Relationships of crystallinity and reaction rates for enzymatic degradation of poly (ethylene terephthalate), PET

Abstract

Biocatalytic degradation of plastic waste is anticipated to play an important role in future recycling systems. However, enzymatic degradation of crystalline poly (ethylene terephthalate) (PET) remains consistently poor. Herein, we employed functional assays to elucidate the molecular underpinnings of this limitation. This included utilizing complementary activity assays to monitor the degradation of PET disks with varying crystallinity (XC), as well as kinetic parameters for soluble PET fragments. The results indicate that a proficient PET-hydrolase, LCCICCG, operates through an endolytic mode of action, and that its activity is limited by conformational constraints in the PET polymer. Such constraints become more pronounced at highXCvalues, and this limits the density of productive sites on the PET surface. Endolytic chain-scissions are the dominant reaction type in the initial stage, and this means that little or no soluble organic product occurs here. However, endolytic cuts gradually and locally promote chain mobility and hence the density of attack sites on the surface. This leads to an upward concave progress curve; a behavior sometimes termed lag-phase kinetics.

From Bulk to Binding: Decoding the Entry of PET into Hydrolase Binding Pockets

“…This is also consistent with a recent study by Schubert et al , in which chain mobility and the number of available sites for endo-type chain scission, which depend on the degree of crystallinity, are postulated as the activity-limiting factor causing the initial lag phase for the release of the soluble product. 7 Another biotechnological approach to mitigate this hindering effect could be to design enzymes to introduce residues that enhance enzyme–PET interactions, which would promote and counterbalance the loss of PET–PET interactions upon entry into the binding cleft.…”

“…PET is an aromatic, semicrystalline thermoplastic that can be enzymatically degraded into monomeric and oligomeric building blocks including 2-hydroxyethylterephthalic acid (MHET), which can then be further hydrolyzed into its monomers terephthalic acid (TPA) and ethylene glycol (EG) by the same or different enzymes. 4,7–9 The monomers can be recovered to synthesize high quality virgin PET (Fig. 1).…”