While searching through a compost heap in a Leipzig cemetery, Christian Sonnendecker and his research team found seven enzymes they had never seen before. They were looking for proteins that could eat PET plastic, the most produced plastic in the world. It is commonly used for bottled water and edibles such as grapes. The scientists didn’t expect much when they brought the samples to the lab, Sonnendecker said when DW visited his lab at the University of Leipzig. It was only the second dump they had rummaged through and they thought PET-eating enzymes were weird. The best of Express PremiumTop qualityPremiumPremiumPremium But in one of the samples they found an enzyme, or polyester hydrolase, called PHL7. And she surprised them. The PHL7 enzyme disintegrated an entire piece of plastic in less than a day.
Two enzymes ‘eat’ plastic: PHL7 vs. LCC
PHL7 appears to ‘eat’ PET plastic much faster than LCC, a standard enzyme used in PET plastic consumption experiments today. To make sure their discovery wasn’t a fluke, Sonnendecker’s team compared PHL7 to LCC, with both enzymes breaking down multiple plastic containers. And they found out that it was true: PHL7 was faster. “I would have thought you would need to sample hundreds of different sites before you found one of these enzymes,” said Graham Howe, an enzymologist at Queens University in Ontario, Canada. Howe, who also studies PET degradation but was not involved in the Leipzig research, seemed stunned by the study published in Chemistry Europe. “Apparently you go to nature and there will be enzymes that do this everywhere,” Howe said.
PET plastic is for everyone
Although PET plastic can be recycled, it does not biodegrade. Like nuclear waste or a nasty comment to your partner, once PET plastic is created, it never really goes away. Can be reshaped into new products; for example, it is not difficult to create a handbag with recycled water bottles. But the quality of the plastic weakens with each cycle. So a lot of PET eventually gets turned into products like rugs and, yes, an inordinate amount of tote bags that end up in landfills. There are two ways to approach solving this problem: the first is to stop the production of all PET plastic. But the material is so common that even if companies stopped producing it immediately, there would still be millions of empty soda bottles, or bags made from those bottles, lying around for thousands of years. The second way is to force the plastic to degrade. Scientists have been trying to find enzymes that do that for decades and in 2012 they found LCC, or “Leaf Branch Compost Cutinase.” LCC was a breakthrough because it showed that PETase, a component of LCC, can be used to degrade PET plastic when combined with another enzyme known as esterase. Esterase enzymes are used to break chemical bonds in a process called hydrolysis. Scientists working at LCC have discovered that the enzyme does not differentiate between natural polymers and synthetic polymers, the latter being plastics. Instead, LCC recognizes PET plastic as a natural substance and eats it as if it were a natural polymer.
Since the discovery of LCC, researchers like Sonnendecker have been searching for new PET-devouring enzymes in nature. LCC is good, they say, but it has limitations. It’s fast for what it is, but it still takes days to break down PET and the reactions have to happen at very high temperatures. Other scientists and researchers have been trying to figure out how to design LCC to make it more efficient. A French company called Carbios is doing it. They are engineering LCC to create a faster and more efficient enzyme. Elsewhere, researchers at the University of Texas at Austin have created a protein that feeds on PET using a machine learning algorithm. They say that its protein can degrade PET plastic in 24 hours. David Zechel, professor of chemistry at Queen’s University, said these approaches always start with something that is known: researchers don’t necessarily find anything new, but rather work to improve what has already been discovered. This type of engineering is important as researchers try to create the optimal enzyme to degrade PET, Zechel said. Sonnendecker’s work shows that “we haven’t even remotely scratched the surface” in terms of the potential of natural enzymes “with respect to PET,” he said.
The bottles still do not biodegrade
Sonnendecker’s newly discovered enzyme also has its limitations. You can break down the containers you buy your grapes in at the supermarket, but you can’t break down a bottle of soda. Not yet. The PET plastic used in beverage bottles is stretched and chemically altered, making it more difficult to biodegrade than the PET used in grape packaging. In testing, Sonnendecker’s team has developed a pretreatment that is applied to PET bottles, making it easier for the enzyme to break down the plastic. But that research has yet to be published. With industry help, the researcher said, technology using PHL7 to break down PET on a large scale could be ready in about four years.