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The rate response of poly(ethylene terephthalate) (PET)-hydrolases to increased substrate crystallinity (X C ) of PET manifests as a rate-lowering effect that varies significantly for different enzymes. Herein, we report the influence of X C on the product release rate of six thermostable PET-hydrolases. All enzyme reactions displayed a distinctive lag phase until measurable product formation occurred. The duration of the lag phase increased with X C . The recently discovered PET-hydrolase PHL7 worked efficiently on "amorphous" PET disks (X C � 10 %), but this enzyme was extremely sensitive to increased X C , whereas the enzymes LCC ICCG , LCC, and DuraPETase had higher tolerance to increases in X C and had activity on PET disks having X C of 24.4 %. Microscopy revealed that the X C -tolerant hydrolases generated smooth and more uniform substrate surface erosion than PHL7 during reaction. Structural and molecular dynamics analysis of the PET-hydrolyzing enzymes disclosed that surface electrostatics and enzyme flexibility may account for the observed differences.

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