Linguists have discovered a new synthetic enzyme that can break down the strong polymer lignin, which is essential for woody plants to maintain their structure. Lignin also has a lot of potential for making renewable materials and energy.

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 was successful in digesting lignin, which had previously resisted attempts to develop it into an economically useful energy source.

As a fuel source, lignin, the second most abundant renewable carbon source on Earth, is generally wasted. Lignin byproducts help give meals a smoky flavor when the wood is burned for cooking. However, instead of collecting carbon for further uses, burning releases it into the atmosphere.

“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 article. “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 helps to create cell walls and give them stiffness. Lignin causes trees to stand, provides hardness to vegetables, and accounts for 20-35 percent of the weight of wood. The wood products industry eliminates lignin as part of the fine papermaking process because it turns yellow when exposed to air. Once removed, it is frequently burned inefficiently to generate fuel and energy.

For more than a century, chemists have attempted and failed to develop valuable products from lignin. There may be a turning point in that long history of disappointment.

“This is the first nature-mimetic enzyme which we know can efficiently digest lignin to produce compounds that can be used as biofuels and for chemical production,” said Chun-Long Chen, a corresponding author.

Fungi and bacteria in nature use their enzymes to break down lignin, which is how a mushroom-covered log decomposes in the forest. Chemical degradation, which requires a lot of heat and consumes more energy than it produces, is far less environmentally friendly than enzyme degradation.

Natural enzymes, on the other hand, deteriorate with time, making them difficult to use in industrial processes. They’re also pricey.

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

While scientists have been unable to put natural enzymes to work for them, they have learnt a lot about how they work over the years. The problems and constraints to the application of lignin-degrading enzymes are outlined in a recent review article by Zhang’s research team.

“Understanding these barriers provides new insights toward designing biomimetic enzymes,” Zhang continued.

The researchers in this work used protein-like compounds called peptoids to substitute the peptides that surround the active site of native enzymes. These peptoids then formed nanoscale crystalline tubes and sheets by self-assembly. In the 1990s, peptidoids were created to mimic the function of proteins. They have various distinct characteristics, including great stability, that enable scientists to address natural enzyme deficits. They provide a high density of active sites in this circumstance, which is impossible to achieve with a normal enzyme.

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

These artificial enzymes are also far more stable and strong than real enzymes, allowing them to perform at temperatures as high as 60 degrees Celsius, which would kill a normal enzyme.

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

It has the potential to be scaled up to a commercial scale if the new bio-mimetic enzyme can be enhanced further to increase conversion yield and generate more selective products. The method opens up new pathways to renewable materials for use such as aviation biofuel and biobased products, among others.

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