Engineering a Plastic-Eating Enzyme

Serendipity: Professor John McGeehan and colleagues inadvertently engineered an enzyme better at degrading plastic than the enzyme which evolved in nature (© Stefan Venter, UPIX Photography)

Professor John McGeehan at the University of Portsmouth and Dr Gregg Beckham at NREL solved the crystal structure of PETase - a recently discovered enzyme that digests PET- and used this 3D information to understand how it works. During this study, they inadvertently engineered an enzyme that is even better at degrading the plastic than the one that evolved in nature.

The researchers are now working on improving the enzyme further to allow it to be used industrially to break down plastics in a fraction of the time. They made the breakthrough when they were examining the structure of a natural enzyme which is thought to have evolved in a waste recycling centre in Japan, allowing a bacterium to degrade plastic as a food source.

PET, patented as a plastic in the 1940s, has not existed in nature for very long, so the team set out to determine how the enzyme evolved and if it might be possible to improve it.

The goal was to determine its structure, but they ended up going a step further and accidentally engineered an enzyme which was even better at breaking down PET plastics.

â??Serendipity often plays a significant role in fundamental scientific research and our discovery here is no exception,â? Professor McGeehan said.

â??Although the improvement is modest, this unanticipated discovery suggests that there is room to further improve these enzymes, moving us closer to a recycling solution for the ever-growing mountain of discarded plastics.â?

Close-up: Electron microscope image of enzyme degrading PET plastic (© Dennis Schroeder / NREL)

The research team can now apply the tools of protein engineering and evolution to continue to improve it.

The University of Portsmouth and NREL collaborated with scientists at the Diamond Light Source in the United Kingdom, a synchrotron that uses intense beams of X-rays 10 billion times brighter than the sun to act as a microscope powerful enough to see individual atoms.

Benjamin Luethi inspecting the I23 beamline detector at the Diamond Light Source which played a major role in the discovery (©Diamond Light Source)

Using their latest laboratory, beamline I23, an ultra-high-resolution 3D model of the PETase enzyme was generated in exquisite detail.

Professor McGeehan said: â??The Diamond Light Source recently created one of the most advanced X-ray beamlines in the world and having access to this facility allowed us to see the 3D atomic structure of PETase in incredible detail. Being able to see the inner workings of this biological catalyst provided us with the blueprints to engineer a faster and more efficient enzyme.â?

PETase mutant is better than the natural PETase

With help from the computational modeling scientists at the University of South Florida and the University of Campinas in Brazil, the team discovered that PETase looks very similar to a cutinase, but it has some unusual features including a more open active site, able to accommodate man-made rather than natural polymers. These differences indicated that PETase may have evolved in a PET-containing environment to enable the enzyme to degrade PET. To test that hypothesis, the researchers mutated the PETase active site to make it more like a cutinase.

And that was when the unexpected happened â?? the researchers found that the PETase mutant was better than the natural PETase in degrading PET.

Significantly, the enzyme can also degrade polyethylene furandicarboxylate, or PEF, a bio-based substitute for PET plastics that is being hailed as a replacement for glass beer bottles.

The research was funded by the University of Portsmouth, NREL and the Biotechnology and Biological Sciences Research Council (BBSRC).

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This project has received funding from the Bio-Based Industries Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement Nº 745828