Poly(ethylene terephthalate) (PET) is a popular plastic for making clear, semi-rigid containers such as water bottles and packaging inserts; it also accounts for 12% of solid waste globally. Researchers hope a newly engineered enzyme can help recycle PET products that might otherwise go in the trash heap.
Recycling plants can mechanically recycle products made from PET alone by melting and recasting the polymer. But dyes, additives, and other polymers can gum up the works, so mixed PET plastics do not get recycled. PET waste “is such a pervasive aspect of our lives that we wanted to find a way to utilize this type of waste stream,” says Hal Alper, a biochemical engineer at the University of Texas at Austin. Now Alper and his colleagues are “giving biology a taste for waste” with a newly engineered enzyme that breaks down a wide variety of PET waste products at 50 °C.
Biochemists previously developed PET degrading enzymes (PETases) derived from the bacteria Ideonella sakaiensis. These engineered enzymes break the bonds within the PET polymer chains to yield reusable monomers. But these existing PETases still struggle to depolymerize postconsumer plastics that contain other ingredients and differ in how uniformly the polymers are organized within the bulk plastic. For the enzymes to work, the PET also has to be heated to at least 65 °C, the temperature where the plastic becomes pliable. Alper and his colleagues wanted to see if engineering a PETase with better thermal stability could make this depolarization more efficient at lower temperatures.
The team started by analyzing known PETases with a machine learning program called MutCompute. Trained on 19,000 protein structures, the MutCompute neural network evaluated the 3D structure of PETases and looked for amino acid residues that seemed out of place in their microenvironment when compared with structures in the training set, Alper says. The program then suggested amino acid substitutions that could be a better fit and potentially make that region more stable, Alper says.
The researchers tested the heat stability and activity of PETases with the suggested mutations. They found that five mutations, none of them in the protein’s active site, yielded an enzyme with up to 38-fold higher activity between 40 and 50 °C when compared with previous PETases (Nature 2022, DOI: 10.1038/s41586-022-04599-z).
To test the new enzyme’s appetite for postconsumer plastics, Alper’s team gathered 51 different PET samples, such as food wrappers, cookie containers, and water bottles, from products they bought at a local grocery store. The new PETase degraded all of them in time frames ranging from hours to weeks. The resaearchers could also take monomers produced from degraded colored PET and easily repolymerize them to regenerate clear, colorless, virgin PET. The efficiency of this new PETase under milder conditions could make a circular plastic economy much more feasible, Alper says.
“They have developed the world’s best PET degrader at 50 °C,” says Richard Gross, a bioinorganic chemist at Rensselaer Polytechnic Institute. Gross, who was not involved in the study, says the new enzyme will need to be tested further in real world conditions before it can be fully vetted for broader use, but, he says, this work is a step towards practical, enzyme-based PET recycling.