Page 54 - Plastics News April 2019

teChnoLogy

'Molecular scissors' for plastic waste

research team has solved the molecular structure a PET bottle, chemically decomposed the PET polymer
A of the enzyme MHETase at BESSY II. MHETase was and synthesised a small chemical fragment from it that
discovered in bacteria and together with a second enzyme binds to MHETase but can no longer be cleaved by it.
-- PETase -- is able to break down the widely used plastic From this 'blocked' MHETase, tiny crystals were grown
PET into its basic building blocks. This 3D structure already for structural investigations at the HZB. "The structural
allowed the researchers to produce a MHETase variant with investigations enabled us to watch MHETase virtually 'at
optimized activity in order to use it, together with PETase, work' and develop strategies for how to optimise this
for a sustainable recycling of PET. The enzyme MHETase enzyme," explains Weber."Thanks to the joint research
is a huge and complex molecule. MHET-molecules from 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
PET plastic dock at the active site inside the MHETase and and ethylene glycol," says Weber. However, neither PETase
are broken down into their basic building blocks. Dr. Gert nor MHETase are particularly efficient yet.
Weber, biochemist and structural biologist from the joint "Plastics have only been around on this scale for a few
Protein Crystallography research group at the Helmholtz- decades -- even bacteria with their rapid successions of
Zentrum Berlin and Freie Universität Berlin says, "In order generations and rapid adaptability have not managed
to see how MHETase binds to PET and decomposes it, you to develop a perfect solution through the evolutionary
need a fragment of plastic that binds to MHETase but is process of trial and error over such a short time," explains
not cleaved by it.” "MHETase is considerably larger than Weber. "Thanks to the clarification of the structure of this
PETase and even more complex. very important enzyme, we have now also been able to
A single MHETase molecule consists of 600 amino acids, plan, produce and biochemically characterise variants that
or about 4000 atoms. MHETase has a surface that is show significantly higher activity than natural MHETase and
about twice as large as the surface of PETase and has are even active against another intermediate product of
therefore considerably more potential to optimise it for PET degradation, BHET," adds Uwe Bornscheuer. In future,
decomposition of PET," explains Dr. Gert Weber .During Uwe Bornscheuer will work on systematically optimising
an interim professorship at the University of Greifswald, the enzymes PETase and MHETase for their task -- the
Weber there contacted the biotechnologist Prof. Uwe decomposition of PET. Gert Weber plans to supplement
Bornscheuer at the Institute of Biochemistry, who these studies with further work on biological structures in
was already involved with plastic-degrading enzymes. order to systematically develop plastic-digesting enzymes
Together, they developed the idea of solving the structure for environmental applications. Access to the measuring
of MHETase and then using this insight to optimise the stations and the IT infrastructure of HZB is indispensable
enzyme for applications in PET recycling. To do this, they for this.Producing these kinds of enzymes in closed
first had to extract the enzyme from bacterial cells and biotechnological cycles, for example, could be a way to
purify it. Within this collaboration, the teams have now really break down PET plastics and other polymers into
succeeded in obtaining the complex three-dimensional their basic building blocks. This would also be the key
architecture of MHETase at BESSY II, the synchrotron to ideal recycling and a long-term solution to the plastic
source at HZB in Berlin. A member of Weber's prior waste problem: production of plastic would be a closed
research team in Greifswald, Dr. Gottfried Palm, cut up cycle and no longer dependent on crude oil.

Plastics News April 2019 54