Breakin’ it down with plastic-degrading enzymes

Matthew Spence and Vanessa Vongsouthi stand behind a sheet of PET plastic film, the breakdown target of their PETase enzyme.

Matthew Spence and Vanessa Vongsouthi stand behind a sheet of PET plastic film, the breakdown target of their PETase enzyme.

Each year, millions of tonnes of Polyethylene Terephthalate­­—better known as PET plastics—are made into disposable items like soft drink bottles and takeaway containers. But despite the convenience of PET products, the Great Pacific Garbage Patch looming in the oceans to our North is an ominous reminder that life in plastic is far from fantastic.

While current recycling processes are quite effective, breaking down PET plastic is still a long and chemically heavy journey. What if there was someone —or something—which could do it for us more efficiently?

This is the question which Vanessa Vongsouthi and Matthew Spence have been investigating in the Jackson Lab of the ANU Research School of Chemistry. For their PhD research, they are both looking at how a naturally occurring molecule, called an ‘enzyme’, might break down plastics.

The enzyme they’re particularly interested in is called ‘PETase’.

“In 2016, it was isolated from a waste facility in a novel species of bacteria,” Vanessa describes. “The bacteria used PET as a carbon and energy source, and PETase was one of the enzymes allowing the bacteria to do that. We thought that was really cool!”

Their projects are a part of a unified global effort into understanding how the bacteria’s PETase protein works: Vanessa is engineering it to make it more stable and tolerant to different heat conditions; Matthew is investigating the enzyme’s molecular evolution to see how it has changed.

“Synthetic plastics haven’t been around for too long, so PETase evolved relatively recently,” Matthew explains. “All of the rules that we understand for other enzymes aren’t quite the same for PETase.”

“We’re looking at its evolution now to understand how it works as a protein and an enzyme. Then we can go on to engineer it better.”

Vanessa says they’re taking a few different approaches to engineering proteins like PETase to improve their efficiency and function. One is repeated design-test cycles. Each time they change PETase’s structure, they take it for a spin and test it to see just how well it works on real PET plastic.

“We literally got a plastic bottle from the office, got a hole puncher, punched a disk out of it and incubated the enzyme with that for five hours,” she says. “You can actually detect the breakdown products of that reaction quite well.”

So far, they have made some progress with the PETase’s thermostability - that is, the protein’s ability to work at more practical temperature conditions without falling apart and losing its function.

“We managed to stabilise the protein by about 20°C, which is quite a lot,” Vanessa adds.

But success doesn’t come without its challenges.

“We realised that we made it far less powerful at degrading the plastic. But in doing that we learned about the trade-off in stability and activity.

“We want to keep it stable but make it more active as well. Each time, we introduce a couple of mutations and see whether those mutations affect its stability and activity.”

Vanessa and Matthew are both excited to be contributing to the worldwide body of knowledge on breaking down plastics.

“We’re always thinking about the end-term application of hopefully being able to recycle or degrade plastic more efficiently than what we can now,” Matthew says.

“It’s not like we’ll finish our PhDs with something you can add to a backyard compost to degrade all of your plastic, but each piece of knowledge we discover will contribute to a broader body of understanding to maybe get us there someday.”

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