A new study published in Engineering reports the structural characterization and mechanism-guided engineering of a promiscuous esterase, Aes72, to improve the enzymatic depolymerization of polyurethane (PU), a widely used synthetic polymer that poses persistent environmental challenges. The research provides detailed structural and mechanistic insights into how Aes72 hydrolyzes urethane bonds and delivers an engineered variant with improved catalytic performance on polyether‑based PU materials.

PU is inherently resistant to biodegradation mainly due to the stability of its urethane bonds. Conventional recycling approaches, including mechanical processing and chemical depolymerization, often involve high energy consumption, complex by‑product streams, and strict feedstock requirements. Biocatalytic technologies offer a milder, solvent‑free alternative for plastic recycling, yet enzymes that efficiently target the urethane bonds dominant in both polyester‑ and polyether‑based PU remain scarce.

In this work, researchers determined the ligand‑free crystal structure of Aes72 at 1.80 Å resolution, revealing a typical α/β hydrolase fold with a conserved catalytic triad of Ser147, Glu241, and His271, along with a characteristic cap domain. Multiscale quantum mechanics/molecular mechanics (QM/MM) simulations uncovered a four‑step concerted catalytic mechanism for urethane bond cleavage, identifying the initial nucleophilic attack as the rate‑determining step. The simulations also indicated that ester moiety cleavage is energetically favored over amide cleavage during the hydrolysis of carbamate groups.

Structure‑guided semi‑rational engineering was then applied to modify the substrate pocket of Aes72. Several single mutants showed improved activity toward the model carbamate substrate bis(4‑hydroxybutyl) (methylenebis(4,1‑phenylene)) dicarbamate (BMC). Combinations of beneficial mutations yielded the double mutant F276A/L141I, which displayed approximately twofold higher catalytic efficacy for BMC hydrolysis compared with the wild‑type enzyme.

Further tests on two polyether‑based PU materials, including a synthetic thermoplastic PEG‑PU and commercial PU foam, confirmed the enhanced performance of F276A/L141I. The variant accelerated the release of the monomer 4,4′‑methylenedianiline (MDA) and induced more substantial molecular weight reduction and surface erosion of PU substrates, as supported by gel permeation chromatography and scanning electron microscopy observations. While the degradation of highly cross‑linked PU foam remained moderate, the improved activity on thermoplastic PU demonstrates the potential of Aes72 engineering for practical waste processing.

These findings establish a clear structure–function relationship for the promiscuous esterase Aes72 in PU biodegradation and support the development of next‑generation biocatalysts for sustainable PU recycling.

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, Min Jiang. Full text of the open access paper: https://doi.org/10.1016/j.eng.2026.02.008. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.