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Synthetic Biology Technology

Researchers produce enzymes which could create ‘perfect’ bio-recycling loop for plastics.

The enzyme MHETase ia a large complex folded molecule. MHET molecules from PET plastic dock on to MHETase at certain locations and are then split into their building blocks. Illustration: Martin Künsting

“MHETase has a surface that is about twice as large as the surface of PETase and has therefore considerably more potential to optimise it for decomposition of PET.”

A Germany-based research team has been able to produce enzymes which can break down PET plastics and other polymers into their basic building blocks.

The researchers from the University of Greifswald (@wissen_lockt) and Helmholtz-Zentrum-Berlin (HZB) have figured out the 3D structure of a plastic-digesting enzyme called MHETase. MHETase can be used in combination with a second enzyme, PETase, to break PET plastic into its basic components, which can then be used to produce new plastic.

According to the researchers, applying these enzymes could create a “perfect” bio-recycling loop for plastics – allowing them to be broken down and reprocessed without waste – and cutting out the need for crude oil to produce virgin materials.

The HZB research, recently published in journal Nature Communications, forms the final piece of a puzzle that researchers have been working to solve since 2016 – when Japanese scientists identified a bacteria that lives on and partially digests PET using PETase and MHETase.

In the following two years, research teams around the world raced to replicate the structure of the enzymes. Last year, the structure of PETase, the more simple of the two, was discovered independently by teams in Korea, China, UK, US and Brazil, reflecting the high level of international interest.

MEHTase, which completes the decomposition process started by PETase, proved more difficult to crack. According to HZB researcher Dr Gert Weber, a single MHETase molecule consists of 600 amino acids, or about 4000 atoms. But he added that it had a great potential to optimise plastic bio-recycling.

Weber said: “MHETase has a surface that is about twice as large as the surface of PETase and has therefore considerably more potential to optimise it for decomposition of PET.”

Weber and biotechnologist Professor Uwe Bornscheuer approached the problem by studying how the enzyme bound to MHET – a smaller building block of PET that is produced by plastic digested by PETase.

In a statement, Weber said that to break down these smaller building blocks into PET’s basic components, the enzyme first needs to “dock” them in a tailor-made 3D structure.

“We can now exactly localise where the MHET molecule docks to MHETase and how MHET is then split into its two building blocks: terephthalic acid and ethylene glycol,” Weber added, explaining that the BESSY II synchrotron was used to produce extremely bright X-rays that shed light on the complex structure.

Weber said: “In order to see how MHETase binds to PET and decomposes it, you need a fragment of plastic that binds to MHETase but is not cleaved by it.”

A member of Weber’s prior research team in Greifswald, Dr. Gottfried Palm, cut up a PET bottle, chemically decomposed the PET polymer and synthesised a small chemical fragment from it that binds to MHETase but can no longer be cleaved by it. From this ‘blocked’ MHETase, tiny crystals were grown for structural investigations at the HZB.

According to the researchers, neither PETases nor MHETase are particularly efficient yet.

Plastics have only been around on this scale for a few decades; even bacteria with their rapid successions of generations and rapid adaptability have not managed to develop a perfect solution through the evolutionary process of trial and error over such a short time,” he explained.

Weber’s team is working to accelerate the process. They have used their 3D structure to create a new version of MHETase that was able to digest MHET and another PET building block, BHET, more efficiently than the natural enzyme.

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