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Researchers Solve the Structure of Key Enzyme for Sustainable Recycling of PET Polymer

The molecular structure of a key enzyme called MHETase has now been solved by researchers from Helmholtz-Zentrum-Berlin (HZB) and the University of Greifswald and Helmholtz-Zentrum-Berlin (HZB) at BESSY II.

The enzyme MHETase is a huge and complex molecule. MHET-molecules from PET plastic dock at the active site inside the MHETase and are broken down into their basic building blocks. (Image credit: M. Künsting/HZB)

Initially discovered in bacteria, the MHETase enzyme along with a second enzyme called PETase is capable of breaking down the extensively used plastic PET into its simple building blocks. Such a 3D structure has already enabled the scientists to create a variant of MHETase with improved activity so that it can be utilized along with PETase, to recycle PET in a sustainable manner. The study results have been reported in the research journal, Nature Communications.

Plastics are known to be excellent materials because they are versatile and nearly eternally long-lasting; however, these very properties also represent a problem because plastic particles can now be found in all the places, after just about a century of creating plastics; for instance, they can be found in the food chain, in the air, in the oceans, and in groundwater. Every year, about 50 million tons of the industrially vital polymer—PET— are created. At present, only a small amount of plastics is being recycled through costly and energy-intensive processes which result in downgraded products or consecutively rely on adding “fresh” crude oil.

Bacteria on PET discovered in 2016

A team of Japanese researchers had identified a bacterium in 2016 that grows on PET and partly feeds on it. The researchers discovered that this microbe has two unique enzymes called MHETase and PETase, which have the ability to digest plastic polymers like PET. Plastic is broken down by PETase into tinier PET building blocks, mainly MHET, which, in turn, is broken down by MHETase into the two simple precursor building blocks of PET called ethylene glycol and terephthalic acid. These two components are extremely useful for producing new PET without having to add any crude oil—for a closed sustainable cycle of production and recovery.

In 2018: Structure of PETase solved

Then, a number of research groups finally and independently solved the structure of PETase in April 2018, and also involved in the experiments was the Diamond Light Source. Nevertheless, PETase is just part of the solution. It is just as significant to define the structure of MHETase—the second enzyme.

Now: 3D architecture of MHETase decoded

MHETase is considerably larger than PETase and even more complex. A single MHETase molecule consists of 600 amino acids, or about 4000 atoms. 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,” explained Dr Gert Weber, biochemist and structural biologist from the joint Protein Crystallography research group at the Helmholtz-Zentrum Berlin and Freie Universität Berlin.

At the time of an interim professorship at the University of Greifswald, Weber contacted Professor Uwe Bornscheuer, a biotechnologist at the Institute of Biochemistry and who was already involved with enzymes that degrade plastic. The duo teamed up to frame the concept of solving the MHETase structure and subsequently applying this understanding to improve the enzyme for use in PET recycling. In order to accomplish this feat, they initially needed to remove the MHETase enzyme from bacterial cells and then purify it. Within this partnership, the research teams have currently succeeded in achieving the intricate 3D architecture of MHETase at the synchrotron source— BESSY II—at HZB in Berlin.

MHETase observed “in action”

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.

Dr Gert Weber, Biochemist and Structural Biologist, Helmholtz-Zentrum Berlin and Freie Universität Berlin.

Dr Gottfried Palm, a member of Weber’s previous research team in Greifswald, first cut up a PET bottle, then chemically decomposed the PET polymer, and finally produced a tiny chemical fragment from it that adheres to MHETase but this piece cannot be cleaved by it anymore. Subsequently, from this “blocked” MHETase, very small crystals were grown for structural analyses at the HZB.

The structural investigations enabled us to watch MHETase virtually ‘at work’ and develop strategies for how to optimise this enzyme.

Dr Gert Weber, Biochemist and Structural Biologist, Helmholtz-Zentrum Berlin and Freie Universität Berlin

Thanks to the joint research group format, we have the means to offer beamtime access on the highly demanded BESSY II MX beamlines for measurements very quickly at any time.

Dr Manfred Weiss, Helmholtz-Zentrum Berlin.

Dr Weiss is responsible for the BESSY II MX beamlines.

MHETase’s 3D architecture, in fact, exhibits certain unique traits—enzymes like MHETase initially adhere to their molecule of interest before a chemical reaction takes place.

For a molecular breakdown, a customized enzyme is required: “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,” stated Weber.

Next steps: Increasing the efficiency

Conversely, both MHETase and PETase are not specifically 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.

Dr Gert Weber, Biochemist and Structural Biologist, Helmholtz-Zentrum Berlin and Freie Universität Berlin.

Thanks to the clarification of the structure of this very important enzyme, we have now also been able to plan, produce and biochemically characterise variants that show significantly higher activity than natural MHETase and are even active against another intermediate product of PET degradation, BHET.

Uwe Bornscheuer Professor and Biotechnologist, Institute of Biochemistry, University of Greifswald.

Going forward, Uwe Bornscheuer will work on methodically improving the enzymes MHETase and PETase for their task—that is, the decomposition of PET.

Gert Weber is now planning to complement these analyses with additional work on biological structures so as to meticulously synthesize plastic-digesting enzymes for environmental use. Access to the IT infrastructure and the measuring stations of HZB is crucial for this.

Outlook: perfect recycling

The development of such kinds of enzymes in closed biotechnological cycles, for instance, may provide a means to truly break down various polymers, including PET plastics, into their simple building blocks. Moreover, this approach would lead to an ideal recycling process and a long-lasting solution to the problem of plastic waste—the development of plastics would be a closed sustainable cycle and would not rely on crude oil anymore.

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