratory and affiliated professor of chemical and chemical engineering at the University of Washington.

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

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

"It is very difficult to produce these enzymes from microorganisms in a significant amount for practical use," Zhang said. "So once you isolate them, they are very fragile and untestable. But these enzymes offer a great opportunity to inspire models that copy their basic design."

Although researchers have failed to use natural enzymes to work for them, over the decades they have learned a lot about how they work. A recent review article from Zhang's research team describes the challenges and barriers to the application of dedegrading lignin enzymes. "Understanding these barriers provides new insights to design biomimetic enzymes," Zhang added.

Peptide scaffold is key

In the current study, researchers replaced the peptides surrounding the active site of natural enzymes with protein-like molecules called peptides. These peptides then self-assemble in crystalline tubes and leaves in nanoscale. Peptides were first developed in the 1990s to mimic protein function. They have several unique features, including high stability, that allow scientists to deal deficiencies of natural enzymes. In this case, they offer a high density of active sites, which is impossible to obtain with a natural enzyme.

"We can precisely organize these active sites and adjust their local environments for catalytic activity," Chen said, "and we have a much higher density of active sites rather than an active site."

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

"This job really opens up new opportunities," Chen said. "This is a significant step in the ability to convert lignin into valuable products using an environmentally benign approach."

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

Research collaboration was facilitated through the WSU-PNNL Bioproduct Institute. Tengyue Jian, Wenchao Yang, Peng Mu, Xin Zhang of PNNL and Yicheng Zhou and Peipei Wang of WSU also contributed to this report.