Near-complete depolymerization of polyesters with nano-dispersed enzymes

Successfully interfacing enzymes and biomachinery with polymers affords on-demand modification and/or programmable degradation during the manufacture, utilization and disposal of plastics, but requires controlled biocatalysis in solid matrices with macromolecular substrates(1-7). Embedding enzyme microparticles speeds up polyester degradation, but compromises host properties and unintentionally acceleratesthe formation of microplastics with partial polymer degradation(6,8,9). Here we showthat by nanoscopically dispersing enzymes with deep active sites, semi-crystalline polyesters can be degraded primarilyvia chain-end-mediated processive depolymerization with programmable latency and material integrity, akin to polyadenylation-induced messenger RNA decay(10). It is also feasible to achieve processivity with enzymes that have surface-exposed active sites by engineering enzyme-protectant-polymer complexes. Poly(caprolactone) and poly(lactic acid) containing less than 2 weight per cent enzymes are depolymerized in days, with up to 98 per cent polymer-to-small-molecule conversion in standard soil composts and household tap water, completely eliminating current needsto separate and landfill their products in compost facilities. Furthermore, oxidases embedded in polyolefins retain their activities. However, hydrocarbon polymers do not closely associate with enzymes, as their polyester counterparts do, and the reactive radicals that are generated cannot chemically modify the macromolecular host. This study provides molecularguidance towards enzyme-polymer pairing and the selection of enzyme protectants to modulate substrate selectivity and optimize biocatalytic pathways. The results also highlight the need for in-depth research in solid-state enzymology, especially in multi-step enzymatic cascades, to tackle chemically dormant substrates without creating secondary environmental contamination and/or biosafety concerns.

Automated summary

By dispersing enzymes with deep active sites, semi-crystalline polyesters can be efficiently degraded in a short period of time, with up to 98% conversion of polyester to small molecules. This method allows for complete elimination of the need to separate and landfill products in compost facilities, with degradation achieved in standard soil composts and household tap water.