Artificial enzyme can break down lignin, a tough, carbon-rich component of wood

US researchers have created an artificial enzyme that can digest lignin, a major component of wood.

“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,” says Chun-Long Chen, a corresponding author on the study and researcher at the US Department of Energy’s Pacific Northwest National Laboratory.

Lignin is a hardy polymer found in woody plants, where it sits in cell walls and provides plants with the rigidity to hold their shape. Lignin makes up about 20% to 35% of the weight of wood and helps give the vegetables in your fridge their firmness.

Artificial enzyme concept a gloved hand holding a jar of lignin
Woody lignin, seen here in purified form, holds significant promise as a renewable biofuel, if it can be efficiently broken down into useful form. Credit: Andrea Starr / Pacific Northwest National Laboratory.

Lignin is the second-most-abundant renewable carbon source on Earth. However, unlike wood’s other major component, cellulose – which is already used to make paper, textiles, building materials, adhesives and food and drug additives – lignin has proved difficult to use as a source of energy or useful materials. It is sometimes burned for cooking or electricity, releasing carbon into the atmosphere.

However, the new enzyme may help unlock lignin’s potential. Certain bacteria and fungi have enzymes that can break down lignin – this is how dead trees are slowly broken down, for example.

Unfortunately for chemists, the natural bacterial and fungal enzymes are difficult to apply to new contexts. The enzymes are expensive to produce at scale, and not very durable.

“It’s really hard to produce these enzymes from microorganisms in a meaningful quantity for practical use,” explains author Xiao Zhang, an associate professor in chemical engineering at Washington State University, US. Zhang is also affiliated with Pacific Northwest National Laboratory.

“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.”

Artificial enzyme concept two male researchers wearing safety glasses and gloves and looking at a chemical flask
Researchers Xiao Zhang (L) and Chun-long Chen (R) examine the products of lignin digestion by their novel biomimetic peptoid catalyst. Credit: Andrea Starr / Pacific Northwest National Laboratory.

Enzymes are proteins, which means they’re made up of chains of amino acids – known as peptides. Zhang and his colleagues used artificial protein-like molecules called peptoids to create their artificial enzyme.

Like a real protein, the peptoid nanomaterials are self-assembling, folding into a specific structure that gives them a particular molecular function. For enzymes, that function is to catalyse a particular chemical reaction – such as breaking apart chemical bonds in the lignin polymer. Artificial enzymes are more stable and heat-resistant than natural proteins, and can be designed to include more active sites that catalyse the chemical reaction, making them more efficient.

In the new study, the researchers demonstrated that an artificial enzyme made with peptoids showed promise in breaking down lignin. If its efficiency can be further improved, it could have industrial applications, including the production of renewable biofuels for aviation.

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

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