Engineered enzyme boosts polyurethane depolymerisation

The team, from Nanjing Tech University, Shandong University, the Tianjin Institute of Industrial Biotechnology and the University of Greifswald, focused on one of the key technical bottlenecks in polyurethane biorecycling: finding enzymes that can attack the urethane linkage itself, rather than only ester groups in more easily degraded polyester-type plastics. Polyurethane is the world’s fifth most-produced synthetic polymer, and existing end-of-life routes such as mechanical recycling and chemical recycling can be energy-intensive and may generate unwanted by-products or require tightly controlled feedstocks.

In the new study, the researchers resolved the ligand-free crystal structure of Aes72 at 1.80 Å resolution and used QM/MM simulations to map the mechanism of urethane-bond hydrolysis. They identified a four-step reaction pathway, with the nucleophilic attack step acting as the rate-determining stage. On the basis of that mechanistic understanding, they then used a semi-rational protein-engineering strategy to redesign the enzyme’s binding pocket.
The resulting double mutant, F276A/L141I, showed roughly a two-fold increase in catalytic efficacy against the model carbamate substrate bis(4-hydroxybutyl) (methylenebis(4,1-phenylene)) dicarbamate (BMC) compared with the wild-type enzyme. More importantly for industry, the engineered variant also delivered improved degradation of thermoplastic polyether PU, producing pronounced chain scission and substantial weight loss in the material during testing.

That matters because polyether-based polyurethane is widely used in applications where hydrolytic stability and toughness are required, yet those same characteristics make waste management difficult. An enzyme system capable of selectively depolymerising such materials under mild conditions could, in principle, offer a route to polyurethane recycling with lower heat input, fewer harsh reagents and better product control than some conventional processes. That said, the researchers note that highly crosslinked thermoset PU foams remain much harder to tackle because of their more complex network structure.

For the polyurethane sector, the work is technically significant because it shifts the discussion from general “biodegradation” claims to a more precise understanding of urethane-bond hydrolysis and enzyme design. In effect, it suggests that future biocatalysts may be tuned for specific PU chemistries and morphologies, rather than treating polyurethane waste as a single class of material. In the nearer term, the findings appear most relevant to thermoplastic polyether PU streams rather than heavily filled, reinforced or crosslinked systems.

The study does not mean enzymatic recycling is ready to displace established polyurethane recycling technologies. Scale-up, reaction rates, product recovery and performance on mixed or contaminated waste streams will still need to be addressed. But by linking structural biology, computational chemistry and protein engineering, the researchers have established a more rigorous platform for developing urethane-cleaving enzymes tailored to polyurethane depolymerisation.

CAPTION: Mechanistic insights into urethane bond hydrolysis by esterase Aes72 and protein engineering CREDIT: Jiawei Liu et al.

Engineering