Recycling is not as guilt-free as it sounds. Only about 30% of the plastic that enters soda bottles is made into new plastic, and it often ends up as a low-strength version. Now the researchers report that they have designed an enzyme that can convert 90% of that same plastic to its pristine starting materials. Work is underway to expand the technology and open a demonstration plant next year.
“This is a huge step forward,” said John McGeehan, who runs the Center for Enzyme Innovation at the University of Portsmouth and was not involved in the work.
Polyethylene terephthalate (PET) is one of the most widely used plastics in the world, with some 70 million tonnes produced each year. PET bottles are already recycled in many places. But the current approach has problems. For starters, recycling companies usually end up with a wide range of different colors of plastic. They then use high temperatures to melt them, producing a gray or black plastic starting material that few companies want to use to package their products.
Instead, the material is usually made into carpets or other low-quality plastic fibers that end up in a landfill or are incinerated. “It’s not really recycling,” says McGeehan.
To get around this concern, scientists looked for enzymes in microbes that break down PET and other plastics. In 2012, researchers from Osaka University found such an enzyme in a compost pile. This enzyme, known as leaf branch compost cutinase (LLC), cuts the links between the two building blocks of PET: terephthalate and ethylene glycol. But LLC, which has evolved to break down the waxy protective coating on the leaves of many plants, slowly breaks only the PET bonds, and disintegrates after only a few days of working at 65 ° C, the temperature at which the PET begins to soften , which allows the enzyme to move more easily through the polymer to reach the links it seeks to break.
To reorganize the LLC, Alain Marty, scientific director of Carbios, a sustainable plastic company, teamed up with Isabelle Andre, expert in enzyme engineering at the University of Toulouse. They began by analyzing the crystal structure of the enzyme, identifying key amino acids at the site where the enzyme binds to chemical bonds between the PET terephthalate groups and ethylene glycol. They also looked for ways to run the enzyme at higher temperatures.
The researchers then generated hundreds of mutant enzymes that modify amino acids at the binding site and add thermal stabilizers. They then mass-produced the mutants in bacteria and screened them for effective PET breakers. After repeating this process for several rounds, they isolated a mutant enzyme that is 10,000 times more effective at breaking PET bonds than the native LLC. It also works without decomposing at 72 ° C, close to the temperature at which PET melts.
In a small reactor designed to test the enzyme, the team discovered that it could break down 90% of 200 grams of PET in 10 hours. The researchers then used the building blocks of the enzyme-generated terephthalate and ethylene glycol to generate new PET and produce plastic bottles as solid as conventional plastic, they report today in Nature.
“It’s very exciting,” said McGeehan. “It shows that it is really viable. “
However, its economic viability remains uncertain. McGeehan notes that a main advantage is that the enzyme has no difficulty in making pure PET building blocks from a mixture of plastics containing other than PET, even PET bottles of different colors . This is because the designed enzyme only breaks the bonds connecting the two PET components, bringing them back to their original shape, while ignoring the dyes and other plastics in the mixture. As a result, he says, businesses and consumers may be willing to pay a little more for recycled plastic as durable and attractive as virgin material.
Marty says Carbios is building a demonstration plant that is expected to recycle hundreds of tonnes of PET per year. The enzyme cannot recycle other major types of plastics, such as polyethylene and polystyrene, which have bonds between building blocks that are more difficult to break. But if successful, it could help society deal with one of the most difficult plastic problems we face.