Your favorite pair of tennies, kicks or trainers may soon become a little more sustainable — thanks to a microbe found in a Danish compost heap and a handful of cleverly edited genes. In a study published Sept. 11 in the journal Chem Catalysis, scientists at Aarhus University in Denmark and the University of Porto in Portugal engineered an enzyme that can chew up the polyurethane foam in shoe soles, breaking it into smaller, reusable pieces.
Polyurethane is everywhere. It pads your sneakers, cushions your mattress and scrubs your dishes. Companies around the globe churn out roughly 22 million tons of the stuff every year — more than 5% of all plastics hitting the market. A tough class of these materials called "thermosets" shows up in shoe soles and heels, and it's notoriously hard to recycle. Most of it ends up piling in landfills.
"We have flooded the environment with so much plastic of different kinds," said Rosie Graham, a co-first author of the study at Aarhus University. "But there are lots of organisms out there that are already using these materials as a source of energy."
So Graham and her colleagues looked to nature for a helper. They studied bacteria that already break down garbage around the world, then zoomed in on the enzymes those organisms use to decompose plastic. One enzyme stood out: CCPUR1, from a bacterium called Chelatococcus composti, first found in compost samples from Denmark.
The real power of enzymes, Graham says, is that they work under gentle conditions — lower temperatures and pressures than today's recycling technologies, which saves energy. But the team wanted their plastic-eater to be even better. When an enzyme breaks down a material, it first grabs hold of it, a bit like two puzzle pieces clicking together. Using complex computer simulations, the researchers studied how CCPUR1 fit with plastic-like materials. Then they edited the DNA of the bacterium to swap out a few key building blocks, called amino acids.
"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?" asked co-first author Pedro Paiva of the University of Porto.
After lots of trial and error, one mutant enzyme proved to have a ravenous appetite. The team cut up pieces from a real shoe sole, mixed them with the modified enzyme, and waited. After three days, this plastic-eater had chewed up about 1.4% of the polyurethane in the material, splitting it into smaller, reusable pieces.
The research is still in its infancy, but it points to a future where enzymes take a real bite out of plastic waste — without any pre-processing of the material. "This is an early demonstration that shows that we can degrade polyurethane with enzymes without processing it beforehand," said Paiva. "It's quite significant."