When in 2016 the first natural enzyme was discovered that could break down PET, researchers were amazed. A manmade material, PET normally will not degrade in the environment, and is a major contributor to the plastic waste littered on land and in the oceans. The enzyme, discovered in Japan, possibly evolved in a waste recycling centre in Japan, where a bacterium developed the capability to degrade plastic as a food source.
It was immediately interesting to scientists who recognised its potential to help solve the end-of-life issues associated with plastic materials.
Further studies showed that the bacterium actually had two enzymes, PETase and MHETase, which could degrade PET. PETase broke the plastic down into smaller PET building blocks and MHETase further splits this into terephthalic acid and ethylene glycol, the two main ingredients of PET.
Scientists around the world started up projects to explore the possibilities of enzymatic depolymerisation, including research teams at the University of Portsmouth and the US Department of Energy’s National Renewable Energy Laboratory (NREL), who were curious about the adaptations contributing to the substrate specificity of PETase.
“In response to the accumulation of plastics in the biosphere, it is becoming increasingly recognized that microbes are adapting and evolving enzymes and catabolic pathways to partially degrade man-made plastics as carbon and energy sources,” they wrote in a later article describing their study.
They discovered that, by means of protein engineering and evolution, the enzyme could be mutated into a form that was better than the natural PETase in degrading PET. Significantly, the enzyme was also found to be able to degrade polyethylene furandicarboxylate, or PEF, a bio-based substitute for PET plastics.
A research team working at the University of Greifswald and Helmholtz-Zentrum-Berlin subsequently solved the molecular structure of enzyme MHETase.
The problem was, that neither PETase nor MHETase were particularly efficient.
Now, the trans-Atlantic team that originally engineered the improved PETase has announced another breakthrough. By combining PETase and MHETase much bigger improvements could be generated.
Simply mixing PETase with MHETase doubled the speed of PET breakdown, and engineering a connection between the two enzymes to create a ‘super-enzyme’, increased this activity by a further three times.
The team was co-led by the same scientists who engineered PETase, Professor John McGeehan, director of the centre for enzyme innovation at the University of Portsmouth, and Dr Gregg Beckham, Senior Research Fellow at the National Renewable Energy Laboratory (NREL) in the US.
Professor McGeehan said, “Gregg and I were chatting about how PETase attacks the surface of the plastics and MHETase chops things up further, so it seemed natural to see if we could use them together, mimicking what happens in nature. We were delighted to see that our new chimeric enzyme is up to three times faster than the naturally evolved separate enzymes, opening new avenues for further improvements.”
The new research combined structural, computational, biochemical and bioinformatics approaches to reveal molecular insights into its structure and how it functions. The study was a huge team effort involving scientists at all levels of their careers.
A company that is even further along in the development of enzymatic depolymerisation of PET France-based Carbios who, in collaboration with the Toulouse Biotechnology Institute, has developed its own ‘PET recycling enzyme’. Carbios’ PET depolymerase enzymatically depolymerises PET that can be recycled into a material with the same properties as fossil-based PET material, and processed into bottles, ‘thereby contributing towards the concept of a circular PET economy’, said the company.
Earlier this year, Carbios announced it had commenced construction on an industrial demonstration plant for the enzymatic recycling of PET plastic, near Lyon, in France’s Chemistry Valley.
The first operations are expected to begin in 2021, after which large-scale industrial deployment is expected to follow. The plant, said Carbios, will validate the technical, environmental, and economic performance of the technology, and produce batches of monomers for technical and regulatory validation of recycled PET by future licensees.