Nancy Huang

Oct 21st 2022

Plastic-Eating Enzyme Could Allow Mountains of Plastic Waste to Be Fully Recycled


People like recycling. Tossing an empty bottle or old mail into the proper bin feels like tidying up and doing your part for the environment. But much of the plastic in recycling bins is never actually recycled, even if it is clean and accepted by the local recycling center, according to NPR. One of the problems is that current methods for recycling plastic are very expensive and produce plastic of inferior quality compared with new plastic, which is cheap to make from oil. As a result, we are inundated with plastic waste.

Plastic-Eating Enzyme Offers Hope

A genetically engineered, plastic-eating enzyme may soon improve plastic recycling on an industrial scale, as the journal Nature reports. The naturally occurring version of this enzyme was discovered in 2016, in bacteria that were living near a plastic recycling facility in Japan, according to the journal Science. This PETase enzyme breaks down a commonly used plastic — called PET, or polyethylene terephthalate — into its component parts, which can then be reassembled into shiny new plastic.

In order for enzymatic degradation of PET to be commercially viable, researchers needed to develop a faster version of PETase. Scientists at the University of Texas at Austin started with the known sequence and structure of naturally occurring PETase plus two genetically engineered versions that had shown improved performance. A machine learning model was trained on a dataset of 19,000 protein structures of similar size and then used to predict which mutations would improve stability and activity of this plastic-eating enzyme. A total of 159 mutation combinations were then produced in bacteria and tested, with a version containing 5 mutations giving the best results.

Enzymatic degradation offers major advantages over the current methods for recycling plastic.

Current Methods for Recycling Plastic

The current methods for recycling plastic involve collecting, transporting, sorting and processing the many different kinds of plastics that are used in everyday life. The process is so intensive that entire loads of plastic might be thrown out if they have been soiled with food or other liquids. In many parts of the United States, otherwise acceptable loads of plastic have been thrown into landfills or incinerated because there were no companies willing to process the material to try and make a profit. The supply of old plastic is much greater than the demand.

Old plastic is processed through washing, shredding, further separation and melting to form plastic pellets, as Recycle Track Systems explains. These plastic pellets might be used to manufacture polyester clothing, carpet or flower pots, but they are rarely used to make new food containers. That’s because plastic gets weaker every time it goes through this recycling process.

Plastics are made of polymers — long chains created by repeating monomer blocks. Every time a piece of plastic is melted for recycling, the polymers get shorter and the plastic becomes weaker. Thus, plastic can only be “recycled” in this way once or twice before becoming trash. In contrast, glass and metal can be recycled again and again without any loss of quality.

The new plastic-eating enzyme provides a way to break PET down to monomer blocks, which can then be used to make shiny new plastic.

New Enzyme on the Block

The new, fast-acting enzyme was named FAST-PETase and found to be highly effective at 50° C (122° F). This relatively low temperature translates into lower costs and less environmental impact for enzymatic degradation. FAST-PETase was tested on small samples of 51 different PET products, including packaging for food, beverages, medications, office supplies, household goods and cosmetics. All the samples were fully degraded within 1 week, with several requiring just 1 or 2 days. In contrast, untreated plastics may take hundreds of years to degrade.

FAST-PETase was able to depolymerize a large plastic cake tray within 48 hours, and a new plastic item was successfully constructed from the waste. Degradation of an entire plastic water bottle was aided by melting the bottle first, followed by 2 weeks at 50° C.

The Advantage of Enzymes

Compared with chemical approaches, enzymes can work at lower temperatures and thus require less energy input. Enzymes are also non-toxic, biodegradable and can be produced in large quantities by microorganisms. Furthermore, enzymes act very specifically on one or a few substrates, making them ideal for mixed waste streams. Examples might include plastics contaminated with food, fabrics that are a mix of cotton and polyester and packaging that includes layers of different materials (such as a potato chip bag with plastic and aluminum).

Obvious applications for FAST-PETase include the recycling of consumer plastics, reducing industrial waste and cleaning up landfills. The study authors are also looking toward environmental remediation.

Our Plastic Mess

According to a study in Science Advances, the world was producing 200 times more plastic in 2015 than it did in the 1950s. Around 9% of that plastic has been recycled, 12% has been incinerated, and 79% has accumulated in landfills or the natural environment. As Cas — a division of the American Chemical Society — describes, over 350 million tons of plastic are being produced each year, including over 50 million tons of PET. Over 8 million tons of plastic end up in the ocean each year and are causing real harm to marine life. These plastics break into microplastics that have been detected in every corner of the world, from the highest peak on land to the deepest trench in the ocean.

While much needs to be done to clean up the mess we’ve made, a plastic-eating enzyme that allows for the endless recycling of plastic can be used to take a bite out of the problem.

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