Researchers from Washington State University (WSU) and the Department of Energy’s Pacific Northwest National Laboratory (PNNL) have devised an artificial enzyme that digests lignin, which has stubbornly resisted previous attempts to develop it into an economically useful energy source. An open-access paper on their work is published in Nature Communications.

Lignin is in all vascular plants, where it forms cell walls and provides plants with rigidity. Lignin allows trees to stand, gives vegetables their firmness and makes up about 20-35% of the weight of wood. Because lignin turns yellow when exposed to air, the wood products industry removes it as part of the fine papermaking process. Once removed, lignin is often inefficiently burned to produce fuel and electricity. Chemists have tried and failed for more than a century to make valuable products from lignin.

Our bio-mimicking enzyme showed promise in degrading real lignin, which is considered to be a breakthrough. We think there is an opportunity to develop a new class of catalysts and to really address the limitations of biological and chemical catalysts.

—Xiao Zhang, co-corresponding author and WSU associate professor

Zhang also holds a joint appointment at PNNL.

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.

—Chun-Long Chen, co-corresponding author and PNNL researcher

Chen is also an affiliate professor in 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 mushroom-covered log decomposes 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 hard to use in an industrial process. They are also expensive.

It’s really hard to produce these enzymes from microorganisms in a meaningful quantity for practical use. 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.

—Xiao Zhang

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.

… we developed a class of self-assembled peptoid/hemin (Pep/hemin) nanomaterials with tunable active sites and microenvironments that mimic peroxidases for lignin depolymerization, by taking advantage of the high tunability of peptoids (or poly-N-substituted glycines) and the uniqueness of their self-assembled crystalline nanomaterials in aligning active sites. Compared to peptides, peptoids can be easily synthesized to achieve a greater side chain diversity while exhibiting much higher chemical and thermal stabilities. Our recent work has shown that tuning amphiphilic peptoids can lead to the formation of hierarchically structured crystalline nanomaterials, including membrane-mimetic 2D nanosheets and nanotubes.

These peptoid-based nanomaterials are highly stable in various pH conditions and at elevated temperatures. New functionalities and applications of peptoid nanomaterials can be easily realized by adjusting the side chain chemistry and by incorporating and aligning different functional groups. Due to their high tunability and stability, peptoid nanomaterials are promising for creating optimal active sites and microenvironments. All these unique properties suggest that peptoid-based crystalline nanomaterials offer great opportunities for developing peroxidase mimetics.

—Jian et al.

Peptoids were first developed in the 1990s to mimic the function of proteins. They have several unique features, including high stability, that allow scientists to address the deficiencies of the 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 tune their local environments for catalytic activity, and we have a much higher density of active sites, instead of one active site.

—Chun-Long Chen

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

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

The research collaboration was facilitated through 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.

The work was funded by the state of Washington’s Joint Center for Aerospace Technology and Innovation, a program that supports industry and university research collaborations to develop innovative technologies in the aerospace industry, and by the Department of Energy, Office of Science, Office of Basic Energy Sciences as part of the Center for the Science of Synthesis Across Scales, an Energy Frontier Research Center located at the University of Washington.

Additional support was provided by the National Science Foundation (1454575) and the Department of Agriculture National Institute of Food and Agriculture (2018-67009-27902). Peptoid synthesis capabilities were supported by the Materials Synthesis and Simulation Across Scales Initiative, a Laboratory Directed Research and Development program at PNNL.

Resources

• Jian, T., Zhou, Y., Wang, P. et al. (2022) “Highly stable and tunable peptoid/hemin enzymatic mimetics with natural peroxidase-like activities.” Nat Commun 13, 3025 doi: 10.1038/s41467-022-30285-9