A new artificial enzyme has been shown to chew through lignin, the tough polymer that helps woody plants maintain their shape. Lignin also contains enormous potential for renewable energy and materials.
Report in the journal nature communicationA team of researchers from Washington State University and the Department of Energy’s Pacific Northwest National Laboratory showed that their artificial enzyme succeeded in digesting lignin, which has stubbornly resisted previous attempts to develop it into an economically useful source of energy.
Lignin, the second most abundant renewable carbon source on Earth, is mostly lost as a fuel source. When wood is burned for cooking, lignin byproducts help impart that smoky flavor to food. But when burned, all that carbon is released into the atmosphere instead of capturing it for other uses.
“Our bio-mimicking enzyme showed promise in breaking down true lignin, which is considered a breakthrough,” said Xiao Zhang, a corresponding author on the paper and an associate professor in WSU’s Gene and Linda Voiland School of Chemical Engineering and Bioengineering. Zhang also has a joint tenure 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 present in all vascular plants, where it forms cell walls and gives plants strength. Lignin makes trees 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, it is often burned inefficiently to produce fuel and electricity.
Chemists have tried and failed for more than a century to make valuable products from lignin. That track record of frustration is about to change.
A better than nature
“This is the first natural mimetic enzyme that we know can efficiently digest lignin to produce compounds that can be used as biofuels and for chemical production,” added Chun-Long Chen, a corresponding author, a Pacific Northwest National Laboratory researcher. , and affiliated with professor of chemical engineering and chemistry at the University of Washington.
In nature, fungi and bacteria are able to break down lignin with their enzymes, and this is how a mushroom-covered trunk perishes in the forest. Enzymes provide a much more environmentally friendly process than chemical degradation, which requires a lot of heat and uses more energy than it produces.
But natural enzymes break down over time, making them difficult to use in an industrial process. They are also expensive.
“It’s really difficult to produce these enzymes from microorganisms in a meaningful amount for practical use,” Zhang said. “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 researchers haven’t been able to use natural enzymes to work for them, they’ve learned a lot about how they work over the decades. A recent review article by Zhang’s research team outlines the challenges and barriers to the application of lignin-degrading enzymes. “Understanding these barriers provides new insights for designing biomimetic enzymes,” Zhang added.
Peptoid scaffold is the key
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 were then self-assembled into nanoscale crystalline tubes and plates. Peptoids were first developed in the 1990s to mimic the function of proteins. They have several unique characteristics, including high stability, which 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 tailor their local environment to catalytic activity,” Chen said, “and we have a much higher density of active sites, rather than one active site.”
As expected, these artificial enzymes are also much more stable and robust than the natural versions, allowing them to operate at temperatures up to 60 degrees Celsius, a temperature that would destroy a natural enzyme.
“This work really opens up new opportunities,” Chen says. “This is an important step towards converting lignin into valuable products in an environmentally friendly way.”
If the new bio-mimetic enzyme can be further improved to increase the conversion yield, to generate more selective products, it has the potential for scaling up to industrial scale. The technology offers new routes to renewable materials for, among other things, aviation biofuels and biobased materials.
The research collaboration was made possible by 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 study.
The work was funded by the Washington State Joint Center for Aerospace Technology and Innovation, a program that supports industry-university 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 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.
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