Our plastic pollution crisis is only getting worse, but scientists may soon have a valuable new tool to chip away at the problem. With a little luck, researchers have happened upon an engineered enzyme with an appetite for some of the most commonly disposed of types of plastic, meaning this waste could conceivably be broken down relatively quickly rather than contaminating the environment for hundreds of years.
We produce hundreds of millions of tons of polyethylene terephthalate (PET) plastic each year for use in things like soda and shampoo bottles. Little of this is recycled, which means we are polluting our environment with materials that take centuries to degrade, much of which washes out into the ocean where it breaks into tiny pieces that are nearly impossible to track, let alone clean up.
Scientists have made some promising discoveries when it comes to putting living organisms to work on this dilemma, with wax worms and bacteria a couple of recent examples. Another is the recently discovered enzyme that consumes PET plastics called PETase, which scientists at the University of Portsmouth and the US Department of Energy's National Renewable Energy Laboratory (NREL) used as a starting point for their groundbreaking research.
The researchers set out to better understand the crystalline structure of PETase, which is believed to have come of age in a Japanese recycling center. What interested the scientists was the evolution of the enzyme, given that PET plastics have only existed in the environment since the 1940s. Their thinking was that if they could understand how it came about in a relatively short space of time, perhaps they could understand how to make it more effective at eating plastic.
They began by using a synchrotron at the Diamond Light Source facility in the UK, which allows them to see individual atoms inside the structure of the enzyme by blasting them with beams of X-ray light 10 billion times brighter than the sun. Through this method they wound up with a ultra-high resolution 3D model of PETase.
"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," says Professor John McGeehan from the University of Portsmouth. "Being able to see the inner workings of this biological catalyst provided us with the blueprints to engineer a faster and more efficient enzyme."
Key to the breakthrough were observations that, at this high resolution, PETase appears very similar to the enzyme cutinase, but with a few notable differences. One of these was a more open active site that better allowed it to accommodate manmade polymers instead of natural variants.
This suggested that PETase had evolved in an environment with PET present, so the researchers mutated the PETase active site to behave more like cutinase, in search of more evidence of this theory. But instead of the mutated PETase proving more ineffective at degrading PET, the team found the opposite, that it actually performed better.
"Serendipity often plays a significant role in fundamental scientific research and our discovery here is no exception," said McGeehan. "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."
The engineered enzyme has the added benefit of being able to degrade polyethylene furandicarboxylate (PEF), a PET alternative that has been floated as a replacement for glass beer bottles. The team is now working to continue refining the engineered enzyme to make it even more effective.
"The engineering process is much the same as for enzymes currently being used in bio-washing detergents and in the manufacture of biofuels – the technology exists and 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 research will be published in the journal Proceedings of the National Academy of Sciences.
Source: University of Portsmouth