“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

Focus-Science and Technology - - DISCOVERIES -

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...

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