Could Enzymes That Digest Plastic Be the Savior of Our Planet?

One of the universe's most fascinating aspects is its ability to repurpose everything. For example, you — a magnificent creation — are made from the remnants of a supernova explosion. Across the cosmos, the universe continuously recycles and reuses, acting as The Ultimate Recycler.

Earth naturally recycles water, carbon, and various nutrients, so it makes sense that we should excel at recycling. However, humans are only moderately efficient. Consider plastic: we extract ancient carbon deposits to produce it — a form of recycling — but since the 1940s, we've created staggering amounts of a material that persists for centuries, harming wildlife and releasing toxins. Less than 10 percent of plastic is typically recycled.

In 2016, a Japanese research team identified a bacteria (Ideonella sakaiensis) that was making progress in recycling plastics, an area where humans have struggled. Poly(ethylene terephthalate) (PET) plastics, commonly found in soda and water bottles, have strong molecular bonds, making it surprising when these bacteria were found thriving in a Japanese landfill.

In the April 17, 2018 issue of Proceedings of the National Academy of Sciences, an international team of researchers detailed the enzyme PETase, produced by this bacteria, which breaks down PET. However, PETase alone isn't enough. Researchers also needed to study the structure of a second enzyme, MHETase, to fully understand the process.

Biochemist and structural biologist Dr. Gert Weber, along with his team from the Helmholtz-Zentrum Berlin and Freie Universität Berlin, played a key role. They discovered that MHETase not only attaches to PET but also breaks it down. Their research was published in the April 2019 issue of Nature Communications.

In an email interview, Weber explained the process: "Both [PETase and MHETase] are hydrolases, a class of enzymes that break ester bonds in PET, releasing the building blocks necessary for re-synthesizing the polymer."

He added, "PETase, which is half the size of MHETase, breaks PET into smaller components called MHET, composed of ethylene glycol and terephthalic acid. MHETase then splits MHET into these two substances, which are essential for creating new polymers."

What does this imply? These two bacterial enzymes are specifically designed to break down PET plastics. While they seem like a potential solution to the planet's overwhelming waste issue, Weber cautions against premature optimism. The enzymes are slow and inefficient. "Both enzymes originate from bacteria," he notes. Given that PET has only existed for about 75 years, these enzymes have evolved rapidly but remain far from optimal.

Weber is optimistic that these plastic-digesting enzymes will improve over time, enabling them to function in environmental applications. However, their impact will be limited. "Traditional PET recycling methods, which account for roughly 18% of all plastics, have significant drawbacks," he explains. "They require extensive pre-sorting, are energy-intensive, and heavily depend on crude oil. Enzymes like PETase and MHETase decompose PET into its basic components, which can then be purified and reused for new PET synthesis. This process can be repeated indefinitely, with minimal carbon loss, low energy requirements, and almost no crude oil consumption."

In essence, if successful, this approach could establish a sustainable, closed-loop system for PET plastic production and recovery. However, there are challenges.

"In the environment, plastics are either already fragmented or break down over time into microplastics," Gerber states. "The smaller the fragments, the more difficult they are to remove. The widespread distribution and fragmentation of plastics make it nearly impossible to address with any single solution. Nature, as demonstrated by PETase and MHETase, may eventually develop enzymes to tackle other types of polymers." His recommendation: Stop producing PET plastics as soon as possible.

This story is part of Covering Climate Now, a global initiative involving over 250 news organizations to enhance climate change reporting.