Poly(ethylene terephthalate) (PET) is used extensively worldwide in plastic products, and its accumulation in the environment has become a global concern. Because the ability to enzymatically degrade PET has been thought to be limited to a few fungal species, biodegradation is not yet a viable remediation or recycling strategy. By screening natural microbial communities exposed to PET in the environment, we isolated a novel bacterium, Ideonella sakaiensis 201-F6, that is able to use PET as its major energy and carbon source. When grown on PET, this strain produces two enzymes capable of hydrolyzing PET and the reaction intermediate, mono(2-hydroxyethyl) terephthalic acid. Both enzymes are required to enzymatically convert PET efficiently into its two environmentally benign monomers, terephthalic acid and ethylene glycol.

http://science.sciencemag.org/content/sci/351/6278/1196.full.pdf

A Bacterium That Degrades and Assimilates Poly(ethylene Terephthalate)

Some bacteria think plastic is fantastic Bacteria isolated from outside a bottle-recycling facility can break down and metabolize plastic. The proliferation of plastics in consumer products, from bottles to clothing, has resulted in the release of countless tons of plastics into the environment. Yoshida et al. show how the biodegradation of plastics by specialized bacteria could be a viable bioremediation strategy (see the Perspective by Bornscheuer). The new species, Ideonella sakaiensis, breaks down the plastic by using two enzymes to hydrolyze PET and a primary reaction intermediate, eventually yielding basic building blocks for growth. Science, this issue p. 1196; see also p. 1154 Two specialized enzymes from a newly isolated bacterium break down plastic into its simplest building blocks. [Also see Perspective by Bornscheuer] Poly(ethylene terephthalate) (PET) is used extensively worldwide in plastic products, and its accumulation in the environment has become a global concern. Because the ability to enzymatically degrade PET has been thought to be limited to a few fungal species, biodegradation is not yet a viable remediation or recycling strategy. By screening natural microbial communities exposed to PET in the environment, we isolated a novel bacterium, Ideonella sakaiensis 201-F6, that is able to use PET as its major energy and carbon source. When grown on PET, this strain produces two enzymes capable of hydrolyzing PET and the reaction intermediate, mono(2-hydroxyethyl) terephthalic acid. Both enzymes are required to enzymatically convert PET efficiently into its two environmentally benign monomers, terephthalic acid and ethylene glycol.

A bacterium that degrades and assimilates poly (ethylene terephthalate) and its enzymes involved in the degradation

Present estimates suggest that of the 359 million tons of plastics produced annually worldwide1, 150–200 million tons accumulate in landfill or in the natural environment2. Poly(ethylene terephthalate) (PET) is the most abundant polyester plastic, with almost 70 million tons manufactured annually worldwide for use in textiles and packaging3. The main recycling process for PET, via thermomechanical means, results in a loss of mechanical properties4. Consequently, de novo synthesis is preferred and PET waste continues to accumulate. With a high ratio of aromatic terephthalate units—which reduce chain mobility—PET is a polyester that is extremely difficult to hydrolyse5. Several PET hydrolase enzymes have been reported, but show limited productivity6,7. Here we describe an improved PET hydrolase that ultimately achieves, over 10 hours, a minimum of 90 per cent PET depolymerization into monomers, with a productivity of 16.7 grams of terephthalate per litre per hour (200 grams per kilogram of PET suspension, with an enzyme concentration of 3 milligrams per gram of PET). This highly efficient, optimized enzyme outperforms all PET hydrolases reported so far, including an enzyme8,9 from the bacterium Ideonella sakaiensis strain 201-F6 (even assisted by a secondary enzyme10) and related improved variants11–14 that have attracted recent interest. We also show that biologically recycled PET exhibiting the same properties as petrochemical PET can be produced from enzymatically depolymerized PET waste, before being processed into bottles, thereby contributing towards the concept of a circular PET economy. Computer-aided engineering produces improvements to an enzyme that breaks down poly(ethylene terephthalate) (PET) into its constituent monomers, which are used to synthesize PET of near-petrochemical grade that can be further processed into bottles.

An engineered PET depolymerase to break down and recycle plastic bottles

Increasing the stream of recycled plastic necessitates an approach beyond the traditional recycling via melting and re-extrusion. Various chemical recycling processes have great potential to enhance recycling rates. In this Review, a summary of the various chemical recycling routes and assessment via life-cycle analysis is complemented by an extensive list of processes developed by companies active in chemical recycling. We show that each of the currently available processes is applicable for specific plastic waste streams. Thus, only a combination of different technologies can address the plastic waste problem. Research should focus on more realistic, more contaminated and mixed waste streams, while collection and sorting infrastructure will need to be improved, that is, by stricter regulation. This Review aims to inspire both science and innovation for the production of higher value and quality products from plastic recycling suitable for reuse or valorization to create the necessary economic and environmental push for a circular economy.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/anie.201915651

Beyond Mechanical Recycling: Giving New Life to Plastic Waste

Poly(ethylene terephthalate) (PET) is one of the most abundantly produced synthetic polymers and is accumulating in the environment at a staggering rate as discarded packaging and textiles. The properties that make PET so useful also endow it with an alarming resistance to biodegradation, likely lasting centuries in the environment. Our collective reliance on PET and other plastics means that this buildup will continue unless solutions are found. Recently, a newly discovered bacterium, Ideonella sakaiensis 201-F6, was shown to exhibit the rare ability to grow on PET as a major carbon and energy source. Central to its PET biodegradation capability is a secreted PETase (PET-digesting enzyme). Here, we present a 0.92 A resolution X-ray crystal structure of PETase, which reveals features common to both cutinases and lipases. PETase retains the ancestral α/β-hydrolase fold but exhibits a more open active-site cleft than homologous cutinases. By narrowing the binding cleft via mutation of two active-site residues to conserved amino acids in cutinases, we surprisingly observe improved PET degradation, suggesting that PETase is not fully optimized for crystalline PET degradation, despite presumably evolving in a PET-rich environment. Additionally, we show that PETase degrades another semiaromatic polyester, polyethylene-2,5-furandicarboxylate (PEF), which is an emerging, bioderived PET replacement with improved barrier properties. In contrast, PETase does not degrade aliphatic polyesters, suggesting that it is generally an aromatic polyesterase. These findings suggest that additional protein engineering to increase PETase performance is realistic and highlight the need for further developments of structure/activity relationships for biodegradation of synthetic polyesters.

/pdf/characterization-and-engineering-of-a-plastic-degrading-2rf8vuokpw.pdf

Characterization and engineering of a plastic-degrading aromatic polyesterase

Petroleum-based plastics have replaced many natural materials in their former applications. With their excellent properties, they have found widespread uses in almost every area of human life. However, the high recalcitrance of many synthetic plastics results in their long persistence in the environment, and the growing amount of plastic waste ending up in landfills and in the oceans has become a global concern. In recent years, a number of microbial enzymes capable of modifying or degrading recalcitrant synthetic polymers have been identified. They are emerging as candidates for the development of biocatalytic plastic recycling processes, by which valuable raw materials can be recovered in an environmentally sustainable way. This review is focused on microbial biocatalysts involved in the degradation of the synthetic plastics polyethylene, polystyrene, polyurethane and polyethylene terephthalate (PET). Recent progress in the application of polyester hydrolases for the recovery of PET building blocks and challenges for the application of these enzymes in alternative plastic waste recycling processes will be discussed.

/pdf/microbial-enzymes-for-the-recycling-of-recalcitrant-2zzb19jclu.pdf

Microbial enzymes for the recycling of recalcitrant petroleum-based plastics: how far are we?

In this study cutinases from Thermobifida cellulosilytica DSM44535 (Thc_Cut1 and Thc_Cut2) and Thermobifida fusca DSM44342 (Thf42_Cut1) hydrolyzing poly(ethylene terephthalate) (PET) were successfully cloned and expressed in E.coli BL21-Gold(DE3). Their ability to hydrolyze PET was compared with other enzymes hydrolyzing natural polyesters, including the PHA depolymerase (ePhaZmcl) from Pseudomonas fluorescens and two cutinases from T. fusca KW3. The three isolated Thermobifida cutinases are very similar (only a maximum of 18 amino acid differences) but yet had different kinetic parameters on soluble substrates. Their kcat and Km values on pNP–acetate were in the ranges 2.4–211.9 s–1 and 127–200 μM while on pNP–butyrate they showed kcat and Km values between 5.3 and 195.1 s–1 and between 1483 and 2133 μM. Thc_Cut1 released highest amounts of MHET and terephthalic acid from PET and bis(benzoyloxyethyl) terephthalate (3PET) with the highest concomitant increase in PET hydrophilicity as indicated by water co...

/pdf/enzymatic-surface-hydrolysis-of-pet-effect-of-structural-4wj448xowc.pdf

Enzymatic surface hydrolysis of PET : effect of structural diversity on kinetic properties of cutinases from thermobifida

Polyethylene terephthalate (PET) is one of the most important synthetic polymers used today. Unfortunately, the polymers accumulate in nature and to date no highly active enzymes are known that can degrade it at high velocity. Enzymes involved in PET degradation are mainly α- and β-hydrolases, like cutinases and related enzymes (EC 3.1.1). Currently, only a small number of such enzymes are well characterized. In this work, a search algorithm was developed that identified 504 possible PET hydrolase candidate genes from various databases. A further global search that comprised more than 16 Gb of sequence information within 108 marine and 25 terrestrial metagenomes obtained from the Integrated Microbial Genome (IMG) database detected 349 putative PET hydrolases. Heterologous expression of four such candidate enzymes verified the function of these enzymes and confirmed the usefulness of the developed search algorithm. In this way, two novel and thermostable enzymes with high potential for downstream application were partially characterized. Clustering of 504 novel enzyme candidates based on amino acid similarities indicated that PET hydrolases mainly occur in the phyla of Actinobacteria, Proteobacteria, and Bacteroidetes Within the Proteobacteria, the Betaproteobacteria, Deltaproteobacteria, and Gammaproteobacteria were the main hosts. Remarkably enough, in the marine environment, bacteria affiliated with the phylum Bacteroidetes appear to be the main hosts of PET hydrolase genes, rather than Actinobacteria or Proteobacteria, as observed for the terrestrial metagenomes. Our data further imply that PET hydrolases are truly rare enzymes. The highest occurrence of 1.5 hits/Mb was observed in sequences from a sample site containing crude oil.IMPORTANCE Polyethylene terephthalate (PET) accumulates in our environment without significant microbial conversion. Although a few PET hydrolases are already known, it is still unknown how frequently they appear and with which main bacterial phyla they are affiliated. In this study, deep sequence mining of protein databases and metagenomes demonstrated that PET hydrolases indeed occur at very low frequencies in the environment. Furthermore, it was possible to link them to phyla that were previously not known to harbor such enzymes. This work contributes novel knowledge on the phylogenetic relationships, the recent evolution, and the global distribution of PET hydrolases. Finally, we describe the biochemical traits of four novel PET hydrolases.

New insights into the function and global distribution of polyethylene terephthalate (PET)-degrading bacteria and enzymes in marine and terrestrial metagenomes

The global production of fossil‐based plastics has grown more than 20‐fold since 1964 to 322 million tons in 2015, and a slowdown of this rate is not expected (Ellen MacArthur Foundation and World Economic Forum, 2014; PlasticsEurope, 2016). Many materials derived from synthetic polymers have already replaced their natural counterparts in all areas of human life. The majority of plastics are short‐lived products which are disposed within 1 year after manufacture. However, only 14% of plastic packaging materials used worldwide is currently collected for recycling while another 14% is incinerated for energy recovery (Ellen MacArthur Foundation and World Economic Forum, 2014). The remaining 72% of plastic packaging is not recovered with 40% land filled and 32% estimated to completely escape the collection system. This part of plastic waste ends up in diverse natural habitats, especially in oceans where it can cause serious environmental damages (Andrady, 2015; Jambeck et al., 2015). Therefore, innovative technologies to improve the recycling of plastics and to reduce the consumption of non‐renewable fossil feed stocks are required.

Polyethylene terephthalate (PET) is the most widely used synthetic polyester. It is a thermoplastic of high‐molecular‐weight composed of terephthalic acid (TPA) and ethylene glycol (EG). PET can exist as both an amorphous and a semi‐crystalline polymer (Webb et al., 2013). Owing to its excellent physical and chemical properties, PET finds numerous applications as textile fibres, packaging materials and beverage bottles. PET is generally referred to as ′polyester′ in the textile industry which consumes the majority of the PET produced globally. In 2014, 49.2 million tons of PET fibres was produced worldwide (Fiber Economics Bureau, 2015). In 2015, the global production of PET resins was 27.8 million tons which was dominantly used for the manufacture of packaging materials and beverage bottles (Plastic Insight, 2016). Almost half of the postconsumer PET bottles worldwide are collected for mechanical recycling to produce polyester fibres (Ellen MacArthur Foundation and World Economic Forum, 2014).

Polyethylene terephthalate made from renewable biomass (bio‐PET) is becoming of industrial interest lately. EG derived from sugarcane ethanol (Tsiropoulos et al., 2015) and TPA derived from sugar beet paraxylene (Collias et al., 2014; Smith, 2015) can be utilized to replace their fossil‐based counterparts to produce bottle‐grade PET. Although fossil feedstocks can be saved by the commercialization of bio‐PET bottles, the challenges for their recycling remain as their recalcitrant properties are the same as those of petroleum‐derived PET bottles (Chen et al., 2016).

Biocatalysis as a green route for recycling the recalcitrant plastic polyethylene terephthalate

Considerable research achievements were made to address the plastic crisis using biotechnology, but this is still limited to polyesters. This Comment aims to clarify important aspects related to myths and realities about plastic biodegradation and suggests distinct strategies for a bio-based circular plastic economy in the future.

Ren Wei

Papers

Microbial enzymes for the recycling of recalcitrant petroleum-based plastics: how far are we?

Enzymatic surface hydrolysis of PET : effect of structural diversity on kinetic properties of cutinases from thermobifida

New insights into the function and global distribution of polyethylene terephthalate (PET)-degrading bacteria and enzymes in marine and terrestrial metagenomes

Biocatalysis as a green route for recycling the recalcitrant plastic polyethylene terephthalate

Possibilities and limitations of biotechnological plastic degradation and recycling