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.

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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 are broken, releasing a large collection of rings that can then be relinked. 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.