Scientists uncover the secrets of a plastic-destroying enzyme

In 2016, a group of Japanese scientists reported on a new species of bacteria they discovered growing on piles of plastic waste. Christened Ideonella sakaiensis, it was able to digest a hardy type of plastic called polyethylene terephthalate, a feat made possible by an enzyme known as terephthalate dioxygenase (TPADO), which targets one of PET’s component chemicals with an uncanny precision.

“We thought this would be a kind of sloppy enzyme” that breaks down many different molecules, Jennifer DuBois, a biochemist at Montana State University, tells Inverse. “But it’s actually quite specific. That was a surprise.”

DuBois and her co-authors recently took a close look at how the enzyme works and published their findings this month in Proceedings of the National Academy of Sciences. The research could help improve chemical recycling as the world discusses ways to stem plastic pollution.

Here’s the background — I. sakaiensis begins its feast by breaking PET into its two main components, ethylene glycol, and terephthalate (TPA). Both are “petrochemicals” made by refining petroleum, and they’re useful to humans, but the bacterium’s internal process isn’t finished yet. This is where TPADO comes in.

The bacterium’s signature enzyme and TPA make for a strangely snug fit, given that TPA is not found in nature. She says TPA’s structure resembles a component of lignin, the substance that makes tree trunks “woody,” so I. sakaiensis may have developed its taste before plastic came along. It may even have recently evolved to eat plastic. “We don’t really know.”

Enzymes are proteins that “speed up” chemical reactions by reducing the amount of energy it takes to get the reaction started, and I. sakaiensis uses TPADO to kickstart a reaction that turns TPA into an intermediate substance called DCD. Then another enzyme-coenzyme team swoops in, removes a hydrogen atom, and turns it into protocatechuic acid, or PCA, a chemical with some value in medicine and industrial processes, in particular making nylon, another plastic product.

“TPA doesn’t have a lot of uses besides making more PET,” says DuBois. The chemical cascade her team observed in I. sakaiensis could help scientists find other ways to recycle it instead. Rather than sending TPA back to the plastics factory, they could end up feeding it to a bioreactor: “a vat of bacteria that have been engineered to do whatever job you want them to do.” In this case, turning TPA to PCA.

Why it matters — A century of producing plastics like PET has created a global problem that has so far been impervious to solutions like recycling, and DuBois hopes this research will contribute to fixing the issue. “If you can find another role for TPA, other than in PET, that's a way of draining the PET out of circulation.” She and other scientists working with the BOTTLE Consortium, a research initiative funded by federal agencies as well as companies like Amazon and Patagonia, are developing new methods of recycling and new types of plastic.

Earlier this month, representatives from 175 countries at a conference in Nairobi, Kenya agreed to begin drawing up terms for a global treaty to curb plastic pollution. Only a small fraction of total plastic waste in the U.S. — about 9 percent — is recycled each year, though for PET the number is higher, about 29 percent in 2018, according to the Environmental Protection Agency. Instead, tens of millions of tons are buried in landfills and washed into the ocean, fragmenting into pieces that are too small to retrieve. Recent research has shown that the amount of microplastics in farmed soil even outweighs the Great Pacific Garbage Patch. For recycling to play a greater role in the big cleanup, it needs new and improved methods as well as governments more willing to put them into practice.

Compared to some other methods of chemical recycling, enzymes tend to be less efficient, Jack Payne, a doctoral candidate in chemistry at the University of Bath who is developing metal-based catalysts for recycling, tells Inverse. This is important because recycled substances like TPA and PCA have to compete on the market with cheap raw materials from sources like oil, so recycling has to move quickly and at a low cost. Understanding how TPADO works on a molecular level can help scientists design enzyme-recycling systems that are more commercially viable, he says.

An enzyme that targets specific types of plastic like PET could be especially useful in dealing with piles of mixed plastics. “You could just chuck a catalyst into it and have it selectively break down all of those plastics…then you’d be able to easily separate them at the end.”

Still, some advocates against plastic waste are skeptical of chemical recycling altogether and say the focus should be kept on stopping pollution before it starts. “Every few years, there’s a new scientific paper about using enzymes or other bio-remediation techniques to address plastic waste, but it has not amounted to much,” Judith Enck, president of Beyond Plastics, a research and advocacy group based in Bennington College, tells Inverse.

What’s next — Enck has criticized the U.S. Department of Energy for promoting research into chemical recycling, which she says could give companies license to continue business as usual, including fossil fuel companies, which are pivoting to petrochemicals. The BOTTLE Consortium receives some funding from companies like Heinz, which is a heavy user of single-use plastics. “The only thing that’s going to solve this problem is making less plastic,” she says.

“Ultimately PET should be replaced by something else,” says DuBois. “But in the meantime, what do you do with it?” She says better methods of recycling will also be necessary to deal with the huge amounts of plastic that are already in the environment. “We all envision a time when we’ll be mining those landfills.”