The study investigates the reaction mechanism of the *Ideonella sakaiensis* PETase enzyme, which is responsible for depolymerizing polyethylene terephthalate (PET). Using quantum mechanical/molecular mechanical (QM/MM) transition path sampling and inertial likelihood maximization (iLMax), the researchers identify optimal reaction coordinates for the acylation and deacylation steps of the two-step serine hydrolase mechanism. The results suggest that deacylation is the rate-limiting step, and the reaction coordinates include elements describing nucleophilic attack, ester bond cleavage, and the "moving-histidine" mechanism. The flexibility of Trp185 is found to promote the reaction, providing a molecular rationale for the decreased activity observed in mutations that restrict Trp185 motion. The study reveals detailed reaction mechanisms that can inform further engineering of enzymes for plastics bioconversion.The study investigates the reaction mechanism of the *Ideonella sakaiensis* PETase enzyme, which is responsible for depolymerizing polyethylene terephthalate (PET). Using quantum mechanical/molecular mechanical (QM/MM) transition path sampling and inertial likelihood maximization (iLMax), the researchers identify optimal reaction coordinates for the acylation and deacylation steps of the two-step serine hydrolase mechanism. The results suggest that deacylation is the rate-limiting step, and the reaction coordinates include elements describing nucleophilic attack, ester bond cleavage, and the "moving-histidine" mechanism. The flexibility of Trp185 is found to promote the reaction, providing a molecular rationale for the decreased activity observed in mutations that restrict Trp185 motion. The study reveals detailed reaction mechanisms that can inform further engineering of enzymes for plastics bioconversion.