Sustainable plastics degradation for recycling

In response to the increase in social awareness about environmental issues, biodegradable plastics are getting more popular. Several types of biodegradable polyesters with different physical properties have emerged to replace traditional petroleum-based polymers. However, there is no efficient recycling strategy in place for most of these plastics. As a solution, we’ have developed a patent-pending, enzyme-based technology that not only enables efficient degradation of polyesters as a recycling approach, but may also serves as a platform to convert post-consumer waste to different value-added chemicals using genetically engineered bacteria. As opposed to a composting process, enzymatic hydrolysis of bioplastics generates carboxylic acids instead of CO2 as the final product.

In our work, over 250 uncharacterized α/β-hydrolases from sequenced microbial genomes and metagenomic libraries were recombinantly expressed in E. coli and purified using metal (Ni) chelate chromatography. Purified proteins were screened for hydrolytic activity against polylactic acid (PLA), polycaprolactone (PCL), and a model polyethylene terephthalate substrate (3PET). Multiple rounds of screening yielded 40 active polyester-degrading enzymes, 14 of which were biochemically characterized. In parallel, the crystal structures of three polyester hydrolases were determined and the active site residues critical for polyester hydrolysis were identified using structure-based, site-directed mutagenesis. The product analyses revealed that polyesters were completely hydrolyzed to water soluble oligomeric species and eventually to monomers. Our results indicate that microbial carboxyl esterases can efficiently hydrolyze various polyesters making them attractive biocatalysts for plastics depolymerization and recycling.

Alternatively, polyester-hydrolyzing enzymes can be used for surface functionalization of bioplastics. Enzymatic hydrolysis of biodegradable plastics creates hydroxyl and carboxyl groups on the surface of polyester material without modifying the properties of the core polymer. The charged groups on the surface contribute to wettability of the polyester and therefore make it more biocompatible for medical and pharmaceutical applications.

Dr. Mahbod Hajighasemi