Combined Adsorption and Michaelis–Menten Approach Reveals Predominant Enzymatic Depolymerization of Crystalline Poly(ethylene terephthalate) by <i>Humicola insolens</i> Cutinase in the Solution Phase

Low conversion of enzymatic hydrolysis of crystalline poly(ethylene terephthalate) (PET) impacts its recycling and end-of-life persistence. Yet, the mechanistic insights into the heterogeneous enzymatic reaction process and the impact of crystallinity remain poorly understood. Here, we combined adsorption and Michaelis–Menten kinetic experiments to examine the influence of PET crystallinity and particle size on depolymerization by a commercial cutinase (HiC). Batch adsorption experiments and modeling revealed that HiC adsorption was unaffected by PET crystallinity. Conventional and inverse Michaelis–Menten kinetic experiments and modeling indicated that the low hydrolysis rates of crystalline PET were more due to slower kinetics rather than low reaction densities. Under the same crystallinity, intrinsic reactivity between different particle sizes was different for amorphous PET. Furthermore, the reaction densities of HiC exceeded adsorption densities by 3–10-fold across all PET, implying that solution phase hydrolysis of oligomers released by initially adsorbed enzyme yielded most monomer products. Finally, prephotoweathering increased initial hydrolysis rates up to an order of magnitude for amorphous PET, highlighting the potential role of UV in PET recycling and environmental persistence. Overall, quantitative insights from this work can guide the rational design of enzymes and recycling processes and inform the end-of-life fate of PET waste.