Recent Advances in the Chemobiological Upcycling of Polyethylene Terephthalate (PET) into Value-Added Chemicals.

Polyethylene terephthalate (PET) is a plastic material commonly applied to beverage packaging used in everyday life. Owing to PET's versatility and ease of use, its consumption has continuously increased, resulting in considerable waste generation. Several physical and chemical recycling processes have been developed to address this problem. Recently, biological upcycling is being actively studied and has come to be regarded as a powerful technology for overcoming the economic issues associated with conventional recycling methods. For upcycling, PET should be degraded into small molecules, such as terephthalic acid and ethylene glycol, which are utilized as substrates for bioconversion, through various degradation processes, including gasification, pyrolysis, and chemical/biological depolymerization. Furthermore, biological upcycling methods have been applied to biosynthesize value-added chemicals, such as adipic acid, muconic acid, catechol, vanillin, and glycolic acid. In this review, we introduce and discuss various degradation methods that yield substrates for bioconversion and biological upcycling processes to produce value-added biochemicals. These technologies encourage a circular economy, which reduces the amount of waste released into the environment.

Biocatalytic valorization of lignin subunit: Screening a carboxylic acid reductase with high substrate preference to syringyl functional group.

Lignin is inexpensive and the most abundant source of biological aromatics. It can be decomposed to three types of subunits, 4-hydroxybenzoic, vanillic and syringic acids, each of which can be valorized to value added compounds. Syringaldehyde is a versatile phenolic aldehyde implicated with multiple bioactive properties as well as intermediates for biofuels. Herein, fourteen microbial carboxylic acid reductases (CARs) were screened for the biocatalysis of the energetically unfavorable reduction of syringic acid to syringaldehyde. Nine CARs were positive to syringic acid reduction, among which Mycobacterium abscessus CAR exhibited the highest analytical yield of the product. By the optimization of the reaction condition, the whole-cell biocatalyst (i.e., recombinant Escherichia coli expressing the gene) successfully converted syringic acid to syringaldehyde with a yield of 90%. Furthermore, structural features of the screened CAR responsible for the specificity toward the syringyl subunit were analyzed that helps to further engineer the biocatalyst for improved performances.

Chemo-Biological Upcycling of Poly(ethylene terephthalate) to Multifunctional Coating Materials.

Chemo-biological upcycling of poly(ethylene terephthalate) (PET) developed in this study includes the following key steps: chemo-enzymatic PET depolymerization, biotransformation of terephthalic acid (TPA) into catechol, and its application as a coating agent. Monomeric units were first produced through PET glycolysis into bis(2-hydroxyethyl) terephthalate (BHET), mono(2-hydroxyethyl) terephthalate (MHET), and PET oligomers, and enzymatic hydrolysis of these glycolyzed products using Bacillus subtilis esterase (Bs2Est). Bs2Est efficiently hydrolyzed glycolyzed products into TPA as a key enzyme for chemo-enzymatic depolymerization. Furthermore, catechol solution produced from TPA via a whole-cell biotransformation (Escherichia coli) could be directly used for functional coating on various substrates after simple cell removal from the culture medium without further purification and water-evaporation. This work demonstrates a proof-of-concept of a PET upcycling strategy via a combination of chemo-biological conversion of PET waste into multifunctional coating materials.