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.

β-Ketoadipic acid production from poly(ethylene terephthalate) waste via chemobiological upcycling

“…The PCA decarboxylation-encoded AroY enzyme for PCA conversion to catechol was inserted into the pKA312 vector. 5 The cat operon genes for catechol-1,2-dioxygenase ( CatA ), muconate cycloisomerase ( CatB ), and muconolactone D isomerase ( CatC ), as well as PcaD , were inserted into the pKE112 vector to generate βKA from catechol through meta-cleavage. 14 For pKE112-catABCpcaD, DNA fragments from each gene were cloned sequentially using EcoRI , KpnI , BamHI , SbfI , and HindIII restriction enzymes.…”

“…While enzymatic hydrolysis offers a promising biotechnological approach to PET degradation under mild conditions, but it has serious limitations, such as the prerequisite for amorphous or low-crystallinity PET necessary for the proper enzyme activity. 5 Recently, a combination of chemical depolymerization of PET and biological upcycling of TPA have been developed to produce value-added chemicals such as protocatechuic acid (PCA), gallic acid, pyrogallol, catechol, muconic acid, and vanillic acid. These chemicals have various industrial applications and can be produced through whole-cell bioconversion.…”

“…In a previous study, TPA inhibited bioconversion and reduced the yield of a biocatalytic reaction by over 1 g L −1 . 5 Therefore, overcoming the TPA-induced inhibition is crucial to develop an efficient biological upcycling process with sustainability.…”

“…These chemicals have various industrial applications and can be produced through whole-cell bioconversion. 4–8…”

Recent Advances and Challenges in Enzymatic Depolymerization and Recycling of PET Wastes

“…Despite a low sequence identity of <50 %, the enzyme structure of Is PETase is rather similar to PET‐hydrolyzing cutinases. Many research groups identified new cutinase‐like (503, [17] 504, [17] 611, [17] Bhr PETase, [18] PET2, [19] PES−H1 (PHL7), [20] MG8 [21] ) and Is PETase‐like (PE−H, [22] Bur PL ( Bb PETase), [23] Rg PETase [24] ) PET hydrolases which expanded the diversity of enzymatic depolymerization of PET [4d–f] (Figure 1).…”