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A new synthetic enzyme has proven that it can break down lignin, the strong polymer that gives woody plants their structure. Lignin is also an excellent source of renewable energy and minerals.

A team of researchers from Washington State University and the Department of Energy’s Pacific Northwest National Laboratory reported in the journal Nature Communications that their artificial enzyme successfully digested lignin, which has previously defied prior efforts to turn it into an economically effective energy source.

The second most abundant renewable carbon source on Earth, lignin, is mostly wasted as a fuel source. Byproducts of wood combustion, such as lignin, contribute to the smokey taste and aroma of food.  Burning, on the other hand, releases all that carbon into the atmosphere rather than storing it for future use.  “Our bio-mimicking enzyme showed promise in degrading real lignin, which is considered to be a breakthrough,” said Xiao Zhang, a corresponding author on the paper and associate professor in WSU’s Gene and Linda Voiland School of Chemical Engineering and Bioengineering. Zhang also holds a joint appointment at PNNL. “We think there is an opportunity to develop a new class of catalysts and to really address the limitations of biological and chemical catalysts.”

Lignin is found in all vascular plants, where it serves as a cell wall and provides the plants with stiffness. Lignin is responsible for the hardness of crops and trees, and it accounts for 20-35% of the weight of wood. Papermakers employ lignin removal to avoid yellowing in fine paper because it turns yellow when exposed to air. Once it is removed, it is typically burnt inefficiently to generate fuel and power.

Chemists have been trying for more than a century but have failed at making viable chemicals from lignin. However, it seems like that pattern of disappointment is going to end.  Researchers at the Pacific Northwest National Laboratory and the University of Washington have discovered the first nature-mimetic enzyme that can successfully digest lignin to create chemicals that may be utilized as biofuels and for chemical manufacturing.

Lignin may be broken down by fungus and bacteria in nature, which is why mushroom-covered logs decay in the forest. In contrast to chemical decomposition, which needs high temperatures and uses more energy than it generates, enzymes provide a more ecologically friendly method of decomposition.  However, since natural enzymes decay over time, using them in industrial processes is difficult. They’re also pricey.

According to Zhang, “It’s really hard to produce these enzymes from microorganisms in a meaningful quantity for practical use” “They’re very fragile and unstable once isolate them. But, the basic design of these enzymes offer a great opportunity to inspire models that copy their basic design.”

Even while scientists have been unable to put natural enzymes to use, they have gained a great deal of knowledge about how they operate over the decades. Lignin degrading enzyme applications face several hurdles, as detailed in a recent review study by Zhang’s team. “Understanding these barriers provides new insights toward designing biomimetic enzymes,” Zhang added.

Peptoids, which are protein-like molecules, were used to substitute the peptides that surround the active site of natural enzymes in the present work. These peptoids subsequently self-assembled into nanocrystalline tubes and sheets. Initially, peptidoids were created in the 1990s to imitate the functions of proteins. ‘ A number of special characteristics, including great stability, enable scientists to solve the shortcomings of natural enzymes. Since they have a high concentration of active sites, this is an advantage over natural enzymes.

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

The artificial enzymes, as predicted, are also considerably more durable and robust than the natural ones, so they can operate at temperatures of up to 60 degrees Celsius, a temperature at which a natural enzyme would be destroyed.

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

Scalability at industrial scale may be possible if the novel bio-mimetic enzyme can be further enhanced to boost conversion yield and create more selective products. The innovative method provides new paths to renewable resources for aviation biofuel and biobased materials, among other uses.

The WSU-PNNL Bioproducts Institute was instrumental in facilitating the research partnership. Researchers from PNNL, WSU, and PNNL’s Xin Zhang and Peng Mu were also involved in the study, as were Yicheng Zhou and Peipei Wang of WSU, as well as Tengyue Jian of WSU.

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The article is paraphrased from the following: New artificial enzyme breaks down tough, woody lignin: Study shows promise for developing a new renewable energy source, Pacific Northwest National Laboratory, May 31, 2022, https://phys.org/news/2022-05-artificial-enzyme-tough-woody-lignin.html