Moist-Solid Biocatalysis Enables Processive Depolymerization of Crystalline PET

Enzymatic depolymerization of polyethylene terephthalate (PET) is typically limited to the amorphous regions of the plastic, therefore requiring energy-intensive pretreatment. Here, using the commercial cutinase HiC, we demonstrate that mechanoenzymatic reactions under moist-solid conditions enable nearly complete depolymerization of highly crystalline PET (42.5% crystallinity) without prior amorphization. The reaction proceeded at the same rate and to the same yield for both high crystallinity and amorphous PET. Remarkably, we show that the limited water availability in moist solids shifts the enzyme equilibrium toward the polymer-bound state, promoting processive cleavage along individual PET chains. Time-course analysis of the reaction products is consistent with processive action (chain end scission). Remarkably, this processivity emerges in an enzyme not previously known to act processively, highlighting the unique catalytic environment created by moist solids. This approach operates at high solids loading (≥40 wt %) and avoids harsh chemicals and conditions, offering an environmentally and economically attractive strategy for PET recycling. The feasibility of scaling up moist-solid depolymerization using industrially relevant equipment is also demonstrated. These findings establish moist-solid biocatalysis as a powerful platform for efficient, scalable, and selective polymer depolymerization.