Comparative biochemistry of four polyester (PET) hydrolases

The potential of bioprocessing in a circular plastic economy has strongly stimulated research in enzymatic degradation of different synthetic polymers. Particular interest has been devoted to the commonly used polyester, poly(ethylene terephthalate) (PET), and  a number of PET hydrolases have been described. However, a kinetic framework for comparisons of PET hydrolases (or other plastic degrading enzymes) acting on the insoluble substrate, has not been established. Here, we propose such a framework and test it against kinetic measurements on four PET hydrolases. The analysis provided values of k_cat and K_M, as well as an apparent specificity constant in the conventional units of M^-1s^-1. These parameters, together with experimental values for the number of enzyme attack sites on the PET surface, enabled comparative analyses. A variant of the PET hydrolase from Ideonella sakaiensis was the most efficient enzyme at ambient conditions, which relied on a high kcat rather than a low K_M. Moreover, both soluble and insoluble PET fragments were consistently hydrolyzed much faster than intact PET. This suggests that interactions between polymer strands slow down PET degradation, while the chemical steps of catalysis and the low accessibility associated with solid substrate were less important for the overall rate. Finally, the investigated enzymes showed a remarkable substrate affinity, and reached half the saturation rate on PET, when the concentration of attack sites in the suspensi.