Hydrolases catalyzed nanosized polyethylene terephthalate depolymerization: New insights from QM/MM analysis

Enzymatic depolymerization has become an increasingly important solution to PET pollution. However, the understanding of depolymerization reaction mechanism of environment-relevant PET waste and the influence of PET size on the depolymerization efficiency are still insufficient. In this work, the nanosized model substrate was used to systematically explore environment-relevant PET depolymerization process with the aid of quantum mechanics/molecular mechanics (QM/MM) techniques. The results supported that the depolymerization process of nanosized PET debris was similar with the routinely used model substrate (PET dimer). However, the enzymatic energy landscape was significantly changed and step iii became the rate-determining step for both IsPETase and LCCICCG with the Boltzmann-weighted average barriers of 15.0 and 12.3 kcal/mol, respectively. The relationships between PET binding position, PET size, and activation energies were further elucidated. This "size effect" was also verified by enzyme kinetic experiments. The influences of "size effect" on depolymerization activity were clarified through hydrogen bond network and distortion/interaction analysis. In addition, the relationship between active site features and activation energy barrier was established. These molecular insights will offer a key basis for rational enzyme engineering strategy to remove PET in a more environmentally friendly and efficient way.