Engineered Enzyme Recycles PET Bottles and Fibers at Moderate Temperatures

Breakthrough in Enzymatic Recycling of PET Plastics

A team of researchers has made a significant advancement in the field of plastic recycling by engineering a novel PET hydrolase enzyme, named PET2-21M. This enzyme demonstrates exceptional efficiency in breaking down bottle-grade polyethylene terephthalate (PET) plastics, offering a promising solution to one of the most pressing environmental challenges of our time.

The study, published in the journal ACS Sustainable Chemistry & Engineering, highlights the potential of this new enzyme to revolutionize the way we handle PET waste. The research was conducted by a multidisciplinary team including Professor Akihiko Nakamura from Shizuoka University, along with collaborators from Kirin Holdings Co., Ltd., and other institutions.

The Challenge of PET Recycling

PET is a widely used synthetic polymer found in bottles, textiles, and packaging materials. It makes up about 83% of the synthetic fiber market. While PET is technically recyclable, traditional mechanical recycling methods often lead to a decline in material quality. Additionally, these methods struggle with complex blends such as PET/cotton and PET/polyurethane (PU).

Chemical recycling offers a more effective way to produce high-purity materials but typically requires harsh conditions and hazardous chemicals, making it less sustainable. Enzymatic recycling, on the other hand, presents an attractive alternative because it can break down PET into its original monomers under milder conditions.

Engineering a More Efficient Enzyme

To enhance the efficiency of the PET-degrading enzyme PET2, the researchers employed a combination of random and targeted mutagenesis. By integrating seven newly identified beneficial mutations with a previously reported variant, they created the highly active PET2-14M enzyme. Further modifications, including surface charge adjustments and structural changes inspired by another enzyme called HotPETase, led to the development of PET2-14M-6Hot.

This process culminated in the creation of PET2-21M, which showed remarkable improvements in catalytic activity compared to the original wild-type PET2. Small-scale assays revealed that PET2-21M produced approximately 28.6 times more product than the original enzyme. In scaled-up experiments, PET2-21M was able to depolymerize around 95% of commercial bottle-grade PET powder within 24 hours at 60 °C, outperforming the benchmark enzyme LCC-ICCG, which required a higher temperature of 72 °C to achieve similar results.

Performance Under Various Conditions

One of the key advantages of PET2-21M is its ability to maintain high degradation efficiency even when enzyme concentrations are reduced. At half the concentration of LCC-ICCG, PET2-21M still achieved around 50% degradation efficiency, nearly doubling the performance of the benchmark enzyme.

Moreover, PET2-21M demonstrated strong performance under higher substrate loading conditions. At 10 mg L⁻¹ enzyme dosage, it achieved a 79% conversion rate at 60 °C, closely rivaling LCC-ICCG’s 95% conversion at 72 °C. Even at lower enzyme dosages, PET2-21M consistently outperformed LCC-ICCG, showcasing its potential to reduce both energy consumption and catalyst costs in industrial settings.

Applications in Textile Recycling

The researchers also evaluated the effectiveness of PET2-14M-6Hot in processing textile waste. When tested on pure PET fibers, PET2-14M-6Hot generated 75.7 mM total degradation products within 24 hours at 60 °C, a 1.4-fold improvement over LCC-ICCG tested at its optimal temperature of 70 °C.

For PET/cotton blends, PET2-14M-6Hot showed superior performance, producing 62.8 mM degradation products compared to 46.7 mM from LCC-ICCG. In challenging PET/PU blends, PET2-14M-6Hot maintained substantial activity at 50 °C, yielding 19.2 mM degradation products—more than double the amount obtained by LCC-ICCG under the same conditions.

Future Prospects

These findings underscore the potential of the engineered PET2 enzyme family to transform industrial PET recycling processes. Their ability to efficiently degrade diverse PET waste streams, including complex textile blends, supports broader applicability and sustainability benefits.

Future research will focus on further optimizing enzyme efficiency at even lower temperatures and in blended materials, aiming to facilitate wider industrial adoption and minimize the environmental impact of global plastic recycling efforts.

Conclusion

The development of PET2-21M and PET2-14M-6Hot marks a major step forward in the quest for sustainable and economically viable solutions to plastic waste. With their enhanced performance and versatility, these enzymes offer a promising pathway toward a more circular and environmentally friendly approach to plastic recycling.