In 2016, a naturally occurring bacterium called Ideonella sakaiensis was discovered in the soil at a recycling center in Japan. This bacterium appeared to subsist on an all-plastic diet—and not just any plastic, but specifically PET (polyethylene terephthalate).
After this initial discovery, a group of scientists from the University of Portsmouth, led by biologist and professor John McGeehan, set out to understand the evolution and structure of one of this bacterium’s enzymes, called PETase. However, they ended up accidentally designing a “mutant” version of this enzyme which was even more effective at breaking down PET plastic.
Recycling PET back into high-quality, reusable plastic is difficult, according to an article from The Economist. The bottles have to be hydrolyzed down to monomers, and then these monomers must be purified and re-polymerized to create new PET. Consequently, most PET eventually ends up in lower-grade applications such as synthetic fibers used for carpets and clothing. It is unlikely that the plastic bottles we recycle will be reborn as new plastic bottles.
While it’s great that such strides have been made, the enzyme isn’t quite ready for commercial or industrial use. According to the article from The Economist mentioned above, just breaking down a few milligrams of plastic in a day would require a liter of PETase enzyme solution. The researchers are optimistic, though, that altering the enzyme further will improve its efficiency to make it commercially valuable. “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,” said McGeehan, quoted in The Independent. Gregg Beckham, one of the co-authors of the study, also commented, “Now that we’ve shown [that PETase is not yet fully optimized to degrade PET], it’s time to apply the tools of protein engineering and evolution to continue to improve it.”
The researchers have been working on engineering further variations of PETase. These iterations are becoming increasingly efficient at breaking down PET into its constituent chemicals to be used to manufacture brand-new plastic bottles. In order to improve the efficiency of PETase, the team plans to focus first on altering the enzyme so that it can function at temperatures greater than 158°F, when PET is most easily broken down. They also hope to transplant the gene for the enzyme into types of bacteria that are more easily cultivated on an industrial scale.
Could a discovery like this eventually be a useful solution to the huge amounts of plastic accumulating in our landfills and oceans?