While scavenging through a compost pile in a Leipzig cemetery, Christian Sonendekar and his research team found seven enzymes they had never seen before.
They were looking for a protein that could eat away at PET plastic – the most widely produced plastic in the world. It is commonly used for bottled water and groceries such as grapes.
When DW visited the laboratory of the University of Leipzig, Sonendekar said the scientists didn’t expect much when they brought the samples back to the lab.
This was only the second dump they rummaged through and thought the enzymes that eat PET were rare.
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But in one sample, they found an enzyme, or polyester hydrolase, called PHL7. And it surprised them. The PHL7 enzyme disintegrated the entire piece of plastic in less than a day.
Two enzymes ‘eat’ the plastic: PHL7 vs LCC
PHL7 ‘eats’ PET plastics faster than LCC, which is a standard enzyme used in experiments eating PET plastics today.
To make sure their discovery was not a fluke, Sonendekar’s team compared PHL7 to LCC, both enzymes that degrade many plastic containers. And they found it to be true: PHL7 was faster.
“I would have thought you had to sample from hundreds of different sites before you could find 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, was intrigued by the study published in Chemistry Europe.
“Obviously, you go into nature and there’s going to be enzymes that do this everywhere,” Howe said.
PET plastic is everyone
Although PET plastic can be recycled, it does not biodegrade. Like nuclear waste or a snarky comment to your partner, once PET becomes plastic, it never really goes away.
It can be turned into new products – it’s not difficult to make a tote bag from recycled water bottles, for example. But the quality of plastic degrades with each cycle.
So, a lot of PET is eventually turned into products like carpets and – yes – the exorbitant number of tote bags end up in landfill sites.
There are two ways to solve this problem: The first is to stop producing all PET plastics.
But the material is so common that even if companies immediately cease production of it, millions of empty soft drink bottles – or tote bags made from those bottles – will be lying around for thousands of years.
Another way is to force the plastic to deteriorate. Scientists have been trying to find enzymes that would do this for decades, and in 2012 they found LCC, or “leaf-branch compost cutinase.”
LCC was a major success because it showed that PETase, a component of LCC, could be used to degrade PET plastic when it was combined with another enzyme called esterase.
Esterase enzymes are used to break chemical bonds in a process called hydrolysis.
Scientists working on LCC have found that the enzyme does not differentiate between natural polymers and synthetic polymers – the latter being plastic. Instead, the LCC recognizes PET plastic as a naturally occurring material and treats it as if it would be a natural polymer.
Since the discovery of LCC, researchers such as Sonnendecker have been looking for new PET-eating enzymes in nature. LCC is good, they say, but it has its limits. It’s fast for what it is, but it still takes several days for PET to break down and the reactions take place at much higher temperatures.
Other scientists and researchers are trying to figure out how to engineer the LCC to be more efficient.
A French company called Carbios is doing this. They are engineering LCC to create a faster, more efficient enzyme.
Elsewhere, researchers at the University of Texas at Austin have created a PET-eating protein using machine learning algorithms. They say their 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 work to improve what has already been discovered. Huh.
This type of engineering is important as researchers try to create the optimal enzyme to degrade PET, Zechel said.
Sondekar’s work shows that “we haven’t even remotely scratched the surface” in terms of the ability of naturally occurring enzymes to “contain” PET.
Bottles Still Don’t Biodegrade
Sonendekar’s newly discovered enzyme also has its limitations. It can break the containers you buy your grapes at the grocery store, but it won’t break the soft drink bottle. Not now.
The PET plastic used in beverage bottles is stretched and chemically altered, making it more difficult to biodegrade than the PET used in grape containers.
In tests, Sonendekar’s team has developed a pre-treatment that is applied to PET bottles, making it easier for enzymes to degrade the plastic. But that research has not yet been published.
With industry help, the researcher said, the technology that uses PHL7 to break down PET on a large scale could be ready in about four years.