‘A huge step forward.’ Mutant enzyme could vastly improve recycling of plastic bottles

Recycling isn’t as guilt-free as it seems. Only about 30% of the plastic that goes into soda bottles gets turned into new plastic, and it often ends up as a lower strength version. Now, researchers report they’ve engineered an enzyme that can convert 90% of that same plastic back to its pristine starting materials. Work is underway to scale up the technology and open a demonstration plant next year.

“This is a huge step forward,” says John McGeehan, who directs the center for enzyme innovation at the University of Portsmouth and who was not involved with the work.

Polyethylene terephthalate (PET) is one of the world’s most commonly used plastics, with some 70 million tons produced annually. PET bottles are already recycled in many places. But the current approach has problems. For starters, recycling companies typically end up with a broad mix of different colors of the plastic. They then use high temperatures to melt those down, producing a gray or black plastic starting material that few companies want to use to package their products.

Instead, the material is typically turned into carpets or other low-grade plastic fibers that eventually end up in a landfill or get incinerated. “It’s not really recycling at all,” McGeehan says.

To get around this concern, scientists have searched for enzymes in microbes that break down PET and other plastics. In 2012, researchers at Osaka University found one such enzyme in a compost heap. That enzyme, known as leaf-branch compost cutinase (LLC), snips the bonds between PET’s two building blocks: terephthalate and ethylene glycol. But LLC, which evolved to break down the waxy protective coating on many plants’ leaves, slowly breaks apart PET bonds only, and falls apart after just a few days of working at 65°C, the temperature at which PET begins to soften, which allows the enzyme to more easily wiggle into the polymer to reach the links it seeks to break.

To re-engineer LLC, Alain Marty, the chief scientific officer at Carbios, a sustainable plastics company, teamed up with Isabelle Andre, an enzyme engineering expert at the University of Toulouse. They started by analyzing the crystal struc... (Read more)

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