A research team has achieved a significant milestone in the field of plastic waste management by elucidating, for the first time, the catalytic mechanism by which the esterase Aes72 hydrolyzes urethane bonds in polyurethane (PU), and by engineering the enzyme to further enhance its catalytic efficiency. The study, published in the journal Engineering, details the structural elucidation and engineering of a promiscuous esterase, Aes72, which demonstrates enhanced capability in breaking down polyether-based PU waste. This advancement offers a promising, environmentally friendly alternative to traditional, energy-intensive recycling methods.
PU is the fifth most-produced synthetic polymer globally, with millions of tonnes manufactured annually. Its widespread use in consumer and industrial products has created significant waste management challenges. Current end-of-life strategies, such as mechanical shredding or chemical recycling, often suffer from high energy consumption, the generation of unwanted by-products, and strict requirements for raw material quality. Consequently, the development of biocatalytic recycling technologies—which operate under mild conditions without organic solvents—has become a priority for researchers aiming to achieve a circular economy.
The research team, led by experts from Nanjing Tech University, Shandong University, the Tianjin Institute of Industrial Biotechnology, and the University of Greifswald, focused on the esterase Aes72. While many enzymes can degrade polyester-type plastics, identifying catalysts that can effectively cleave the urethane bonds found in diverse PU wastes remains a formidable challenge. By resolving the ligand-free crystal structure of Aes72 at a high resolution of 1.80 Å, the researchers gained critical insights into the enzyme's architecture.
Using advanced multiscale quantum mechanics/molecular mechanics (QM/MM) simulations, the team mapped the catalytic mechanism of urethane bond cleavage. They identified a four-step reaction process, pinpointing the nucleophilic attack as the rate-determining step. Armed with this mechanistic understanding, the scientists employed a semi-rational design strategy to engineer the enzyme's binding pocket.
The resulting double mutant, F276A/L141I, exhibited a remarkable two-fold increase in catalytic efficacy toward the model substrate bis(4-hydroxybutyl) (methylenebis(4,1-phenylene)) dicarbamate (BMC) compared to the wild-type enzyme. Furthermore, the variant demonstrated significantly enhanced degradation performance on polyether-based PU materials. In experiments, the engineered Aes72 variant led to pronounced chain scission and substantial weight loss in thermoplastic polyether–PU, confirming its potential for industrial application.
This finding provides essential mechanistic insights into the structure–function relationship of the promiscuous esterase Aes72 in PU degradation. While the study highlights that the degradation of highly cross-linked thermoset PU foams remains a challenge due to their complex structure, the successful engineering of Aes72 establishes a vital foundation for future efforts. By combining structural biology with computational design, this research paves the way for more potent, bio-based catalysts, bringing the scientific community closer to achieving sustainable and efficient recycling of diverse plastic wastes.
The paper, "Structural Elucidation and Mechanisms-Guided Engineering of a Promiscuous Esterase for Enhanced Polyurethane Depolymerization," is authored by Jiawei Liu, Mingna Zheng, Yuan Wen, Wei Xia, Xu Han, Jie Zhou, Weidong Liu, Ren Wei, Yanwei Li, Weiliang Dong, and Min Jiang. It was published in the journal Engineering. Full text of the open access paper: https://doi.org/10.1016/j.eng.2026.02.008
