Synopsis

• Confronting the global plastic waste challenge, especially regarding difficult-to-recycle blended PET fibres, need more sustainable recycling techniques.
• Researchers developed a new PET hydrolase, PET2-21M, and initiated large-scale manufacturing in yeast. This enzyme dramatically boosted PET bottle-grade PET breakdown.
• Concurrently, its direct precursor PET2-14M-6Hot effectively decomposed difficult blended fibres (PET/cotton, PET/PU) at moderate temperatures.
• This innovation presents a potential, energy-efficient approach for a circular plastics economy, expediting industrial-scale recycling of various polymer wastes.

A research team led by Professor Akihiko Nakamura from the Research Institute of Green Science and Technology at Shizuoka University, in collaboration with Researchers Takashi Matsuzaki and Toshiyuki Saeki from Kirin Holdings Co., Ltd., Professor Ryota Iino from the Institute for Molecular Science, and Professor Nobuyasu Koga from the Institute for Protein Research at Osaka University, has successfully engineered a novel PET hydrolase enzyme, PET2-21M, significantly enhancing the biodegradation of bottle-grade polyethylene terephthalate (PET) plastics. Significant activity was also exhibited towards PET/cotton and PET/polyurethane (PU) textile blends, namely with the closely related variation PET2-14M-6Hot. This notable advancement tackles the pressing worldwide issue of recycling PET trash by providing a sustainable and effective alternative to traditional recycling methods.

PET is a commonly employed synthetic polymer, notable in bottles, textiles, and packaging materials, accounting for over 83% of the synthetic fibre market. Notwithstanding its inherent recyclability, conventional mechanical recycling techniques sometimes lead to a decline in material quality and demonstrate restricted efficacy for intricate composite materials like PET/cotton and PET/PU. Chemical recycling, although capable of yielding high-purity materials, generally requires severe conditions and environmentally detrimental chemicals, hence constraining its practical sustainability.

Enzymatic recycling has emerged as a compelling option due to its capacity to depolymerise PET into its original monomeric components under gentler aqueous conditions. Researchers implemented a comprehensive engineering technique to improve the PET-degrading efficiency of the enzyme PET2. They methodically utilised both random and targeted mutagenesis, integrating seven newly discovered advantageous mutations with the previously documented manufactured variant PET2-7M, culminating in the highly efficient PET2-14M enzyme. Supplementary surface modifications that imparted positive charges to enhance substrate binding, along with deliberate modifications in the substrate-binding cleft inspired by the enzyme HotPETase as a structural model, resulted in the development of PET2-14M-6Hot. The final developed form PET2-21M was produced by additional optimisation. Additionally, substantial outputs of PET2-14M-6Hot and PET2-21M were accomplished in the yeast host, Komagataella phaffii. Significantly, PET2-14M-6Hot achieved yields of up to 691 mg L⁻¹ after 137 hours of incubation, exhibiting great expression efficiency devoid of glycosylation-induced variability.

The PET2-21M exhibited markedly improved catalytic activity relative to the wild-type enzyme PET2, with preliminary small-scale experiments indicating a total product yield roughly 28.6 times higher. Subsequent scaled-up experiments in 300 mL reactors further corroborated these enhancements; specifically, PET2-21M depolymerised approximately 95% of commercial bottle-grade PET powder (20 g L⁻¹) within 24 hours at 60 °C, whereas the benchmark enzyme LCC-ICCG necessitated its optimal temperature of 72 °C to achieve a similar conversion of 91%.

The advantage of PET2-21M was especially apparent under diminished enzyme loading circumstances. When the enzyme concentration was reduced to 2.5 mg L⁻¹, PET2-21M sustained approximately 50% degradation efficiency, nearly double the performance of LCC-ICCG, which attained just 26% conversion under the same conditions. This underscores PET2-21M’s significant capacity to reduce catalytic demands and related expenses.

Significantly, PET2-21M maintained its competitive edge under elevated substrate loading conditions (40 g L⁻¹). At an enzyme concentration of 10 mg L⁻¹, PET2-21M attained a 79% conversion at 60 °C, closely competing with LCC-ICCG’s 95% conversion at its elevated optimum temperature of 72 °C. Moreover, when the enzyme dosage was decreased to 5 mg L⁻¹, PET2-21M continued to surpass LCC-ICCG, achieving a 44% conversion in contrast to LCC-ICCG’s 29%. The strong performance at moderate temperatures and lower enzyme-to-substrate ratios establishes PET2-21M as a highly viable option for industrial PET recycling, potentially facilitating significant reductions in energy usage and catalyst costs.

The recycling capability of designed PET hydrolases for textile waste was assessed by comparing PET2-14M-6Hot with the benchmark enzyme LCC-ICCG on pure PET fibres and textile mixtures. At 60 °C, PET2-14M-6Hot produced 75.7 mM of total degradation products from pure PET fibres within 24 hours, indicating a 1.4-fold enhancement compared to LCC-ICCG evaluated at its ideal temperature of 70 °C. Likewise, PET2-14M-6Hot demonstrated superior catalytic performance on PET/cotton (65/35 wt%) blends, yielding 62.8 mM products compared to 46.7 mM by LCC-ICCG, with negligible interference from cotton fibres.

In the difficult PET/PU textile blends (85/15 wt%), both enzymes demonstrated diminished activity beyond the glass-transition temperature (Tg = 55 °C) of PU. At a reduced reaction temperature of 50 °C, PET2-14M-6Hot exhibited significant catalytic activity, producing 19.2 mM of degradation products—over twice the 8.2 mM achieved by LCC-ICCG under the same conditions. This highlights PET2-14M-6Hot’s exceptional ability to treat intricate mixed textiles that have historically defied enzymatic breakdown.

The findings validate the substantial potential of the modified PET2 enzyme family for industrial-scale enzymatic recycling. Their capacity to effectively decompose various PET waste streams, including complex textile mixtures at moderate temperatures, significantly enhances the usability and sustainability advantages in PET recycling methods.

These findings signify a significant progression towards achieving a more sustainable and economically feasible circular plastics industry. The modified PET2 enzymes exhibit an exceptional capacity to depolymerise PET and intricate fibre blends at moderate temperatures, presenting considerable potential for practical industrial recycling processes, especially in managing challenging blended textile waste. Future research endeavours aim to enhance enzyme effectiveness at reduced reaction temperatures and inside blended materials, thereby promoting wider industrial usage and decreasing the environmental impact of global plastic recycling initiatives.

Original Publication
Authors: Takashi Matsuzaki, Toshiyuki Saeki, Fuhito Yamazaki, Natsuka Koyama, Tatsunori Okubo, Daiki Hombe, Yui Ogura, Yoshihito Hashino, Rie Tatsumi-Koga, Nobuyasu Koga, Ryota Iino and Akihiko Nakamura.
Journal: ACS Sustainable Chemistry & Engineering
DOI: 10.1021/acssuschemeng.5c01602
Method of Research: Experimental study
Subject of Research: Not applicable
Article Title: Development and Production of Moderate-Thermophilic PET Hydrolase for PET Bottle and Fiber Recycling
Article Publication Date: 27-Jun-2025

Original Source: https://www.ims.ac.jp/en/news/2025/07/0707.html