Your favorite pair of tennies, kicks, or trainers may soon become a little more sustainable. In a study publishing September 11 in the Cell Press journal Chem Catalysis, researchers have developed a new kind of enzyme based on a bacterium found in compost that can degrade the polyurethane material in shoe foams. The results could one day help recycle plastic waste from shoes, mattresses, kitchen sponges, and more.
"We have flooded the environment with so much plastic of different kinds," says co-first author Rosie Graham of Aarhus University in Denmark. "But there are lots of organisms out there that are already using these materials as a source of energy."
Across the globe, companies churn out roughly 22 million tons of polyurethane every year, more than 5% of all plastics hitting the market. A special class of these polyurethane materials, known as "thermosets," shows up in shoe soles, heels, and more.
To address these products piling up in landfills, Graham and her colleagues drew inspiration from nature. They studied the bacteria and other microorganisms being used around the world to break down garbage, then took a close look at the specific enzymes those organisms were using to decompose plastics. One enzyme called CCPUR1 from the bacterium Chelatococcus composti, a microbe found in samples of compost from Denmark, stood out for its ability to degrade polyurethane.
"The real plus about using enzymes for recycling is that they work under mild conditions, so at lower temperatures and pressures than currently used in most recycling technologies today," says Graham.
When an enzyme degrades a substrate, such as polyurethane, it first grabs hold of that substrate, a bit like two puzzle pieces clicking together. The researchers wanted to see if they could make puzzle pieces that fit together even better to more effectively break down plastics. So, they put CCPUR1 to the test. They first employed complex computer simulations to explore how the enzyme interacts with plastic-like materials, then used a series of biochemistry tools to tweak the structure of enzymes.
The researchers then turned to the DNA of C. composti. They edited that code to switch out a few of the key amino acids that make up the CCPUR1 enzyme.
"What if we try to change specific amino acids to larger ones that could better fill the empty space between the enzyme and the substrate?" says co-first author Pedro Paiva of the University of Porto in Portugal.
After a lot of trial and error, the researchers landed on a winner. One of their mutant enzymes had a ravenous appetite for polyurethane. The team cut up pieces from the soles of a shoe and mixed them with the modified enzyme. After three days, this plastic-eater had chewed up about 1.4% of the polyurethane in that material—splitting the waste product into smaller, reusable pieces.
"The research highlights the power of enzymes," says co-first author Rosie Graham of Aarhus University in Denmark.
The research is still in its infancy, but the results show how enzymes could one day take a bite out of plastic waste, says the team.
"This is an early demonstration that shows that we can degrade polyurethane with enzymes without processing it beforehand," says Paiva. "It's quite significant."
