A possible solution to Plastic pollution – PET degrading enzymes and the plastic pact

Last month an exciting scientific paper was published that coincided with several new policies centring around reforming the UK’s plastic consumption. The paper was titled “Characterization and engineering of a plastic-degrading aromatic polyesterase” and has caused quite a stir with everyone getting excited about this promising discovery, and what it could mean for the very prominent plastic issue which is in the public eye.

It began in 2016 with a team of scientists led by Yoshida from Japan. They discovered that a species of bacteria isolated from outside a plastic bottle recycling plant, Iedonella sakaiensis 201-F6, had the ability to digest single-use plastic drinks bottles made of polyethylene terephthalate (PET).

Polyethylene plastic, although it has only been widely used since the sixties, has become one of the most ubiquitous for packaging such as plastic bags and bottles as well as in clothing and carpeting, with around 100 million tonnes of it produced annually. Plastics are complex polymers that have the characteristics of being very durable, insoluble and take an incredibly long time to decompose naturally. This along with its semi-crystalline structure makes it a strong reliable material but is also what makes it pose such a huge threat to the environment.

The bacteria utilise a cutinase-like enzyme called PETase which breaks down the ester bonds in PET, degrading the plastic into smaller soluble fragments. The bacteria can then use these fragments as a source of food and carbon. Prof John McGeehan at the University of Portsmouth and his many colleagues were looking into the possible connection between the PETase and cutinase and noticed a feature common to each – the alpha/beta-hydrolase fold. They found that PETase showed a more open cleft, and by mutating two active-site residues they inadvertently improved its ability to break down PET by 20%. In other terms, this meant that the enzyme broke down plastic in days instead of the centuries it can take in the oceans.

An exciting possible use of this enzyme would be in bio-recycling -. PETase’s optimum temperature is 30°C which makes it suitable for use in bio-reactors. The enhanced enzyme produced attacks a more accessible part of PET’s structure, meaning PET’s semi-crystalline structure is not such a roadblock to catabolism. Another possibility is to transplant the enzyme into thermophilic/extremophile bacteria (bacteria that survive and thrive in high temperatures). PET changes state at around 70°C to a liquid, which theoretically would assist the enzyme and further speed up the reaction.

Beyond PET we manufacture and use a huge number of polyesters, there is a prospect that in the future PETase could be adapted and its substrate specificity changed. This could open a door into bio-recycling of other kinds of plastics.  A study found that of the whopping 8.3 billion tons of plastic that have been produced since the sixties and found that an infinitesimal 9% has been recycled.

It goes to show how reliant we are on plastic and how bad the pollution has become that a bacterium has naturally evolved that degrades it. While it is an exciting discovery in terms of how we can harness it, it also means that we need to consider that plastic isn’t as reliable as we initially thought and could threaten structures and items that we believed to be durable.

As aforementioned before I jumped headfirst into PETase, this paper overlapped with some changes in the UK that surround our plastic consumption and subsequent pollution. In the last week over 60 businesses have signed up to support the UK Plastics Pact, including huge retailers Tesco, Sainsbury’s, Unilever, Nestle and Coca Cola while other companies follow suit in different campaigns. For example, Starbucks has vowed to trial paper or biodegradable straws in place of their plastic counterparts in many their stores. Although the Plastic Pact is voluntary and carries no enforcement mechanism, and it still would be easier to manufacture more plastic, the public opinion carries a weighty incentive and can really drive the change. Gove has also promised that by 2025 100% of plastics will be reusable/recyclable/compostable, and all plastic packaging should have around 30% recycled content. These are huge statements to make and, if abided by, will have a huge impact on how this island treats plastic.

I believe that a huge driver of these changes we are seeing is owed to Attenborough’s documentary Blue Planet 2. It brought incredible scenes and unseen footage of parts of this beautiful blue planet, but Attenborough also used this to hammer home the importance of plastic pollution and the negative impacts, particularly for the ocean. From a turtle trapped in floating plastic to a whale calf that died due to poisoned milk, and a baby albatross that died from ingesting a plastic toothpick, these are just a few examples that left the public heartbroken.

Sir David Attenborough has always been a personal hero of mine, and his continued push to educate the public on the beauty of nature while also opening eyes to the negative impacts of the human lifestyle is fantastic. Even the queen declared a personal war on plastic after working with Attenborough on a conservation documentary about wildlife in the commonwealth. Without this having reached the huge numbers and impacted the public as it has it is unclear if we would be seeing such huge changes and talks within the commonwealth. New Zealand, Sri Lanka, Vanuata and Ghana have joined the UK in the Commonwealth Clean Oceans alliance so far with hopefully more to follow.

A study launched a few years ago that tried to estimate the amount of plastic in the oceans and predicted that by the mid-century the oceans could, ton for ton, contain more plastic waste than fish. This is a terrifying statistic and is understandably a call to arms for change, a change that I am not so quietly hopeful is happening.

As Attenborough said in the closing episode of Blue Planet 2, the future of all life now depends on us.

A clip about the future of our oceans || BBC Blue Planet 2

Some of the papers used as references/further reading

McGeehan et al 2018, Characterization and engineering of a plastic-degrading aromatic polyesterase       –       http://www.pnas.org/content/early/2018/04/16/1718804115

Yoshida et al 2016, A bacterium that degrades and assimilates poly(ethylene terephthalate)        –       http://science.sciencemag.org/content/351/6278/1196?ijkey=ce09a50b9fc446cd80fdfbab1f240a7cc4c186ea&keytype2=tf_ipsecsha

Jamber, Geye, Law 2017, The Production, Use, and Fate of all Plastics Ever Made       –       https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5517107/

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