Characterization of a new cutinase fromThermobifida albafor PET-surface hydrolysis

Current state and sustainable management of waste polyethylene terephthalate bio-disposal: enzymatic degradation to upcycling

Poly (ethylene terephthalate) (PET) is a widely used plastic that leads to significant environmental pollution due to its durability. Enzymatic degradation of PET presents an eco-friendly disposal approach, with potential scalability for industrial applications. This review examines key crucial factors influencing PET enzymatic degradation, including the catalytic efficiency of PET hydrolase, production scalability of PET hydrolase, and recyclability of degraded PET. We outline major advancements in PET hydrolase development, including discovery techniques, functional enhancement strategies, and degradation optimization. Additionally, it assesses the preparation methodologies for PET hydrolase, covering bacterial expression systems, high-density fermentation technologies, and approaches for sustainable catalytic use. The review also discusses upcycling processes for PET hydrolysates, focusing on repolymerization into new plastics or bioconversion into valuable chemicals. Successful achievement of waste PET bio-disposal in industrial-scale n hinges on balancing degradation costs with revenue from upcycling products. Aim at this target, the review further points out the critical challenges, and proposes targeted solutions and expectations.

Unveiling the toxicity of micro-nanoplastics: A systematic exploration of understanding environmental and health implications

The surge in plastic production has spurred a global crisis as plastic pollution intensifies, with microplastics and nanoplastics emerging as notable environmental threats. Due to their miniature size, these particles are ubiquitous across ecosystems and pose severe hazards as they are ingested and bioaccumulate within organisms. Although global plastic production has reached an alarming 400.3 MTs, recycling efforts remain limited, with only 18.5 MTs being recycled. Currently, out of the total plastic waste, 49.6 % is converted into energy, 27 % is recycled, and 23.5 % is recovered as material, indicating a need for better waste management practices to combat the escalating pollution levels. Research studies on micro-nanoplastics have primarily concentrated on their environmental presence and laboratory-based toxicity studies. This review critically examines the sources and detection methods for micro-nanoplastics, emphasising their toxicological effects and ecological impacts. Organisms like zebrafish and rats serve as key models for studying these particle's bioaccumulative potential, showcasing adverse effects that extend to DNA damage, oxidative stress, and cellular apoptosis. Studies reveal that micro-nanoplastics can permeate biological barriers, including the blood-brain barrier, neurological imbalance, cardiac, respiratory, and dermatological disorders. These health risks, particularly relevant for humans, underscore the urgency for broader, real-world studies beyond controlled laboratory conditions. Additionally, the review discusses innovative energy-harvesting technologies as sustainable alternatives for plastic waste utilisation, particularly valuable for energy-deficient regions. These strategies aim to simultaneously address energy demands and mitigate plastic waste. This approach aligns with global sustainability goals, providing a promising avenue for both pollution reduction and energy generation. The review calls for further research to enhance detection techniques, assess long-term environmental impacts, and explore sustainable solutions that integrate energy recovery with pollution mitigation, especially in regions most affected by both energy shortages and increased plastic waste.

Discovery of two novel cutinases from a gut yeast of plastic-eating mealworm for polyester depolymerization

Identification of novel plastic-degrading enzymes is crucial for developing enzymatic degradation and recycling strategies for plastic waste. Here, we report the discovery of two novel cutinases, SiCut1 and SiCut2, from a yeast strain Sakaguchia sp. BIT-D3 was isolated from the gut of plastic-eating mealworms. Their amino acid sequences share less than 25% identity with all previously described cutinases and reveal a conserved S-D-H catalytic triad with a unique GYSKG motif. Their recombinant proteins were successfully overexpressed in Pichia pastoris. The pH range for both enzymes was 4.0 to 11.0 and the temperature range for SiCut1 and SiCut2 was 10°C to 50°C and 10°C to 70°C, respectively. Both enzymes showed strong activity against apple cutin and short-chain fatty acid esters of p-nitrophenol and glycerol, substantiating their classification as true cutinases. SiCut1 and SiCut2 have been demonstrated to exhibit efficient degradation of polycaprolactone (PCL) film, polybutylene succinate (PBS) film, and polyester-polyurethane (PUR) foam. Molecular docking and molecular dynamics simulations were used to elucidate the underlying mechanisms of the observed catalytic activity and thermal stability. This study shows that SiCut1 and SiCut2 are novel yeast-derived cutinases with the potential for depolymerization and recycling of plastic waste.IMPORTANCEThe identification of novel plastic-degrading enzymes is critical in addressing the pervasive problem of plastic pollution. This study presents two unique cutinases, SiCut1 and SiCut2, derived from the yeast Sakaguchia sp. BIT-D3 isolated from the gut of plastic-feeding mealworms. Despite sharing less than 25% sequence identity with known cutinases, both enzymes exhibit remarkable degradation capabilities against various polyester plastics, including polycaprolactone (PCL) film, polybutylene succinate (PBS) film, and polyester-polyurethane (PUR) foam. Our results elucidate the catalytic mechanisms of SiCut1 and SiCut2 and provide insights into their potential applications in enzymatic degradation and recycling strategies. By harnessing the gut microbiota of plastic-degrading organisms, this research lays the foundation for innovative enzyme-based solutions to reduce plastic waste and promote sustainable practices in waste management.

Efficient Bioprocess for Mixed PET Waste Depolymerization Using Crude Cutinase

In recent years, several plastic-degrading enzymes with efficient depolymerization abilities for PET have been reported. Here, we report a bioprocess for mixed PET waste depolymerization using crude extracellularly expressed enzymes in E. coli. The enzymes, namely FastPETase, LCC, and LCCICCG, were screened to depolymerize amorphous PET powder and films of different sizes and crystallinity. FastPETase, LCC, and LCCICCG achieved approximately 25, 34, and 70% depolymerization, respectively, when applied to 13 g L-1 of PET film, powder, or mixed waste in optimized enzyme conditions without any pH control. The yield of terephthalic acid in the hydrolytic process was maximum for LCCICCG followed by LCC and FastPETase. Finally, extracellular LCCICCG-producing E. coli cells were cultivated using minimal media supplemented with 0.1% ammonium chloride and 1% glycerol as nitrogen and carbon sources in a bioreactor with a final protein content and specific activity of 119 ± 5 mg L-1 and 1232 ± 18 U mg-1, respectively. Nearly complete depolymerization of 13 g L-1 PET and 23.8 g L-1 post-consumer PET was achieved in 50 h using crude LCCICCG supernatant, without enzyme purification, at 62 °C. A bioprocess was thus developed to depolymerize 100 g L-1 mixed PET trays and bottle waste (MW1 and MW2), reaching 78% and 50% yield at 62 °C with a crude enzyme loading of 2.32 mg g-1 PET in 60 h. The results demonstrate an easy PET depolymerization strategy that could be exploited in large-scale facilities for efficient plastic waste treatment.

Evaluating cutinase from Fusarium oxysporum as a biocatalyst for the degradation of nine synthetic polymer

Plastic poses a significant environmental impact due to its chemical resilience, leading to prolonged and degradation times and resulting in widespread adverse effects on global flora and fauna. Cutinases are essential enzymes in the biodegradation process of synthetic polymers like polyethylene terephthalate (PET), which recognized organisms can break down. Here, we used molecular dynamics and binding free energy calculations to explore the interaction of nine synthetic polymers, including PET, with Cutinase from Fusarium oxysporum (FoCut). According to our findings, the polymers poly(ethylene terephthalate) (PET), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), poly(butylene succinate) (PBS), poly(butylene adipate-co-terephthalate) (PBAT) and poly(ε-caprolactone) (PCL) can bind to the Cutinase enzyme from F. oxysporum, indicating potential biodegradation activity for these polymers. PET exhibited the highest binding affinity (- 34.26 kcal/mol). Besides PET, the polymers PHBH, PBS, PBAT, and PCL also demonstrated significant affinities for the FoCut enzyme, with binding values of - 18.44, - 29.71, - 22.78, and - 22.26 kcal/mol, respectively. Additionally, analysis of the phylogenetic tree of cutinases produced by different organisms demonstrated that even though the organisms belong to different kingdoms, the cutinase from F. oxysporum (FoCut) showed biological similarity in its activity in degrading polymers with the cutinase enzyme from the bacterium Kineococcus radiotolerans and the fungus Moniliophthora roreri. Furthermore, the phylogenetic analysis demonstrated that the PETase enzyme has a very high similarity with the bacterial cutinase enzyme than with the fungal cutinase, therefore demonstrating that the PETase enzyme from Ideonella sakaiensis can easily be a modified bacterial cutinase enzyme that created a unique feature in biodegrading only the pet polymer through an evolutionary process due to its environment and its biochemical need for carbon. Our data demonstrate that bacterial cutinase enzymes have the same common ancestor as the PETase enzyme. Therefore, cutinases and PETase are interconnected through their biological similarity in biodegrading polymers. We demonstrated that important conserved regions, such as the Ser-Asp-His catalytic triad, exist in the enzyme's catalytic site and that all Cut enzymes from different organisms have the same region to couple with the polymer structures.

Polyesterase activity is widespread in the family IV carboxylesterases from bacteria

Enzyme-based depolymerization of plastics, including polyesters, has emerged as a promising approach for plastic waste recycling and reducing environmental plastic pollution. Currently, most of the known polyester-degrading enzymes are represented by a few natural and engineered PETases from the carboxylesterase family V. To identify novel groups of polyesterases, we selected 25 proteins from the carboxylesterase family IV, which share 22 % to 80 % sequence identity to the metagenomic thermophilic polyesterase IS12. All purified proteins were found to be active against chromogenic para-nitrophenyl esters with a preference for short acyl chains. Screening for polyesterase activity using emulsified polyesters demonstrated the presence of hydrolytic activity against bis(benzoyloxyethyl) terephthalate (3PET), polycaprolactone (PCL), and polylactic acid (PLA) in all tested proteins. Biochemical characterization of four selected polyesterases revealed high thermostability in CBA10055, whereas the mesophilic GEN0105 exhibited higher polyesterase activity. Two ancestral variants of GEN0105 showed higher thermostability and activity against PCL and PLA, but reduced activity with amorphous PET. Furthermore, six established PETases were found to be highly active against PCL and PLA. Thus, our results indicate that polyesterase activity is widespread in the family IV carboxylesterases, and that most polyesterases are promiscuous being able to degrade different polyesters.

The biochemical mechanisms of plastic biodegradation

Since the invention of the first synthetic plastic, an estimated 12 billion metric tons of plastics have been manufactured, 70% of which was produced in the last 20 years. Plastic waste is placing new selective pressures on humans and the organisms we depend on, yet it also places new pressures on microorganisms as they compete to exploit this new and growing source of carbon. The limited efficacy of traditional recycling methods on plastic waste, which can leach into the environment at low purity and concentration, indicates the utility of this evolving metabolic activity. This review will categorize and discuss the probable metabolic routes for each industrially relevant plastic, rank the most effective biodegraders for each plastic by harmonizing and reinterpreting prior literature, and explain the experimental techniques most often used in plastic biodegradation research, thus providing a comprehensive resource for researchers investigating and engineering plastic biodegradation.

Isolation and purification of esterase enzyme from marine bacteria associated with biodegradation of polyvinyl chloride (PVC)

Polyvinyl chloride (PVC) is the third most produced synthetic plastic and releases the most harmful and lethal environmental component after incineration and landfilling. Few studies on microbial degradation of PVC have been reported but very little knowledge about the enzymes. In the present study, esterase enzyme was isolated and partially purified from marine bacterial isolates (T-1.3, BP-4.3 and S-237 identified as Vibrio sp., Alteromonas sp., and Cobetia sp., respectively) having the capability of PVC degradation. Initially, a plate assay was carried out for testing esterase production by studying bacteria using 1-naphthyl acetate as substrate. Enzyme assay showed higher production of esterase i.e. 0.57 U mL-1 (2nd day), 0.46 U mL-1 (2nd day) and 0.55 U mL-1 (5th day) by bacterial isolate Vibrio sp., Alteromonas sp. and Cobetia sp., respectively incubated with PVC. Other enzymes like lipase, laccase and manganese peroxidase were much less or negligible compared to esterase enzyme production. Sephadex G-50 column purification had shown 58.62, 42.35 and 223.70 units mg-1 of a specific activity by esterase for bacterial isolates Vibrio sp., Alteromonas sp. and Cobetia sp., respectively. Further, Sephadex G-50 column purification removed all the contamination and gave a clear appearance of the band at 38, 20 and 20 KD for bacterial isolates Vibrio sp., Alteromonas sp., and Cobetia sp., respectively. Esterase has shown maximum stability at a range of pH between 6.0 to 7.5, temperature between 30 to 35 °C and salinity concentration between 3 to 3.5 M for all bacterial isolates. In conclusion, esterase enzyme has promising potential to degrade PVC which can contribute to the decline the plastic pollution in an eco-friendly manner from the environment.

In Silico Analysis and Biochemical Characterization of Streptomyces PET Hydrolase with Bis(2-Hydroxyethyl) Terephthalate Biodegradation Activity

Polyethylene terephthalate (PET), one of the most widely used plastics in the world, causes serious environmental problems. Recently, scientists have been focused on the enzymatic degradation of PET, an environmentally friendly method that offers an attractive approach to the degradation and recycling of PET. In this work, PET hydrolase from Streptomyces sp. W2061 was biochemically characterized, and the biodegradation of PET was performed using the PET model substrate bis (2-hydroxyethyl terephthalate) (BHET). PET hydrolase has an isoelectric point of 5.84, and a molecular mass of about 50.31 kDa. The optimum pH and temperature were 7.0 and 40°C, respectively. LC-MS analysis of the enzymatic products showed that the PET hydrolase successfully degraded a single ester bond of BHET, leading to the formation of MHET. Furthermore, in silico characterization of the PET hydrolase protein sequence and its predicted three-dimensional structure was designed and compared with the well-characterized IsPETase from Ideonella sakaiensis. The structural analysis showed that the (Gly-x1-Ser-x2-Gly) serine hydrolase motif and the catalytic triad (Ser, Asp, and His) were conserved in all sequences. In addition, we integrated molecular dynamics (MD) simulations to analyze the variation in the structural stability of the PET hydrolase in the absence and presence of BHET. These simulations showed the formation of a stable complex between the PET hydrolase and BHET. To the best of our knowledge, this is the first study on Streptomyces sp. W2061 to investigate the BHET degradation activity of PET hydrolase, which has potential application in the biodegradation of plastics in the environment.

Nature-Inspired Strategies for Sustainable Degradation of Synthetic Plastics

Synthetic plastics have become integral to our daily lives, yet their escalating production, limited biodegradability, and inadequate waste management contribute to environmental contamination. Biological plastic degradation is one promising strategy to address this pollution. The inherent chemical and physical properties of synthetic plastics, however, pose challenges for microbial enzymes, hindering the effective degradation and the development of a sustainable biological recycling process. This Perspective explores alternative, nature-inspired strategies designed to overcome some key limitations in currently available plastic-degrading enzymes. Nature's refined degradation pathways for natural polymers, such as cellulose, present a compelling framework for the development of efficient technologies for enzymatic plastic degradation. By drawing insights from nature, we propose a general strategy of employing substrate binding domains to improve targeting and multienzyme scaffolds to overcome enzymatic efficiency limitations. As one potential application, we outline a multienzyme pathway to upcycle polyethylene into alkenes. Employing nature-inspired strategies can present a path toward sustainable solution to the environmental impact of synthetic plastics.

Discovery and characterization of two novel polyethylene terephthalate hydrolases: One from a bacterium identified in human feces and one from the Streptomyces genus

Polyethylene terephthalate (PET) is widely used for various industrial applications. However, owing to its extremely slow breakdown rate, PET accumulates as plastic trash, which negatively affects the environment and human health. Here, we report two novel PET hydrolases: PpPETase from Pseudomonas paralcaligenes MRCP1333, identified in human feces, and ScPETase from Streptomyces calvus DSM 41452. These two enzymes can decompose various PET materials, including semicrystalline PET powders (Cry-PET) and low-crystallinity PET films (gf-PET). By structure-guided engineering, two variants, PpPETaseY239R/F244G/Y250G and ScPETaseA212C/T249C/N195H/N243K were obtained that decompose Cry-PET 3.1- and 1.9-fold faster than their wild-type enzymes, respectively. The co-expression of ScPETase and mono-(2-hydroxyethyl) terephthalate hydrolase from Ideonella sakaiensis (IsMHETase) resulted in 1.4-fold more degradation than the single enzyme system. This engineered strain degraded Cry-PET and gf-PET by more than 40% and 6%, respectively, after 30 d. The concentrations of terephthalic acid (TPA) in the Cry-PET and gf-PET degradation products were 37.7% and 25.6%, respectively. The discovery of these two novel PET hydrolases provides opportunities to create more powerful biocatalysts for PET biodegradation.

Lipases for targeted industrial applications, focusing on the development of biotechnologically significant aspects: A comprehensive review of recent trends in protein engineering

Lipases are remarkable biocatalysts, adept at catalyzing the breakdown of diverse compounds into glycerol, fatty acids, and mono- and di-glycerides via hydrolysis. Beyond this, they facilitate esterification, transesterification, alcoholysis, acidolysis, and more, making them versatile in industrial applications. In industrial processes, lipases that exhibit high stability are favored as they can withstand harsh conditions. However, most native lipases are unable to endure adverse conditions, making them unsuitable for industrial use. Protein engineering proves to be a potent technology in the development of lipases that can function effectively under challenging conditions and fulfill criteria for various industrial processes. This review concentrated on new trends in protein engineering to enhance the diversity of lipase genes and employed in silico methods for predicting and comprehensively analyzing target mutations in lipases. Additionally, key molecular factors associated with industrial characteristics of lipases, including thermostability, solvent tolerance, catalytic activity, and substrate preference have been elucidated. The present review delved into how industrial traits can be enhanced through directed evolution (epPCR, gene shuffling), rational design (FRESCO, ASR), combined engineering strategies (i.e. CAST, ISM, and FRISM) as protein engineering methodologies in contexts of biodiesel production, food processing, and applications of detergent, pharmaceutics, and plastic degradation.

Polyester degradation by soil bacteria: identification of conserved BHETase enzymes in Streptomyces

The rising use of plastic results in an appalling amount of waste which is scattered into the environment. One of these plastics is PET which is mainly used for bottles. We have identified and characterized an esterase from Streptomyces, annotated as LipA, which can efficiently degrade the PET-derived oligomer BHET. The Streptomyces coelicolor ScLipA enzyme exhibits varying sequence similarity to several BHETase/PETase enzymes, including IsPETase, TfCut2, LCC, PET40 and PET46. Of 96 Streptomyces strains, 18% were able to degrade BHET via one of three variants of LipA, named ScLipA, S2LipA and S92LipA. SclipA was deleted from S. coelicolor resulting in reduced BHET degradation. Overexpression of all LipA variants significantly enhanced BHET degradation. All variants were expressed in E. coli for purification and biochemical analysis. The optimum conditions were determined as pH 7 and 25 °C for all variants. The activity on BHET and amorphous PET film was investigated. S2LipA efficiently degraded BHET and caused roughening and indents on the surface of PET films, comparable to the activity of previously described TfCut2 under the same conditions. The abundance of the S2LipA variant in Streptomyces suggests an environmental advantage towards the degradation of more polar substrates including these polluting plastics.

Plastic bio-mitigation by Pseudomonas mendocina ABF786 and simultaneous conversion of its CO2 byproduct to microalgal biodiesel

Bio-mitigation of plastics by microorganisms generates carbon dioxide (CO2) that can be utilized for algal biomass generation. Pseudomonas mendocina ABF786, reportedly the most efficient plastic-degrading bacteria, was screened using the modified most probable number technique. This study highlights the use of an integrative prototype for the production of microalgal biomass (Chlorella vulgaris) in combination with bio-mitigation of plastics, which serves a dual purpose: (i) increased plastic-degradation capability by microorganisms (53%-85% increase in plastic weight loss) due to removal of CO2 feedback inhibition and (ii) increased algal biomass generation (200%-237%) due to supply of extra CO2 from plastic degradation to the algal cultivation flask. Whole-genome sequencing and functional annotation confirmed that all the genes involved in the mineralization of plastic to CO2 are present within the genome of P. mendocina ABF786. Using two or more microbial cultures for remediation may increase the process efficiency.

Engineered Geobacillus lipolytic enzymes - Attractive polyesterases that degrade polycaprolactones and simultaneously produce esters

Plastic pollution is one of the biggest environmental problems plaguing the modern world. Polyester-based plastics contribute significantly to this ecological safety concern. In this study, lipolytic biocatalysts GD-95RM and GDEst-lip developed based on lipase/esterase produced by Geobacillus sp. 95 strain were applied for the degradation of polycaprolactone films (Mn 45.000 (PCL45000) and Mn 80.000 (PCL80000)). The degradation efficiency was significantly enhanced by the addition of short chain alcohols. Lipase GD-95RM (1 mg) can depolymerize 264.0 mg and 280.7 mg of PCL45000 and PCL80000, films respectively, in a 24 h period at 30 °C, while the fused enzyme GDEst-lip (1 mg) is capable of degrading 145.5 mg PCL45000 and 134.0 mg of PCL80000 films in 24 h. The addition of ethanol (25 %) improves the degradation efficiency ~2.5 fold in the case of GD-95RM. In the case of GDEst-lip, 50 % methanol was found to be the optimal alcohol solution and the degradation efficiency was increased by ~3.25 times. The addition of alcohols not only increased degradation speeds but also allowed for simultaneous synthesis of industrially valuable 6-hydroxyhexonic acid esters. The suggested system is an attractive approach for removing of plastic waste and supports the principles of bioeconomics.

Harvesting marine plastic pollutants-derived renewable energy: A comprehensive review on applied energy and sustainable approach

The inevitable use of plastics in the existing standard of life makes its way to ecosystems, predominantly into the marine ecosystem. Recent research on energy recycling from marine discarded plastics through biological, chemical, and thermal processes is summarized, which degrade plastic debris and transform it into energy-efficient products. In a system-oriented approach, different boundaries like carbon efficiency, global warming potential, cumulative energy demand, and cost of the product have been evaluated. Even these technologies may successfully reduce the yearly volume of marine plastics by up to 89% while reducing greenhouse gas emissions by 30%. Conversely, recycling a ton of marine discarded plastics may save 915 cubic feet of landfill space, 6500 kWh of energy, and barrels of oil. Energy may be recovered up to 79% from waste plastics using various techniques. Up to 84% liquid fuel had been generated, with a maximum calorific power of 45 MJ/kg. It has been shown that in Asian countries, the power generation capacity of throw-away facemask wastes regularly varies from 2256 kWh/day to 18.52 million kWh/day. Hence, the conversion of marine plastics into biofuel, syngas, biochar, hydrocarbons, electricity, and value-added functional materials by various biotechnological and chemical processes like biodegradation, pyrolysis, gasification, methanolysis, and hydrolysis should be improvised as a source of alternative energy in the immediate future. Our review signifies the potential benefits of energy harvesting technologies from marine plastics pollutants to overcome the growing challenge of energy demands and provide a long-term solution to underdeveloped and developing countries as a sustainable source of energy. Endorsing current strategies to harvest energy from marine plastic wastes that enhance power generation technologies will help in building a more sustainable and greener environment that imparts a healthy and circular economy while shielding natural resources.

Modulating biofilm can potentiate activity of novel plastic-degrading enzymes

Plastic pollution is an increasing global issue desperately requiring a solution. Only 9% of all plastic waste has been recycled, and whilst recycling gives a second life to plastic, it is costly and there are limited downstream uses of recycled plastic, therefore an alternative is urgently needed. Biodegradation of plastic by microorganisms is a developing field of interest with the potential for bioreactors to be used alongside recycling to degrade plastic that may otherwise be sent to landfill. Here, we have identified two novel polyethylene terephthalate (PET) degrading enzymes through genomic mining and characterised their activity, including their ability to degrade PET. One of the main roadblocks facing the development of microbial enzymes as a plastic biodegradation solution, is that their efficiency is too low to facilitate development as bioremediation tools. In an innovative approach to tackle this roadblock, we hypothesised that enhancing a bacteria's ability to attach to and form a biofilm on plastic could maximise the local concentration of the enzyme around the target substrate, therefore increasing the overall rate of plastic degradation. We found that increasing biofilm levels, by manipulating the levels of the second messenger, Cyclic-di-GMP, led to increased levels of polyester degradation in cells expressing novel and well characterised polyester-degrading enzymes. This indicates that modulating biofilm formation is a viable mechanism to fast track the development of bacterial plastic bioremediation solutions.

Discovery and rational engineering of PET hydrolase with both mesophilic and thermophilic PET hydrolase properties

Excessive polyethylene terephthalate (PET) waste causes a variety of problems. Extensive research focused on the development of superior PET hydrolases for PET biorecycling has been conducted. However, template enzymes employed in enzyme engineering mainly focused on IsPETase and leaf-branch compost cutinase, which exhibit mesophilic and thermophilic hydrolytic properties, respectively. Herein, we report a PET hydrolase from Cryptosporangium aurantiacum (CaPETase) that exhibits high thermostability and remarkable PET degradation activity at ambient temperatures. We uncover the crystal structure of CaPETase, which displays a distinct backbone conformation at the active site and residues forming the substrate binding cleft, compared with other PET hydrolases. We further develop a CaPETase^M9 variant that exhibits robust thermostability with a T_m of 83.2 °C and 41.7-fold enhanced PET hydrolytic activity at 60 °C compared with CaPETase^WT. CaPETase^M9 almost completely decompose both transparent and colored post-consumer PET powder at 55 °C within half a day in a pH-stat bioreactor.

Discovery and mechanism-guided engineering of BHET hydrolases for improved PET recycling and upcycling

Although considerable research achievements have been made to address the plastic crisis using enzymes, their applications are limited due to incomplete degradation and low efficiency. Herein, we report the identification and subsequent engineering of BHETases, which have the potential to improve the efficiency of PET recycling and upcycling. Two BHETases (ChryBHETase and BsEst) are identified from the environment via enzyme mining. Subsequently, mechanism-guided barrier engineering is employed to yield two robust and thermostable ΔBHETases with up to 3.5-fold enhanced kcat/KM than wild-type, followed by atomic resolution understanding. Coupling ΔBHETase into a two-enzyme system overcomes the challenge of heterogeneous product formation and results in up to 7.0-fold improved TPA production than seven state-of-the-art PET hydrolases, under the conditions used here. Finally, we employ a ΔBHETase-joined tandem chemical-enzymatic approach to valorize 21 commercial post-consumed plastics into virgin PET and an example chemical (p-phthaloyl chloride) for achieving the closed-loop PET recycling and open-loop PET upcycling.

Discovering untapped microbial communities through metagenomics for microplastic remediation: recent advances, challenges, and way forward

Microplastics (MPs) are ubiquitous pollutants persisting almost everywhere in the environment. With the increase in anthropogenic activities, MP accumulation is increasing enormously in aquatic, marine, and terrestrial ecosystems. Owing to the slow degradation of plastics, MPs show an increased biomagnification probability of persistent, bioaccumulative, and toxic substances thereby creating a threat to environmental biota. Thus, remediation of MP-pollutants requires efficient strategies to circumvent the mobilization of contaminants leaching into the water, soil, and ultimately to human beings. Over the years, several microorganisms have been characterized by the potential to degrade different plastic polymers through enzymatic actions. Metagenomics (MGs) is an effective way to discover novel microbial communities and access their functional genetics for the exploration and characterization of plastic-degrading microbial consortia and enzymes. MGs in combination with metatranscriptomics and metabolomics approaches are a powerful tool to identify and select remediation-efficient microbes in situ. Advancement in bioinformatics and sequencing tools allows rapid screening, mining, and prediction of genes that are capable of polymer degradation. This review comprehensively summarizes the growing threat of microplastics around the world and highlights the role of MGs and computational biology in building effective response strategies for MP remediation.

Global plastic upcycling during and after the COVID-19 pandemic: The status and perspective

Plastic pollution has become one of the most pressing environmental issues worldwide since the vast majority of post-consumer plastics are hard to degrade in the environment. The coronavirus disease (COVID-19) pandemic had disrupted the previous effort of plastic pollution mitigation to a great extent due to the overflow of plastic-based medical waste. In the post-pandemic era, the remaining challenge is how to motivate global action towards a plastic circular economy. The need for one package of sustainable and systematic plastic upcycling approaches has never been greater to address such a challenge. In this review, we summarized the threat of plastic pollution during COVID-19 to public health and ecosystem. In order to solve the aforementioned challenges, we present a shifting concept, regeneration value from plastic waste, that provides four promising pathways to achieve a sustainable circular economy: 1) Increasing reusability and biodegradability of plastics; 2) Transforming plastic waste into high-value products by chemical approaches; 3) The closed-loop recycling can be promoted by biodegradation; 4) Involving renewable energy into plastic upcycling. Additionally, the joint efforts from different social perspectives are also encouraged to create the necessary economic and environmental impetus for a circular economy.

Degradation of PET Bottles by an Engineered Ideonella sakaiensis PETase

Extensive plastic production has become a serious environmental and health problem due to the lack of efficient treatment of plastic waste. Polyethylene terephthalate (PET) is one of the most used polymers and is accumulating in landfills or elsewhere in nature at alarming rates. In recent years, enzymatic degradation of PET by Ideonella sakaiensis PETase (IsPETase), a cutinase-like enzyme, has emerged as a promising strategy to completely depolymerize this polymer into its building blocks. Here, inspired by the architecture of cutinases and lipases homologous to IsPETase and using 3D structure information of the enzyme, we rationally designed three mutations in IsPETase active site for enhancing its PET-degrading activity. In particular, the S238Y mutant, located nearby the catalytic triad, showed a degradation activity increased by 3.3-fold in comparison to the wild-type enzyme. Importantly, this structural modification favoured the function of the enzyme in breaking down highly crystallized (~31%) PET, which is found in commercial soft drink bottles. In addition, microscopical analysis of enzyme-treated PET samples showed that IsPETase acts better when the smooth surface of highly crystalline PET is altered by mechanical stress. These results represent important progress in the accomplishment of a sustainable and complete degradation of PET pollution.

Enzymes' Power for Plastics Degradation

Plastics are everywhere in our modern way of living, and their production keeps increasing every year, causing major environmental concerns. Nowadays, the end-of-life management involves accumulation in landfills, incineration, and recycling to a lower extent. This ecological threat to the environment is inspiring alternative bio-based solutions for plastic waste treatment and recycling toward a circular economy. Over the past decade, considerable efforts have been made to degrade commodity plastics using biocatalytic approaches. Here, we provide a comprehensive review on the recent advances in enzyme-based biocatalysis and in the design of related biocatalytic processes to recycle or upcycle commodity plastics, including polyesters, polyamides, polyurethanes, and polyolefins. We also discuss scope and limitations, challenges, and opportunities of this field of research. An important message from this review is that polymer-assimilating enzymes are very likely part of the solution to reaching a circular plastic economy.

Degradation of poly(butylene adipate-co-terephthalate) films by Thermobifida fusca FXJ-1 isolated from compost

In recent years, Poly(butylene adipate-co-terephthalate) (PBAT) films were wildly used due to its biodegradable properties. However, there are few reports of strains that can high efficiently degrade PBAT. Thermobifida fusca FXJ-1, a thermophilic actinomycete, was screened and identified from compost. FXJ-1 can efficiently degrade PBAT at 55 °C in MSM medium. The degradation rates of the pure PBAT film (PF), PBAT film used for mulching on agricultural fields (PAF), and PBAT-PLA-ST film (PPSF) were 82.87 ± 1.01%, 87.83 ± 2.00% and 52.53 ± 0.54%, respectively, after nine days of incubation in MSM medium. Cracking areas were monitored uniformly distributed on the surfaces of three kinds of PBAT-based films after treatment with FXJ-1 using scanning electron microscopy. The LC-MS results showed that PBAT might be degraded into adipic acid, terephthalic acid, butylene adipate, butylene terephthalate and butylene adipate-co-terephthalate, and these products are involved in the cleavage of ester bonds. We also found that amylase produced by FXJ-1 played an important role in the degradation of PPSF. FXJ-1 also showed an efficient PBAT-based films degradation ability in simulating compost environment, which implied its potential application in PBAT and starch-based film degradation by industrial composting.

The PET-Degrading Potential of Global Metagenomes: From In Silico Mining to Active Enzymes

Against the background of the steadily increasing amount of plastic waste in the sea and on land, it is more important than ever to find ways out of this situation. In recent years, microorganisms have been discovered that are capable of degrading artificial polymers such as polyethylene terephthalate (PET). Even if the turnover rates of the enzymes responsible for this reaction may be too low to solve the global plastic pollution problem, it is still of great societal interest to find microorganisms that are able to degrade the polymer. The corresponding enzymes, PET esterases (PETases) can be used in biotechnological processes and could contribute to a resource-saving circular economy. In this chapter, we present a sequence-based in silico screening method to find new PETases in metagenomic datasets. This method can easily be adapted to find other enzyme classes. We also list a number of assays that can be used to test the enzymes for activity on PET as well as other substrates.

Mechanoenzymatic reactions for the hydrolysis of PET†

Recent advances in the enzymatic degradation of poly(ethylene terphthalate) (PET) have led to a number of PET hydrolytic enzymes and mutants being developed. With the amount of PET building up in the natural world, there is a pressing need to develop scalable methods of breaking down the polymer into its monomers for recycling or other uses. Mechanoenzymatic reactions have gained traction recently as a green and efficient alternative to traditional biocatalytic reactions. For the first time we report increased yields of PET degradation by whole cell PETase enzymes by up to 27-fold by utilising ball milling cycles of reactive aging, when compared with typical solution-based reactions. This methodology leads to up to a 2600-fold decrease in the solvent required when compared with other leading degradation reactions in the field and a 30-fold decrease in comparison to reported industrial scale PET hydrolysis reactions.

Mechanoenzymatic reactions are described for the degradation of different PET materials using whole cell PETases.

Sourcing thermotolerant poly(ethylene terephthalate) hydrolase scaffolds from natural diversity

Enzymatic deconstruction of poly(ethylene terephthalate) (PET) is under intense investigation, given the ability of hydrolase enzymes to depolymerize PET to its constituent monomers near the polymer glass transition temperature. To date, reported PET hydrolases have been sourced from a relatively narrow sequence space. Here, we identify additional PET-active biocatalysts from natural diversity by using bioinformatics and machine learning to mine 74 putative thermotolerant PET hydrolases. We successfully express, purify, and assay 51 enzymes from seven distinct phylogenetic groups; observing PET hydrolysis activity on amorphous PET film from 37 enzymes in reactions spanning pH from 4.5-9.0 and temperatures from 30-70 °C. We conduct PET hydrolysis time-course reactions with the best-performing enzymes, where we observe differences in substrate selectivity as function of PET morphology. We employed X-ray crystallography and AlphaFold to examine the enzyme architectures of all 74 candidates, revealing protein folds and accessory domains not previously associated with PET deconstruction. Overall, this study expands the number and diversity of thermotolerant scaffolds for enzymatic PET deconstruction.

A critical review of microplastic degradation and material flow analysis towards a circular economy

The resilience and low cost of plastics has made their usage ubiquitous, but is also the cause of their prevalence and longevity as waste. Plastic pollution has become a great concern to the health and wellbeing of ecosystems around the world; microplastics are a particular threat, due to their high mobility, ease of ingestion by wildlife, and ability to adsorb and carry toxic contaminants. Material flow analysis has been widely applied to examine stocks and flows of materials in other industries, and has more recently been applied to plastics to examine areas where waste can reach the environment. However, while much research has gone into the environmental fate of microplastics, degradation strategies have been a lesser focus, and material flow analysis of microplastics has suffered from lack of data. Furthermore, the variety of plastics, their additives, and any contaminants pose a significant challenge in degrading (and not merely fragmenting) microplastic particles. This review discusses the current degradation strategies and solutions for dealing with existing and newly-generated microplastic waste along with examining the status of microplastics-based material flow analysis, which are critical for evaluating the possibility of incorporating microplastic waste into a circular economy. The degradation strategies are critically examined, identifying challenges and current trends, as well as important considerations that are frequently under-reported. An emphasis is placed on identifying missing data or information in both material flow analysis and degradation methods that could prove crucial in improving understanding of microplastic flows, as well as optimizing degradation strategies and minimizing any negative environmental impact.

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

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.

Microbial degradation of polyethylene terephthalate: a systematic review

Plastic pollution levels have increased rapidly in recent years, due to the accumulation of plastic waste, including polyethylene terephthalate (PET). Both high production and the lack of efficient methods for disposal and recycling affect diverse aquatic and terrestrial ecosystems owing to the high accumulation rates of plastics. Traditional chemical and physical degradation techniques have caused adverse effects on the environment; hence, the use of microorganisms for plastic degradation has gained importance recently. This systematic review was conducted for evaluating the reported findings about PET degradation by wild and genetically modified microorganisms to make them available for future work and to contribute to the eventual implementation of an alternative, an effective, and environmentally friendly method for the management of plastic waste such as PET. Both wild and genetically modified microorganisms with the metabolic potential to degrade this polymer were identified, in addition to the enzymes and genes used for genetic modification. The most prevalent wild-type PET-degrading microorganisms were bacteria (56.3%, 36 genera), followed by fungi (32.4%, 30 genera), microalgae (1.4%; 1 genus, namely Spirulina sp.), and invertebrate associated microbiota (2.8%). Among fungi and bacteria, the most prevalent genera were Aspergillus sp. and Bacillus sp., respectively. About genetically modified microorganisms, 50 strains of Escherichia coli, most of them expressing PETase enzyme, have been used. We emphasize the pressing need for implementing biological techniques for PET waste management on a commercial scale, using consortia of microorganisms. We present this work in five sections: an Introduction that highlights the importance of PET biodegradation as an effective and sustainable alternative, a section on Materials and methods that summarizes how the search for articles and manuscripts in different databases was done, and another Results section where we present the works found on the subject, a final part of Discussion and analysis of the literature found and finally we present a Conclusion and prospects.

Can polymer-degrading microorganisms solve the bottleneck of plastics' environmental challenges?

Increasing world population and industrial activities have enhanced anthropogenic pollution, plastic pollution being especially alarming. So, plastics should be recycled and/or make them biodegradable. Chemical and physical remediating methods are usually energy consuming and costly. In addition, they are not ecofriendly and usually produce toxic byproducts. Bioremediation is a proper option as it is cost-efficient and environmentally friendly. Plastic production and consumption are increasing daily, and, as a consequence, more microorganisms are exposed to these nonbiodegradable polymers. Therefore, investigating new efficient microorganisms and increasing the knowledge about their biology can pave the way for efficient and feasible plastic bioremediation processes. In this sense, omics, systems biology and bioinformatics are three important fields to analyze the biodegradation pathways in microorganisms. Based on the above-mentioned technologies, researchers can engineer microorganisms with specific desired properties to make bioremediation more efficient.

Identification of BgP, a Cutinase-Like Polyesterase From a Deep-Sea Sponge-Derived Actinobacterium

Many marine bacteria produce extracellular enzymes that degrade complex molecules to facilitate their growth in environmental conditions that are often harsh and low in nutrients. Marine bacteria, including those inhabiting sea sponges, have previously been reported to be a promising source of polyesterase enzymes, which have received recent attention due to their potential ability to degrade polyethylene terephthalate (PET) plastic. During the screening of 51 marine bacterial isolates for hydrolytic activities targeting ester and polyester substrates, a Brachybacterium ginsengisoli B129SM11 isolate from the deep-sea sponge Pheronema sp. was identified as a polyesterase producer. Sequence analysis of genomic DNA from strain B129SM11, coupled with a genome "mining" strategy, allowed the identification of potential polyesterases, using a custom database of enzymes that had previously been reported to hydrolyze PET or other synthetic polyesters. This resulted in the identification of a putative PET hydrolase gene, encoding a polyesterase-type enzyme which we named BgP that shared high overall similarity with three well-characterized PET hydrolases-LCC, TfCut2, and Cut190, all of which are key enzymes currently under investigation for the biological recycling of PET. In silico protein analyses and homology protein modeling offered structural and functional insights into BgP, and a detailed comparison with Cut190 revealed highly conserved features with implications for both catalysis and substrate binding. Polyesterase activity was confirmed using an agar-based polycaprolactone (PCL) clearing assay, following heterologous expression of BgP in Escherichia coli. This is the first report of a polyesterase being identified from a deep-sea sponge bacterium such as Brachybacterium ginsengisoli and provides further insights into marine-derived polyesterases, an important family of enzymes for PET plastic hydrolysis. Microorganisms living in association with sponges are likely to have increased exposure to plastics and microplastics given the wide-scale contamination of marine ecosystems with these plastics, and thus they may represent a worthwhile source of enzymes for use in new plastic waste management systems. This study adds to the growing knowledge of microbial polyesterases and endorses further exploration of marine host-associated microorganisms as a potentially valuable source of this family of enzymes for PET plastic hydrolysis.

Toward Microbial Recycling and Upcycling of Plastics: Prospects and Challenges

Annually, 400 Mt of plastics are produced of which roughly 40% is discarded within a year. Current plastic waste management approaches focus on applying physical, thermal, and chemical treatments of plastic polymers. However, these methods have severe limitations leading to the loss of valuable materials and resources. Another major drawback is the rapid accumulation of plastics into the environment causing one of the biggest environmental threats of the twenty-first century. Therefore, to complement current plastic management approaches novel routes toward plastic degradation and upcycling need to be developed. Enzymatic degradation and conversion of plastics present a promising approach toward sustainable recycling of plastics and plastics building blocks. However, the quest for novel enzymes that efficiently operate in cost-effective, large-scale plastics degradation poses many challenges. To date, a wide range of experimental set-ups has been reported, in many cases lacking a detailed investigation of microbial species exhibiting plastics degrading properties as well as of their corresponding plastics degrading enzymes. The apparent lack of consistent approaches compromises the necessary discovery of a wide range of novel enzymes. In this review, we discuss prospects and possibilities for efficient enzymatic degradation, recycling, and upcycling of plastics, in correlation with their wide diversity and broad utilization. Current methods for the identification and optimization of plastics degrading enzymes are compared and discussed. We present a framework for a standardized workflow, allowing transparent discovery and optimization of novel enzymes for efficient and sustainable plastics degradation in the future.

A flexible kinetic assay efficiently sorts prospective biocatalysts for PET plastic subunit hydrolysis

Esterase enzymes catalyze diverse hydrolysis reactions with important biological, commercial, and biotechnological applications. For the improvement of these biocatalysts, there is a need for widely accessible, inexpensive, and adaptable activity screening assays that identify enzymes with particular substrate specificities. Natural systems for biopolymer bioconversion, and likely those designed to mimic them, depend on cocktails of enzymes, each of which specifically targets the intact material as well as water-soluble subunits of varying size. In this work, we have adapted a UV/visible assay using pH-sensitive sulfonphthalein dyes for the real-time quantification of ester hydrolysis of bis-(2-hydroxyethyl) terephthalate (BHET), a subunit of polyethylene terephthalate (PET) plastic. We applied this method to a diverse set of known PET hydrolases and commercial esterases in a microplate format. The approach identified four PET hydrolases and one commercial esterase with high levels of specificity for BHET hydrolysis. Five additional PET hydrolases and three commercial esterases, including a thermophilic enzyme, effectively hydrolyzed both BHET and its monoester product MHET (mono-(2-hydroxyethyl) terephthalate). Specific activities were discernible within one hour and reactions reached an unequivocal endpoint well within 24 hours. The results from the UV/visible method correlated well with conventional HPLC analysis of the reaction products. We examined the suitability of the method toward variable pH, temperature, enzyme preparation method, mono- and multi-ester substrate type, and level of sensitivity versus stringency, finding the assay to be easily adaptable to diverse screening conditions and kinetic measurements. This method offers an accurate, easily accessible, and cost-effective route towards high-throughput library screening to support the discovery, directed evolution, and protein engineering of these critical biocatalysts.

Fluorescence-activated droplet sorting of PET degrading microorganisms

Enzymes that can decompose synthetic plastics such as polyethylene terephthalate (PET) are urgently needed. Still, a bottleneck remains due to a lack of techniques for detecting and sorting environmental microorganisms with vast diversity and abundance. Here, we developed a fluorescence-activated droplet sorting (FADS) pipeline for high-throughput screening of PET-degrading microorganisms or enzymes (PETases). The pipeline comprises three steps: generation and incubation of droplets encapsulating single cells, picoinjection of fluorescein dibenzoate (FDBz) as the fluorogenic probe, and screening of droplets to obtain PET-degrading cells. We characterized critical factors associated with this method, including specificity and sensitivity for discriminating PETase from other enzymes. We then optimized its performance and compatibility with environmental samples. The system was used to screen a wastewater sample from a PET textile mill. We successfully obtained PET-degrading species from nine different genera. Moreover, two putative PETases from isolates Kineococcus endophyticus Un-5 and Staphylococcus epidermidis Un-C2-8 were genetically derived, heterologously expressed, and preliminarily validated for PET-degrading activities. We speculate that the FADS pipeline can be widely adopted to discover new plastic-degrading microorganisms and enzymes in various environments and may be utilized in the directed evolution of degrading enzymes using synthetic biology.

Residue-Specific Incorporation of the Non-Canonical Amino Acid Norleucine Improves Lipase Activity on Synthetic Polyesters

Environmentally friendly functionalization and recycling processes for synthetic polymers have recently gained momentum, and enzymes play a central role in these procedures. However, natural enzymes must be engineered to accept synthetic polymers as substrates. To enhance the activity on synthetic polyesters, the canonical amino acid methionine in Thermoanaerobacter thermohydrosulfuricus lipase (TTL) was exchanged by the residue-specific incorporation method for the more hydrophobic non-canonical norleucine (Nle). Strutural modelling of TTL revealed that residues Met-114 and Met-142 are in close vicinity of the active site and their replacement by the norleucine could modulate the catalytic activity of the enzyme. Indeed, hydrolysis of the polyethylene terephthalate model substrate by the Nle variant resulted in significantly higher amounts of release products than the Met variant. A similar trend was observed for an ionic phthalic polyester containing a short alkyl diol (C5). Interestingly, a 50% increased activity was found for TTL [Nle] towards ionic phthalic polyesters containing different ether diols compared to the parent enzyme TTL [Met]. These findings clearly demonstrate the high potential of non-canonical amino acids for enzyme engineering.

Occurrence, toxicity and remediation of polyethylene terephthalate plastics. A review

Polyethylene terephthalate is a common plastic in many products such as viscose rayon for clothing, and packaging material in the food and beverage industries. Polyethylene terephthalate has beneficial properties such as light weight, high tensile strength, transparency and gas barrier. Nonetheless, there is actually increasing concern about plastic pollution and toxicity. Here we review the properties, occurrence, toxicity, remediation and analysis of polyethylene terephthalate as macroplastic, mesoplastic, microplastic and nanoplastic. Polyethylene terephthalate occurs in groundwater, drinking water, soils and sediments. Plastic uptake by humans induces diseases such as reducing migration and proliferation of human mesenchymal stem cells of bone marrow and endothelial progenitor cells. Polyethylene terephthalate can be degraded by physical, chemical and biological methods.

Potential Use of Microbial Enzymes for the Conversion of Plastic Waste Into Value-Added Products: A Viable Solution

The widespread use of commercial polymers composed of a mixture of polylactic acid and polyethene terephthalate (PLA-PET) in bottles and other packaging materials has caused a massive environmental crisis. The valorization of these contaminants via cost-effective technologies is urgently needed to achieve a circular economy. The enzymatic hydrolysis of PLA-PET contaminants plays a vital role in environmentally friendly strategies for plastic waste recycling and degradation. In this review, the potential roles of microbial enzymes for solving this critical problem are highlighted. Various enzymes involved in PLA-PET recycling and bioconversion, such as PETase and MHETase produced by Ideonella sakaiensis; esterases produced by Bacillus and Nocardia; lipases produced by Thermomyces lanuginosus, Candida antarctica, Triticum aestivum, and Burkholderia spp.; and leaf-branch compost cutinases are critically discussed. Strategies for the utilization of PLA-PET's carbon content as C1 building blocks were investigated for the production of new plastic monomers and different value-added products, such as cyclic acetals, 1,3-propanediol, and vanillin. The bioconversion of PET-PLA degradation monomers to polyhydroxyalkanoate biopolymers by Pseudomonas and Halomonas strains was addressed in detail. Different solutions to the production of biodegradable plastics from food waste, agricultural residues, and polyhydroxybutyrate (PHB)-accumulating bacteria were discussed. Fuel oil production via PLA-PET thermal pyrolysis and possible hybrid integration techniques for the incorporation of thermostable plastic degradation enzymes for the conversion into fuel oil is explained in detail.

A Novel Actinobacterial Cutinase Containing a Non-Catalytic Polymer-Binding Domain

The single putative cutinase-encoding gene from the genome of Kineococcus radiotolerans SRS30216 was cloned and expressed in Escherichia coli as a secreted fusion protein, designated YebF-KrCUT, where YebF is the extracellular carrier protein. The 294-amino acid sequence of KrCUT is unique among currently characterized cutinases by having a C-terminal extension that consists of a short (Pro-Thr)-rich linker and a 55-amino-acid region resembling the substrate binding domain of poly(hydroxybutyrate) (PHB) depolymerases. Phylogenetically, KrCUT takes a unique position among known cutinases and cutinase-like proteins of bacterial and fungal origin. A modeled structure of KrCUT, although displaying a typical α/ß hydrolase fold, shows some unique loops close to the catalytic site. The 39-kDa YebF-KrCUT fusion protein and a truncated variant thereof were purified to electrophoretic homogeneity and functionally characterized. The melting temperatures (T_m) of KrCUT and its variant KrCUT206 devoid of the putative PHB-binding domain were established to be very similar at 50-51°C. Cutinase activity was confirmed by the appearance of characteristic cutin components, C₁₆ and C₁₈ hydroxyl fatty acids, in the mass chromatograms following incubation of KrCUT with apple cutin as substrate. KrCUT also efficiently degraded synthetic polyesters such as polycaprolactone and poly(1,3-propylene adipate). Although incapable of PHB depolymerization, KrCUT could efficiently bind PHB, confirming the predicted characteristic of the C-terminal region. KrCUT also potentiated the activity of pectate lyase in the degradation of pectin from hemp fibres. This synergistic effect is relevant to the enzyme retting process of natural fibres. IMPORTANCE. To date only a limited number of cutinases have been isolated and characterized from nature, the majority being sourced from phytopathogenic fungi and thermophilic bacteria. The significance of our research relates to the identification and characterization of a unique member of microbial cutinases, of name KrCUT, that was derived from the genome of the Gram-positive Kineococcus radiotolerans SRS30216, a highly radiation-resistant actinobacterium. Given the wide-ranging importance of cutinases in applications such as the degradation of natural and synthetic polymers, in the textile industry, in laundry detergents, or in biocatalysis (e.g., transesterification reactions), our results could foster new research leading to broader biotechnological impacts. This study also demonstrated that genome mining or prospecting is a viable means to discover novel biocatalysts as environmentally friendly and biotechnological tool.

Perspectives on the Role of Enzymatic Biocatalysis for the Degradation of Plastic PET

Plastics are highly durable and widely used materials. Current methodologies of plastic degradation, elimination, and recycling are flawed. In recent years, biodegradation (the usage of microorganisms for material recycling) has grown as a valid alternative to previously used methods. The evolution of bioengineering techniques and the discovery of novel microorganisms and enzymes with degradation ability have been key. One of the most produced plastics is PET, a long chain polymer of terephthalic acid (TPA) and ethylene glycol (EG) repeating monomers. Many enzymes with PET degradation activity have been discovered, characterized, and engineered in the last few years. However, classification and integrated knowledge of these enzymes are not trivial. Therefore, in this work we present a summary of currently known PET degrading enzymes, focusing on their structural and activity characteristics, and summarizing engineering efforts to improve activity. Although several high potential enzymes have been discovered, further efforts to improve activity and thermal stability are necessary.

Emerging Strategies in Polyethylene Terephthalate Hydrolase Research for Biorecycling

The research on polyethylene terephthalate (PET) hydrolyzing enzymes started in 2005; several studies are now nearing the objective of their application in biorecycling of PET, which is an urgent environmental issue. The thermostability of PET hydrolases must be higher than 70 °C, which has already been established by several thermophilic cutinases, as higher thermostability results in higher activity. Additionally, pretreatment of waste PET to more enzyme-attackable forms is necessary for PET biorecycling. This Minireview summarizes research on enzymatic PET hydrolysis from two viewpoints: 1) improvement of PET hydrolases by focusing on their thermostabilities by mutation of enzyme genes, their expression in several hosts, and their modifications; and 2) processing of waste PET to readily biodegradable forms. Finally, the outlook of PET biorecycling is described.

Analytical methods for the investigation of enzyme-catalyzed degradation of polyethylene terephthalate

The polyester PET (poly(ethylene terephthalate)) plastic is chemically inert and remarkably persistent, posing relevant and global pollution concerns due to its accumulation in ecosystems across the globe. In past years, research focused on identifying bacteria active on PET and on the specific enzymes responsible for its degradation. Here, the enzymatic degradation of PET can be considered as an 'erosion process' that takes place on the surface of an insoluble material and results in an unusual, substrate-limited kinetic condition. In this review, we report on the most suitable models to evaluate the kinetics of PET-hydrolyzing enzymes, which takes into consideration the amount of enzyme adsorbed on the substrate, the enzyme-accessible ester bonds, and the product inhibition effects. Careful kinetic analysis is especially relevant to compare enzymes from different sources and evolved variants generated by protein engineering studies as well. Furthermore, the analytical methods most suitable to screen natural bacteria and recombinant variant libraries generated by protein engineering have been also reported. These methods rely on different detection systems and are performed both on model compounds and on different PET samples (e.g., nanoparticles, microparticles, and waste products). All this meaningful information represents an optimal starting point and boosts the process of identifying systems able to biologically recycle PET waste products.

A comprehensive and critical review on key elements to implement enzymatic PET depolymerization for recycling purposes

Plastics production and recycling chains must be refitted to a circular economy. Poly(ethylene terephthalate) (PET) is especially suitable for recycling because of its hydrolysable ester bonds and high environmental impact due to employment in single-use packaging, so that recycling processes utilizing enzymes are a promising biotechnological route to monomer recovery. However, enzymatic PET depolymerization still faces challenges to become a competitive route at an industrial level. In this review, PET characteristics as a substrate for enzymes are discussed, as well as the analytical methods used to evaluate the reaction progress. A comprehensive view on the biocatalysts used is discussed. Subsequently, different strategies pursued to improve enzymatic PET depolymerization are presented, including enzyme modification through mutagenesis, utilization of multiple enzymes, improvement of the interaction between enzymes and the hydrophobic surface of PET, and various reaction conditions (e.g., particle size, reaction medium, agitation, and additives). All scientific developments regarding these different aspects of PET depolymerization are crucial to offer a scalable and competitive technology. However, they must be integrated into global processes from upstream to downstream, discussed here at the final sections, which must be evaluated for their economic feasibility and life cycle assessment to check if PET recycling chains can be broadly incorporated into the future circular economy.

Strategic Possibility Routes of Recycled PET

The polyethylene terephthalate (PET) application has many challenges and potential due to its sustainability. The conventional PET degradation was developed for several technologies to get higher yield products of ethylene glycol, bis(2-hydroxyethyl terephthalate) and terephthalic acid. The chemical recycling of PET is reviewed, such as pyrolysis, hydrolysis, methanolysis, glycolysis, ionic-liquid, phase-transfer catalysis and combination of glycolysis-hydrolysis, glycolysis-methanolysis and methanolysis-hydrolysis. Furthermore, the reaction kinetics and reaction conditions were investigated both theoretically and experimentally. The recycling of PET is to solve environmental problems and find another source of raw material for petrochemical products and energy.

Protein engineering of stable IsPETase for PET plastic degradation by Premuse

Poly(ethylene terephthalate) (PET) is used widely by human beings, but is very difficult to degrade. Up to now, the PET degradation effect of PETase from Ideonella sakaiensis 201-F6 (IsPETase) variants with low stability and activity was not ideal. In this study, a mutation design tool, Premuse, was developed to integrate the sequence alignment and quantitative selection of the preferred mutations based on natural sequence evolution. Ten single point mutants were selected from 1486 homologous sequences using Premuse, and then two mutations (W159H and F229Y) with improved stability were screened from them. The derived double point mutant, W159H/F229Y, exhibited a strikingly enhanced enzymatic performance. Its T_m and catalytic efficiency values (k_cat/K_m) respectively increased by 10.4 °C and 2.0-fold using p-NPP as the substrate compared with wild type. The degradation activity for amorphous PET was increased by almost 40-fold in comparison with wild type at 40 °C in 24 h. Additionally, the variant could catalyze biodegradation of PET bottle preform at a mean rate of 23.4 mg_PET/h/mg_enzyme. This study allowed us to design the mutation more efficiently, and provides a tool for achieving biodegradation of PET pollution under mild natural environments.

IsPETase Is a Novel Biocatalyst for Poly(ethylene terephthalate) (PET) Hydrolysis

Poly(ethylene terephthalate) (PET) is one of the most widely used synthetic polyesters, but also a major cause of plastic pollution. Because the chemical degradation of PET would be uneconomical and rather burdensome, considerable efforts have been devoted to exploring enzymatic processes for the disposal of PET waste. Many PET-hydrolyzing enzymes have been reported in recent decades, some of which demonstrate excellent potential for industrial applications. This review sets out to summarize the state of investigation into IsPETase, a cutinase-like enzyme from Ideonella sakaiensis possessing ability to degrade crystalline PET, and to gain further insight into the structure-function relationship of IsPETase. Benefiting from the continuing identification of novel cutinase-like proteins and growing availability of the engineered IsPETase, we may anticipate future developments in this type of enzyme would generate suitable biocatalyst for industrial use.

Cutinases from thermophilic bacteria (actinomycetes): From identification to functional and structural characterization

Thermophilic cutinases are mainly obtained from thermophilic actinomycetes, and are categorized into two groups, i.e., those with higher (>70°C) or lower (<70°C) thermostabilities. The thermostabilities of cutinases are highly relevant to their ability to degrade polyethylene terephthalate (PET). Many crystal structures of thermophilic cutinases have been solved, showing that their overall backbone structures are identical, irrespective of their ability to hydrolyze PET. One of the unique properties of cutinases is that metal ion-binding on the enzyme's surface both elevates their melting temperatures and activates the enzyme. In this chapter, we introduce the methodology for the identification and cloning of thermophilic cutinases from actinomycetes. For detailed characterization of cutinases, we describe the approach to analyze the intricate dynamics of the enzyme, based on its crystal structures complexed with metal ions and model substrates using a combination of experimental and computational techniques.