On the Binding Mode and Molecular Mechanism of Enzymatic Polyethylene Terephthalate Degradation

Enzymatic degradation of polyethylene terephthlate (PET) by polyester hydrolases is currently subject to intensive research, as it is considered as a potential eco-friendly recycling method for plastic waste. However, the substrate-binding mode and the molecular mechanism of enzymatic PET hydrolysis are still under intense investigation, and controversial hypotheses have been presented. To help unravel the inherent mechanism of biocatalytic PET degradation at the atomic level, we performed solid-state NMR measurements of a cutinase from Thermobifida fusca (TfCut2) embedded in trehalose glasses together with chemically synthesized, amorphous C-13(=O)-labeled oligomeric PET. The resulting ternary enzyme-PET-trehalose glassy system enabled advanced solid-state NMR methods for real-time tracking of the enzymatic PET degradation and the investigation of PET chain dynamics. Combined with enhanced-sampling molecular dynamics simulations, specific enzyme-substrate interactions during the degradation process could also be monitored. Our results demonstrate that the PET chain is first cleaved by TfCut2 in blocks of at least one repeat unit and further to terephthalic acid and ethylene glycol. Moreover, the second step (formation of final hydrolysis products) appears to be rate-limiting in such reactions. The observed dynamic changes and interfacial protein contacts of C-13-labeled PET carbonyl groups suggest that only one PET repeat unit is bound to the enzyme during the degradation process while the rest of the PET chain is only loosely confined to the active site. These results, not accessible by using conventional solution enzyme samples and small nonhydrolyzable substrates, provide a better understanding of the biocatalytic PET degradation mechanism of polyester hydrolases.