'Accidental' enzyme guzzles plastic bottles
Image: Electron microscope image of enzyme degrading PET plastic [Schroeder/NREL]
Using X-ray analysis, researchers have resolved the structure of the plastic-digesting enzyme, PETase, and accidentally engineered a 'mutant' enzyme that is even better at breaking down the polyethylene terephthalate used for drinks bottles.
Professor John McGeehan, Director of the Institute of Biological and Biomedical Sciences at the University of Portsmouth, UK, and colleagues are now looking to speed up the enzyme process for industrial use.
“Serendipity often plays a significant role in fundamental scientific research and our discovery here is no exception,” says McGeehan. "[We believe] there is room to further improve these enzymes, moving us closer to a recycling solution for the ever-growing mountain of discarded plastics.”
Crystal structure of PETase (in green) with a docked PET polymer (in yellow) bound to the active site. The research team used this 3D information to better understand how PETase works, which led to engineering an enzyme even better at degrading plastic. [John McGeehan/University of Portsmouth]
While plastic-degrading organisms already exist, enzyme action is too slow to be truly useful on an industrial scale.
Given this, McGeehan and colleagues from Portsmouth and the US Department of Energy's National Renewable Energy Laboratory first used electron microscopy to study PETase digesting PET samples taken from soda bottles.
As NREL senior scientist, Dr Bryon Donohoe, says: "After 96 hours you could see clearly that the PETase was degrading PET."
Electron microscopy of enzyme-substrate interactions, [Schroeder/NREL].
Collaborating with scientists from UK synchrotron Diamond Light Source, the researchers determined the structure of PETase using the long-wavelength macromolecular crystallography beamline I23, and reconstructed a 3D atomic model of the enzyme.
"The large, curved area detector in combination with the vacuum environment on the I23 beamline is ideal for this work as it allows high resolution structure determinations at low X-ray doses limiting detrimental radiation damage effects to the crystals,” highlights principal I23 beamline scientist, Dr Armin Wagner. "[The PETase] protein crystals diffracted really well and we were able to achieve very high resolution."
"While most of the known protein structures have been determined to resolutions between 1.5 and 3.0 Å , [the resolution] we could achieve from for one of the investigated structures [was] 0.92 Å," he adds. "The high resolution 3D structure allowed us to get a clear picture of where the enzyme grips its target which then informed the next step of computational modelling to investigate the mechanism of PET degradation further."
With help from the computational modeling scientists at the University of South Florida and the University of Campinas in Brazil, the researchers discovered that PETase looks very similar to a cutinase, an enzyme that breaks down the insoluble plant compound, cutin.
However, PETase has additional features including a more open active site, able to accommodate man-made rather than natural polymers.
The researchers reckoned these differences indicated that PETase may have evolved in a PET-containing environment to enable the enzyme to degrade PET.
To test this hypothesis, they mutated the PETase active site, to make it more like a cutinase, and discovered that their PETase mutant was some 20% more effective at degrading PET than the natural PETase.
Importantly, 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.
Right now, the new enzyme takes a few days to start breaking down PET, but McGeehan and colleagues are confident they can accelerate enzyme reactions for industrial-scale processes.
Professor John McGeehan used intense beams of X-rays at UK synchrotron, Diamond Light Source, to determine the structure of PETase. [Stefan Venter, UPIX Photography]
"It’s well within the possibility that in the coming years we will see an industrially viable process to turn PET and potentially other substrates like PEF, PLA, and PBS, back into their original building blocks so that they can be sustainably recycled,” says McGeehan.
“The detail that the team were able to draw out from the results achieved on the I23 beamline at Diamond will be invaluable in looking to tailor the enzyme for use in large-scale industrial recycling processes," adds Diamond Light Source chief executive, Professor Andrew Harrison. "The impact of such an innovative solution to plastic waste would be global. It is fantastic that UK scientists and facilities are helping to lead the way.”
Research is published in PNAS.