Plastics have a lot of properties that have made them fixtures of modern societies. They can be molded into any shape we'd like, they're tough yet flexible, and they come in enough variations that we can tune the chemistry to suit different needs. The problem is that they're tough enough that they don't break down on their own, and incinerating them is relatively inefficient. As a result, they've collected in our environment as both bulk plastics and the seemingly omnipresent microplastic waste.

For natural materials, breaking down isn't an issue, as microbes have evolved ways of digesting them to obtain energy or useful chemicals. But many plastics have only been around for decades, and we're just now seeing organisms that have evolved enzymes to digest them. Figuring they could do one better, researchers in France have engineered an enzyme that can efficiently break down one of the most common forms of plastic. The end result of this reaction is a raw material that can be reused directly to make new plastic bottles.

An unwanted PET

The plastic in question is polyethylene terephthalate, or PET. PET has a variety of uses, including as thin films with very high tensile strength (marketed as mylar). But its most notable use is in plastic drink bottles, which are a major component of environmental plastic waste. PET was first developed in the 1940s, and the first living organism that can break down and use the carbon in PET was described in 2016—found in sediment near a plastic recycling facility, naturally.

While microbes like this could solve the plastic waste issue, they don't make plastics any more sustainable since the carbon backbone of PET ends up being broken down completely. That means we have to constantly supply new material to replace PET containers as they're broken down—material that currently comes from petrochemicals. The French team was interested in creating a circular PET process, in which existing material gets broken down in a way that allows it to be immediately reused to make new PET products.

PET is a long collection of carbon rings linked by oxygen and carbon atoms. To break it down in a way that allows recycling, these carbon-oxygen links haven't been broken, releasing a large collection of rings that can then be re-linked. The microbes that currently digest PET break down that ring as well, making them unsuitable for recycling.

But a number of enzymes that can break the links in PET have already been identified. These all function to break down the waxy coating on the surfaces of leaves, called "cutin" (making these enzymes cutinases). These provided the starting materials for the new work. To begin with, the researchers took a panel of cutinases and tested their activities in breaking down PET. The one with the highest activity turned out to have a name that indicated where it was originally found: in a compost pile (it's called "leaf-branch compost cutinase").

I’m melting

To understand the researchers' next steps, we have to understand a bit about PET itself. While all versions of PET have the same chemical formula, the material can solidify into two forms: a tightly packed crystalline form and a more loose, disordered form. Most materials made of PET have different amounts of these two forms, as their ratios can allow manufacturers to tune the material's properties. The tight packing of the crystalline form, however, makes it difficult to digest for even the most efficient enzyme. Fortunately, there's a partial solution: heating any form of PET causes some of the crystalline PET to melt into a disordered form, allowing more of it to be digested.

That, unfortunately, creates a problem, as the enzymes themselves often melt and are inactivated at the temperatures involved (65°C, or 150°F). In addition, these enzymes evolved to break down a different polymer and wouldn't be expected to work as well on PET, which is chemically distinct from anything on plants' leaves. These were the two big hurdles faced by the researchers.

To get the enzyme to work better on PET, the researchers looked up the cutinase structure and ran chemical simulations to figure out where PET would interact with the enzyme. They found it fit into a groove on the enzyme's surface that included the location where the PET would be cut. To improve PET's fit into this groove, the researchers created a large panel of mutant versions of the enzyme that, in different combinations, changed every single amino acid on the inside of the groove. While most of these nearly eliminated the enzyme's activity, a few actually improved it and were used for further studies.

The second problem was the issue of the enzyme's ability to tolerate high temperatures. Here, studies with related enzymes provided a hint: many were stabilized by interacting with a metal ion that holds two parts of the enzyme together. Starting with the original version of the enzyme, the researchers engineered in two amino acids that could form a chemical bond between those two parts (for those who know biochemistry, that's a disulfide bridge). This version was more stable at high temperatures than the original one.

By combining all these changes, the researchers created two versions that they then tested on PET obtained by shredding drink bottles.

Cheap and effective

Given this source of PET, the original enzyme could digest about half in 20 hours. The researchers' best modified version only needed 15 hours to hit 85 percent digestion. Optimizing the conditions, they were able to hit 90 percent breakdown of PET in under 10 hours. While there was still some crystalline PET left over, they found that they could take 1,000kg of PET waste and produce 863kg of raw materials from it. Put in different terms, their redesigned enzyme is more efficient at digesting PET than our digestive enzymes are at breaking down starches.

They then used this raw material to make new PET products using standard industrial reactions. The new product's ability to withstand pressure was only 5 percent off from the value measured for PET made from standard chemical sources. Appearance wise, it was within 10 percent of the PET produced the regular way.

How much would using recycled PET cost compared to starting with petrochemical feedstocks? The authors estimate that, if the protein can be made for about $25 a kilogram, then the cost of the process will end up being about 4 percent of what you can get with for the PET made from it. While that might not be as cheap as petrochemicals—especially now, after oil prices have collapsed—it's going to be relatively immune to future price shocks and is far more sustainable.

Nature, 2020. DOI: 10.1038/s41586-020-2149-4 (About DOIs).