Content area
Full Text
Plastic waste poses an ecological challenge1-3 and enzymatic degradation offers one, potentially green and scalable, route for polyesters waste recycling4. Poly(ethylene terephthalate) (PET) accounts for 12% of global solid waste5, and a circular carbon economy for PET is theoretically attainable through rapid enzymatic depolymerization followed by repolymerization or conversion/valorization into other products6-10. Application of PET hydrolases, however, has been hampered by their lack of robustness to pH and temperature ranges, slow reaction rates and inability to directly use untreated postconsumer plastics11. Here, we use a structure-based, machine learning algorithm to engineer a robust and active PET hydrolase. Our mutant and scaffold combination (FAST-PETase: functional, active, stable and tolerant PETase) contains five mutations compared to wild-type PETase (N233K/R224Q/S121E from prediction and D186H/R280A from scaffold) and shows superior PET-hydrolytic activity relative to both wild-type and engineered alternatives12 between 30 and 50°C and a range of pH levels. We demonstrate that untreated, postconsumer-PET from 51 different thermoformed products can all be almost completely degraded by FAST-PETase in 1 week. FAST-PETase can also depolymerize untreated, amorphous portions of a commercial water bottle and an entire thermally pretreated water bottle at 50 °C. Finally, we demonstrate a closed-loop PET recycling process by using FAST-PETase and resynthesizing PET from the recovered monomers. Collectively, our results demonstrate a viable route for enzymatic plastic recycling at the industrial scale.
Enzymatic depolymerization of PET was first reported in 2005 and has been nascently demonstrated using 19 distinct PET-hydrolysing enzymes (PHEs) derived from esterases, lipases and cutinases4,11,13. However, most of these enzymes only show appreciable hydrolytic activity at high reaction temperatures (that is, at or exceeding the PET glass transition temperature of roughly 70 °C) and with highly processed substrates. For example, an engineered leaf-branch compost cutinase (LCC) can degrade 90% of pretreated postconsumer-PET (pc-PET) in 10h at 72°C and a pH of 8.0 (ref. 12). Most other PHEs similarly show poor activity at moderate temperatures14 and more neutral pH conditions15, greatly restricting in situ/microbially enabled degradation solutions for PET waste. This limitation is of critical concern as 40% of plastic waste bypasses collection systems and resides in natural environments16. In addition, converting untreated postconsumer plastic waste at near ambient temperatures would lower net operating costs.
Although the PHE from the PET-assimilating bacterium Ideonella...