While scrambling through a compost heap at a cemetery in Leipzig, Christian Sonnendecker and his research team found seven enzymes they had never seen before.
They were looking for proteins that would eat PET plastic – the most produced plastic in the world. It is often used for bottled water and groceries such as grapes.
The scientists did not expect much when they brought the samples back to the laboratory, Sonnendecker said when DW visited her laboratory from the University of Leipzig.
It was only the second dump they had rummaged through and they thought pet food enzymes were rare.
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But in one of the samples they found an enzyme, or polyester hydrolase, called PHL7. And it shocked them. The PHL7 enzyme disintegrates a whole piece of plastic in less than a day.
Two enzymes ‘eat’ plastic: PHL7 vs. LCC
PHL7 seems to ‘eat’ PET plastic times faster than LCC, a standard enzyme used today in PET plastic food experiments.
To ensure that their discovery was not a coincidence, Sonnendecker’s team compared PHL7 with LCC, using both enzymes that degrade multiple plastic containers. And they found that it was true: PHL7 was faster.
“I would have thought you would have to sample hundreds of different sites before you find any of these enzymes,” said Graham Howe, an enzymeologist at Queens University in Ontario, Canada.
Howe, who is also studying PET degradation but was not involved in the Leipzig study, seemed surprised 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 everyone
Although PET plastic can be recycled, it is not biodegradable. Like nuclear waste as a nasty remark to your partner, once PET plastic is made, it never really goes away.
It can be transformed into new products – it is not difficult to make a bag from, for example, recycled water bottles. But the quality of the plastic weakens with each cycle.
That said, a lot of PET is eventually transformed into products like carpets and – yes – an exorbitant number of tote bags that end up in landfills.
There are two ways to look at solving this problem: The first is to stop the production of all PET plastic.
But the material is so simple that even if companies stop producing it right away, there will still be millions of millions of empty soft drink bottles – or tote bags made from those bottles – for thousands of years.
The second way is to force the plastic to degrade. Scientists have been trying to find enzymes that will do this for decades and in 2012 they discovered LCC, or “leaf-branch compost cutinase.”
LCC was a major breakthrough because it showed that PETase, a component of LCC, can be used to degrade PET plastic when it is combined with another enzyme known as an esterase.
Esterase enzymes are used to break down 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 substance and eats it as a natural polymer.
Engineering the enzyme
Since the discovery of LCC, researchers like Sonnendecker have been searching for new PET food enzymes in nature. LCC is good, they say, but it has limitations. It is fast for what it is, but it still takes days to degrade PET and the reactions must occur at very high temperatures.
Other scientists and researchers have been trying to figure out how to construct LCC to make it more efficient.
A French company called Carbios does that. They are engineering LCC to make a faster, more efficient enzyme.
Elsewhere, researchers at the University of Texas at Austin have created a pet food protein using a machine learning algorithm. They say their protein can degrade PET plastic in 24 hours.
David Zechel, a professor of chemistry at Queen’s University, said that this approach always starts with something that is known – the researchers do not necessarily find something new, but work to improve what has already been discovered.
This type of engineering is important when researchers are trying to find the optimal enzyme to degrade PET, Zechel said.
Sonnendecker’s work shows that “we have not even remotely scratched the surface” in terms of the potential of naturally occurring enzymes “with respect to PET,” he said.
Bottles are not yet biodegradable
Sonnendecker’s newly discovered enzyme also has its limitations. It can break down the containers in which you buy your grapes in the bakery, but it can not break down a bottle of soft drink. Not yet.
The PET plastic used in beverage bottles is stretched and chemically modified, making it harder to biodegrade than the PET used in grape containers.
In testing, Sonnendecker’s team has developed a pre-treatment that is applied to PET bottles, making it easier for the enzyme to degrade the plastic. But that research has yet to be published.
With the help of the industry, the researcher said, technology using PHL7 to degrade PET on a large scale could be ready in about four years.