Hydrolase-Catalyzed Depolymerization Mechanism toward Crystalline and Amorphous Polyethylene Terephthalate

Enzyme-based recycling of the extensively utilized polyethylene terephthalate (PET) is considered as a promising eco-friendly strategy to manage PET waste. Although efficient depolymerization of amorphous PET has been achieved, highly efficient depolymerization of the well-ordered crystalline region of PET presents a huge challenge. Herein, leveraged to molecular dynamics simulations and quantum mechanics/molecular mechanics calculations, we study the depolymerization mechanism of IsPETase toward both crystalline and amorphous PET to unravel the origin of the efficient depolymerization of the amorphous region. We demonstrated that crystalline PET is not well accommodated within the active pocket, and significant distortion energy is needed during its depolymerization. Poor stabilization within the oxyanion hole for crystalline PET was identified and confirmed to lead to higher energy barriers. In addition, after identifying the rate-determining step, we reveal that IsPETase prefers to depolymerize trans than gauche conformation of the PET polymer with a 2.4 kcal mol(-1) lower energy barrier. Differences in structural/charge distributions within the active site of the enzyme between trans and gauche conformations were proposed to be responsible for this preference. The structure-charge-activity relationship for the rate-determining step was built. Our results will aid the rational engineering of PETases with high depolymerization efficiency toward the crystalline region of PET for fighting the challenges of industrial applications.