It Is Dangerous to Go Alone: Strategies to Optimize PET Biocatalysis and Upcycling through Enzymatic Synergism

Current mainstream methods of plastic recycling are inadequate, producing lower-quality polymers, or are cost-inefficient, requiring expensive reagents operating under harsh conditions. Enzymatic biodegradation of poly-(ethylene terephthalate) (PET) plastic was first extensively described in 2005, and several PET degrading enzymes have been identified since then. Recently discovered or developed PET-degrading enzymes are inhibited by the intermediate reaction product, mono-(2-hydroxyethyl) terephthalate (MHET). Therefore, the enzymatic conversion of PET into its original components, terephthalic acid (TPA) and ethylene glycol (EG), is still inefficient. The synergistic cooperation between PET hydrolases and enzymes capable of hydrolyzing released intermediate products can increase reaction efficiency, reduce separation costs, and enable the full recovery of the basic components of PET for their potential conversion into value-added chemicals. This review aims to provide an overview of enzymes capable of degrading intermediate products and potentially solving the heterogeneous product solution problem, their structures and activities, importance, and potential applications in combination with PET hydrolases.