Catalytic mechanisms, engineering, and cascade biocatalysis of mono(2-hydroxyethyl) terephthalate hydrolases for...

Polyethylene terephthalate (PET) is a widely used synthetic polyester produced from terephthalic acid (TPA) and ethylene glycol (EG), and its environmental accumulation has become a global concern. Enzymatic depolymerization has emerged as a sustainable strategy for PET recycling. However, during PET hydrolysis, the major intermediate mono(2-hydroxyethyl) terephthalate (MHET) accumulates and causes product inhibition of PET hydrolases, thereby limiting complete depolymerization. MHET hydrolases overcome this bottleneck by converting MHET into the monomers TPA and EG and therefore play a crucial role in PET biodegradation. In recent years, the discovery of new MHET hydrolases, together with protein engineering and dual-enzyme cascade systems combining PET hydrolases and MHET hydrolases, has significantly improved enzymatic PET degradation efficiency. This review summarizes MHET hydrolases from diverse biological sources and then focuses on the representative enzyme IsMHETase from Ideonella sakaiensis, including its catalytic mechanism, structure-function relationships, and protein engineering strategies. The enzymatic cascade between IsMHET and PET hydrolases is further examined, with emphasis on surface display and immobilization approaches. Current challenges and future research directions are also outlined, highlighting opportunities for scalable PET biodegradation and upcycling.