“There’s a big misconception that your plastic bottle gets turned into another plastic bottle”
Scientists are engineering a plastic-eating enzyme. Portsmouth University’s Prof John McGeehan, who is involved in the research, explains how it could help to transform recycling
Why is plastic such a big environmental problem?
A plastic bottle normally comes from oil and it’s made of two components: one is called ethylene glycol, a very common chemical; the other is terephthalic acid. You link those together with an ester bond, and lots of ester bonds in a row generate a long chain – a polyester. Polyethylene terephthalate or PET [a common type of polyester] – the stuff plastic bottles are made of – is incredibly difficult to break down. When chemists made it around 50 years ago, they couldn’t have realised it would end up being a scourge in our oceans, lasting for hundreds of years.
Where was the plastic-eating enzyme discovered?
In 2016, a Japanese group spectacularly found bacteria in a recycling dump that were essentially living off the plastic, digesting it. What we think has happened is that these bacteria swapped eating natural polyesters for human-made ones, just by mutating one of the enzymes they were making.
How did you study the enzyme?
We took the gene for this enzyme and made lots in the lab. We took it to the Diamond Light Source, a massive X-ray microscope, and were able to generate a beautiful 3D structure. That allowed us to compare it.
There’s a polyester on plant leaves called cutin that’s really similar to some human-made things. It is as if the enzyme [in the bacteria] has evolved from a cutin-digesting enzyme and then became a plastic-digesting enzyme just by changing the shape of its surface a little bit. Getting the 3D structure helps you see how the enzyme works: it breaks the bonds and turns those long chains into their original building blocks.
Why engineer a better version of this enzyme?
It’s incredible that bacteria evolved to do this, but the enzyme is still quite slow. It takes weeks for these processes to happen. If you’re going to make it a recycling solution, then you have to get those times down to hours in order to be economically viable.
How would the enzyme be used in recycling?
There’s a big misconception that when you throw your plastic bottle into a recycling bin, it gets turned into another plastic bottle. That rarely happens because when you make it into plastic pellets during the recycling process, it loses some
of its properties and you have to then use it for a lower-value material, like a fibre for clothing or carpet. Eventually, it’s effectively worthless and ends up in landfill or being incinerated. This releases CO2, which is not good either.
Our idea would be that you have a large vat of plastic bottles, pour the enzyme solution in and digest it to its original building blocks. That would allow us to remake the plastic from scratch, basically closing the loop on the process and making it 100 per cent recyclable. That would be our goal.
ABOVE: Once they are in the environment, plastics can break down into tinier pieces that are harder to clear up and are ingested by animals RIGHT: Every minute, a million plastic bottles are bought around the world, yet only a small proportion of...