Science Highlight

12.04.2019

"Molecular scissors" for plastic waste

A research team from the University of Greifswald and Helmholtz-Zentrum-Berlin (HZB) has solved the molecular structure of the important enzyme MHETase at BESSY II. MHETase was discovered in bacteria and together with a second enzyme - PETase - is able to break down the widely used plastic PET into its basic building blocks. This 3D structure already allowed the researchers to produce a MHETase variant with optimized activity in order to use it, together with PETase, for a sustainable recycling of PET. The results have been published in the research journal Nature Communications.

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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“, explains biochemist and structural biologist Dr. Gert Weber from the joint Protein Crystallography research group at the Helmholtz-Zentrum Berlin and Freie Universität Berlin. During an interim professorship at the University of Greifswald, Weber there contacted the biotechnologist Prof. Uwe Bornscheuer at the Institute of Biochemistry, who was already involved with plastic-degrading enzymes. Together, they developed the idea of solving the structure of MHETase and then using this insight to optimise the enzyme for applications in PET recycling. To do this, they first had to extract the enzyme from bacterial cells and purify it. Within this collaboration, the teams have now succeeded in obtaining the complex three-dimensional architecture of MHETase at BESSY II, the synchrotron source 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”, explains Weber. 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. “The structural investigations enabled us to watch MHETase virtually ‘at work’ and develop strategies for how to optimise this enzyme”, explains Weber.

“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”, says Dr. Manfred Weiss, who is responsible for the BESSY II MX beamlines. The three-dimensional architecture of MHETase actually displays some special features: enzymes such as MHETase bind to their target molecule first before a chemical reaction occurs. For breakdown of a molecule you need a tailor-made enzyme: “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”, says Weber.

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