An Overview into Polyethylene Terephthalate (PET) Hydrolases and Efforts in Tailoring Enzymes for Improved Plastic Degradation

Abstract

Plastic or microplastic pollution is a global threat affecting ecosystems, with the current generation reaching as much as 400 metric tons per/year. Soil ecosystems comprising agricultural lands act as microplastics sinks, though the impact could be unexpectedly more far-reaching. This is troubling as most plastic forms, such as polyethylene terephthalate (PET), formed from polymerized terephthalic acid (TPA) and ethylene glycol (EG) monomers, are non-biodegradable environmental pollutants. The current approach to use mechanical, thermal, and chemical-based treatments to reduce PET waste remains cost-prohibitive and could potentially produce toxic secondary pollutants. Thus, better remediation methods must be developed to deal with plastic pollutants in marine and terrestrial environments. Enzymatic treatments could be a plausible avenue to overcome plastic pollutants, given the near-ambient conditions under which enzymes function without the need for chemicals. The discovery of several PET hydrolases, along with further modification of the enzymes, has considerably aided efforts to improve their ability to degrade the ester bond of PET. Hence, this review emphasizes PET-degrading microbial hydrolases and their contribution to alleviating environmental microplastics. Information on the molecular and degradation mechanisms of PET is also highlighted in this review, which might be useful in the future rational engineering of PET-hydrolyzing enzymes.

Keywords: PET hydrolases; biodegradation; plastic waste; polyethylene terephthalate; recycling.

Figure 1

Recycling treatments to treat PET wastes.

Figure 2

The three-dimensional protein folds of the different hydrolases that reportedly degrade PET (a) T. fusca cutinase structure with one disulfide bond (Cys241-Cys249) (PDB ID: 4CG3); (b) I. sakaiensis IsPETase with two disulfide bonds (Cys239-Cys203, Cys273-Cys289) (PDB ID: 5XJH); (c) I. sakaiensis MHETase with a large lid domain and five disulfide bonds (Cys51-Cys92, Cys224-Cys529, Cys303-Cys302, Cys340-Cys348, and Cys-577-Cys599) (PDB ID: 6QGA); (d) C. antarctica lipase with a α₅-helix lid (PDB ID: 4K6G); (e) P. aestusnigri carboxylesterase (PDB ID: 6SBN); (f) T. halotolerans esterase (GenBank: AFA45122.1). Catalytic triad residues are highlighted in cyan, cysteine residues are presented by black sticks, and lid domains are in blue, respectively.

Figure 3

Schematic depiction of PET degradation catalyzed by PETase or cutinase. Polyethylene terephthalate (PET) is hydrolyzed by PETase/cutinase to produce bis(2-hydroxyethyl) terephthalate (BHET), mono(2-hydroxyethyl) terephthalate (MHET), terephthalic acid (TPA), and ethylene glycol (EG). MHET is hydrolyzed again by the second enzyme, MHETase, to yield terephthalic acid (TPA) and ethylene glycol (EG).