Identification and comparison of cutinases for synthetic polyester degradation

Review of nomenclature and methods of analysis of polyethylene terephthalic acid hydrolyzing enzymes activity

Enzymatic degradation of polyethylene terephthalic acid (PET) has been gaining increasing importance. This has resulted in a significant increase in the search for newer enzymes and the development of more efficient enzyme-based systems. Due to the lack of a standard screening process, screening new enzymes has relied on other assays to determine the presence of esterase activity. This, in turn, has led to various nomenclatures and methods used to describe them and measure their activity. Since all PET-hydrolyzing enzymes are α/β hydrolases, they catalyze a serine nucleophilic attack and cleave an ester bond. They are lipases, esterases, cutinases and hydrolases. This has been used interchangeably, leading to difficulties while comparing results and evaluating progress. This review discusses the varied enzyme nomenclature being adapted, the different assays and analysis methods reported, and the strategies used to increase PET-hydrolyzing enzyme efficiency. A section on the various ways to quantify PET hydrolysis is also covered.

ANCUT1, a novel thermoalkaline cutinase from Aspergillus nidulans and its application on hydroxycinnamic acids lipophilization

One of the four cutinases encoded in the Aspergillus nidulans genome, ANCUT1, is described here. Culture conditions were evaluated, and it was found that this enzyme is produced only when cutin is present in the culture medium, unlike the previously described ANCUT2, with which it shares 62% amino acid identity. The differences between them include the fact that ANCUT1 is a smaller enzyme, with experimental molecular weight and pI values of 22 kDa and 6, respectively. It shows maximum activity at pH 9 and 60 °C under assayed conditions and retains more than 60% of activity after incubation for 1 h at 60 °C in a wide range of pH values (6-10) after incubations of 1 or 3 h. It has a higher activity towards medium-chain esters and can modify long-chain length hydroxylated fatty acids constituting cutin. Its substrate specificity properties allow the lipophilization of alkyl coumarates, valuable antioxidants and its thermoalkaline behavior, which competes favorably with other fungal cutinases, suggests it may be useful in many more applications.

Identification and characterization of a fungal cutinase-like enzyme CpCut1 from Cladosporium sp. P7 for polyurethane degradation

Plastic degradation by biological systems emerges as a prospective avenue for addressing the pressing global concern of plastic waste accumulation. The intricate chemical compositions and diverse structural facets inherent to polyurethanes (PU) substantially increase the complexity associated with PU waste management. Despite the extensive research endeavors spanning over decades, most known enzymes exhibit a propensity for hydrolyzing waterborne PU dispersion (i.e., the commercial Impranil DLN-SD), with only a limited capacity for the degradation of bulky PU materials. Here, we report a novel cutinase (CpCut1) derived from Cladosporium sp. P7, which demonstrates remarkable efficiency in the degrading of various polyester-PU materials. After 12-h incubation at 55°C, CpCut1 was capable of degrading 40.5% and 20.6% of thermoplastic PU film and post-consumer foam, respectively, while achieving complete depolymerization of Impranil DLN-SD. Further analysis of the degradation intermediates suggested that the activity of CpCut1 primarily targeted the ester bonds within the PU soft segments. The versatile performance of CpCut1 against a spectrum of polyester-PU materials positions it as a promising candidate for the bio-recycling of waste plastics.IMPORTANCEPolyurethane (PU) has a complex chemical composition that frequently incorporates a variety of additives, which poses significant obstacles to biodegradability and recyclability. Recent advances have unveiled microbial degradation and enzymatic depolymerization as promising waste PU disposal strategies. In this study, we identified a gene encoding a cutinase from the PU-degrading fungus Cladosporium sp. P7, which allowed the expression, purification, and characterization of the recombinant enzyme CpCut1. Furthermore, this study identified the products derived from the CpCut1 catalyzed PU degradation and proposed its underlying mechanism. These findings highlight the potential of this newly discovered fungal cutinase as a remarkably efficient tool in the degradation of PU materials.

Harnessing biocatalysis to achieve selective functional group interconversion of monomers

Polymeric materials are ubiquitous to modern life. However, reliance of petroleum for polymeric building blocks is not sustainable. The synthesis of macromolecules from recalcitrant polymer waste feedstocks, such as plastic waste and lignocellulosic biomass, presents an opportunity to bypass the use of petroleum-based feedstocks. However, the deconstruction and transformation of these alternative feedstocks remained limited until recently. Herein, we highlight examples of monomers liberated from the deconstruction of recalcitrant polymers, and more extensively, we showcase the state-of-the-art in biocatalytic technologies that are enabling synthesis of diverse upcycled monomeric starting materials for a wide variety of macromolecules. Overall, this review emphasizes the importance of functional group interconversion as a promising strategy by which biocatalysis can aid the diversification and upcycling of monomers.

Recent advances in microbial and enzymatic engineering for the biodegradation of micro- and nanoplastics

This review examines the escalating issue of plastic pollution, specifically highlighting the detrimental effects on the environment and human health caused by microplastics and nanoplastics. The extensive use of synthetic polymers such as polyethylene (PE), polyethylene terephthalate (PET), and polystyrene (PS) has raised significant environmental concerns because of their long-lasting and non-degradable characteristics. This review delves into the role of enzymatic and microbial strategies in breaking down these polymers, showcasing recent advancements in the field. The intricacies of enzymatic degradation are thoroughly examined, including the effectiveness of enzymes such as PETase and MHETase, as well as the contribution of microbial pathways in breaking down resilient polymers into more benign substances. The paper also discusses the impact of chemical composition on plastic degradation kinetics and emphasizes the need for an approach to managing the environmental impact of synthetic polymers. The review highlights the significance of comprehending the physical characteristics and long-term impacts of micro- and nanoplastics in different ecosystems. Furthermore, it points out the environmental and health consequences of these contaminants, such as their ability to cause cancer and interfere with the endocrine system. The paper emphasizes the need for advanced analytical methods and effective strategies for enzymatic degradation, as well as continued research and development in this area. This review highlights the crucial role of enzymatic and microbial strategies in addressing plastic pollution and proposes methods to create effective and environmentally friendly solutions.

Setting new standards: Multiphasic analysis of microplastic mineralization by fungi

Plastic materials provide numerous benefits. However, properties such as durability and resistance to degradation that make plastic attractive for variable applications likewise foster accumulation in the environment. Fragmentation of plastics leads to the formation of potentially hazardous microplastic, of which a considerable amount derives from polystyrene. Here, we investigated the biodegradation of polystyrene by the tropical sooty mold fungus Capnodium coffeae in different experimental setups. Growth of C. coffeae was stimulated significantly when cultured in presence of plastic polymers rather than in its absence. Stable isotope tracing using 13C-enriched polystyrene particles combined with cavity ring-down spectroscopy showed that the fungus mineralized polystyrene traces. However, phospholipid fatty acid stable isotope probing indicated only marginal assimilation of polystyrene-13C by C. coffeae in liquid cultures. NMR spectroscopic analysis of residual styrene contents prior to and after incubation revealed negligible changes in concentration. Thus, this study suggests a plastiphilic life style of C. coffeae despite minor usage of plastic as a carbon source and the general capability of sooty mold fungi to stimulate polystyrene mineralization, and proposes new standards to identify and unambiguously demonstrate plastic degrading capabilities of microbes.

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.

Recent advances in the biodegradation of polyethylene terephthalate with cutinase-like enzymes

Polyethylene terephthalate (PET) is a synthetic polymer in the polyester family. It is widely found in objects used daily, including packaging materials (such as bottles and containers), textiles (such as fibers), and even in the automotive and electronics industries. PET is known for its excellent mechanical properties, chemical resistance, and transparency. However, these features (e.g., high hydrophobicity and high molecular weight) also make PET highly resistant to degradation by wild-type microorganisms or physicochemical methods in nature, contributing to the accumulation of plastic waste in the environment. Therefore, accelerated PET recycling is becoming increasingly urgent to address the global environmental problem caused by plastic wastes and prevent plastic pollution. In addition to traditional physical cycling (e.g., pyrolysis, gasification) and chemical cycling (e.g., chemical depolymerization), biodegradation can be used, which involves breaking down organic materials into simpler compounds by microorganisms or PET-degrading enzymes. Lipases and cutinases are the two classes of enzymes that have been studied extensively for this purpose. Biodegradation of PET is an attractive approach for managing PET waste, as it can help reduce environmental pollution and promote a circular economy. During the past few years, great advances have been accomplished in PET biodegradation. In this review, current knowledge on cutinase-like PET hydrolases (such as TfCut2, Cut190, HiC, and LCC) was described in detail, including the structures, ligand-protein interactions, and rational protein engineering for improved PET-degrading performance. In particular, applications of the engineered catalysts were highlighted, such as improving the PET hydrolytic activity by constructing fusion proteins. The review is expected to provide novel insights for the biodegradation of complex polymers.

Role of organisms and their enzymes in the biodegradation of microplastics and nanoplastics: A review

Microplastic (MP) and Nanoplastic (NP) contamination have become a critical ecological concern due to their persistent presence in every aspect of the ecosystem and their potentially harmful effects. The current approaches to eradicate these wastes by burning up and dumping adversely impact the environment, while recycling has its own challenges. As a result, applying degradation techniques to eliminate these recalcitrant polymers has been a focus of scientific investigation in the recent past. Biological, photocatalytic, electrocatalytic, and, recently, nanotechnologies have been studied to degrade these polymers. Nevertheless, it is hard to degrade MPs and NPs in the environment, and these degradation techniques are comparatively inefficient and require further development. The recent research focuses on the potential use of microbes to degrade MPs and NPs as a sustainable solution. Therefore, considering the recent advancements in this important research field, this review highlights the utilization of organisms and enzymes for the biodegradation of the MPs and NPs with their probable degradation mechanisms. This review provides insight into various microbial entities and their enzymes for the biodegradation of MPs. In addition, owing to the lack of research on the biodegradation of NPs, the perspective of applying these processes to NPs degradation has also been looked at. Finally, a critical evaluation of the recent development and perspective for future research to improve the effective removal of MPs and NPs in the environment through biodegradation is also discussed.

Enzymatic post-consumer poly(ethylene terephthalate) (PET) depolymerization using commercial enzymes

Poly(ethylene terephthalate) (PET) is a synthetic polymer widely used globally. The high PET resistance to biotic degradation and its improper destination result in the accumulation of this plastic in the environment, largely affecting terrestrial and aquatic animals. This work investigated post-consumer PET (PC-PET) degradation using five commercial hydrolase enzymes (Novozym 51032, CalB, Palatase, Eversa, Lipozyme TL). Humicola insolens cutinase (HiC, Novozym 51032) was the most active among the enzymes studied. Several important reaction parameters (enzyme type, dual enzyme system, enzyme concentration, temperature, ultrasound treatment) were evaluated in PC-PET hydrolysis using HiC. The concentration and the proportion (molar ratio) of hydrolysis products, terephthalic acid (TPA), mono(2-hydroxyethyl) terephthalate (MHET), and bis(2-hydroxyethyl) terephthalate (BHET), were significantly changed depending on the reaction temperature. The TPA released at 70 °C was 3.65-fold higher than at 50 °C. At higher temperatures, the conversion of MHET into TPA was favored. The enzymatic PET hydrolysis by HiC was very sensitive to the enzyme concentration, indicating that it strongly adsorbs on the polymer surface. The concentration of TPA, MHET, and BHET increased as the enzyme concentration increased, and a maximum was achieved using 40-50 vol % of HiC. The presented results add relevant data to optimizing enzyme-based PET recycling technologies.

Boosting Degradation of Biodegradable Polymers

Biodegradation of polymers in composting conditions is an alternative end-of-life (EoL) scenario for contaminated materials collected through the municipal solid waste management system, mainly when mechanical or chemical methods cannot be used to recycle them. Compostability certification requirements are time-consuming and expensive. Therefore, approaches to accelerate the biodegradation of these polymers in simulated composting conditions can facilitate and speed up the evaluation and selection of potential compostable polymer alternatives and inform faster methods to biodegrade these polymers in real composting. This review highlights recent trends, challenges, and future strategies to accelerate biodegradation by modifying the polymer properties/structure and the compost environment. Both abiotic and biotic methods show potential for accelerating the biodegradation of biodegradable polymers. Abiotic methods, such as the incorporation of additives, reduction of molecular weight, reduction of size and reactive blending, are potentially the most straightforward, providing a level of technology that allows for easy adoption and adaptability. Novel methods, including the concept of self-immolative and triggering the scission of polymer chains in specific conditions, are increasingly sought. In terms of biotic methods, dispersion/encapsulation of enzymes during the processing step, biostimulation of the environment, and bioaugmentation with specific microbial strains during the biodegradation process are promising to accelerate biodegradation.

Screening the Degradation of Polymer Microparticles on a Chip

Enzymatic degradation of polymers has advantages over standard degradation methods, such as soil burial and weathering, which are time-consuming and cannot provide time-resolved observations. We have developed a microfluidic device to study the degradation of single microparticles. The enzymatic degradation of poly (1,4-butylene adipate-co-terephthalate) (PBAT) microparticles was studied using Novozym 51032 cutinase. PBAT microparticles were prepared via an oil-in-water emulsion solvent removal method, and their morphology and chemical composition were characterized. Then, microparticles with varying diameters of 30-60 μm were loaded into the microfluidic chip. Enzyme solutions at different concentrations were introduced to the device, and changes in the size and transparency of PBAT microparticles were observed over time. The physicochemical properties of degraded products were analyzed by FT-IR, NMR, mass spectrometry, and differential scanning calorimetry. The degradation process was also performed in bulk, and the results were compared to those of the microfluidic method. Our analysis confirms that the degradation process in both bulk and microfluidic methods was similar. In both cases, degradation takes place on aliphatic and soft segments of PBAT. Our findings serve as a proof of concept for a microfluidic method for easy and time-resolved degradation analysis, with degradation results comparable to those of conventional bulk methods.

Comparison of the enzymatic depolymerization of polyethylene terephthalate and AkestraTM using Humicola insolens cutinase

The enzymatic depolymerization of synthetic polyesters has become of great interest in recycling plastics. Most of the research in this area focuses on the depolymerization of polyethylene terephthalate (PET) due to its widespread use in various applications. However, the enzymatic activity on other commercial polyesters is less frequently investigated. Therefore, AkestraTM attracted our attention, which is a copolymer derived from PET with a partially biobased spirocyclic acetal structure. In this study, the activity of Humicola insolens cutinase (HiCut) on PET and AkestraTM films and powder was investigated. HiCut showed higher depolymerization activity on amorphous PET films than on Akestra™ films. However, an outstanding performance was achieved on AkestraTM powder, reaching 38% depolymerization in 235h, while only 12% for PET powder. These results are consistent with the dependence of the enzymes on the crystallinity of the polymer since Akestra™ is amorphous while the PET powder has 14% crystallinity. On the other hand, HiCut docking studies and molecular dynamic simulations (MD) suggested that the PET-derived mono (hydroxyethyl)terephthalate dimer (MHET)2 is a hydrolyzable ligand, producing terephthalic acid (TPA), while the Akestra™-derived TPA-spiroglycol ester is not, which is consistent with the depolymerization products determined experimentally. MD studies also suggest ligand-induced local conformational changes in the active site.

Biodegradation of Biodegradable Polymers in Mesophilic Aerobic Environments

Finding alternatives to diminish plastic pollution has become one of the main challenges of modern life. A few alternatives have gained potential for a shift toward a more circular and sustainable relationship with plastics. Biodegradable polymers derived from bio- and fossil-based sources have emerged as one feasible alternative to overcome inconveniences associated with the use and disposal of non-biodegradable polymers. The biodegradation process depends on the environment's factors, microorganisms and associated enzymes, and the polymer properties, resulting in a plethora of parameters that create a complex process whereby biodegradation times and rates can vary immensely. This review aims to provide a background and a comprehensive, systematic, and critical overview of this complex process with a special focus on the mesophilic range. Activity toward depolymerization by extracellular enzymes, biofilm effect on the dynamic of the degradation process, CO2 evolution evaluating the extent of biodegradation, and metabolic pathways are discussed. Remarks and perspectives for potential future research are provided with a focus on the current knowledge gaps if the goal is to minimize the persistence of plastics across environments. Innovative approaches such as the addition of specific compounds to trigger depolymerization under particular conditions, biostimulation, bioaugmentation, and the addition of natural and/or modified enzymes are state-of-the-art methods that need faster development. Furthermore, methods must be connected to standards and techniques that fully track the biodegradation process. More transdisciplinary research within areas of polymer chemistry/processing and microbiology/biochemistry is needed.

Recent advances in the breakdown of microplastics: strategies and future prospectives

Microplastics pollution is becoming a major environmental issue, and exposure to microplastics has been associated with numerous adverse results to both the ecological system and humans. This work summarized the state-of-the-art developments in the breakdown of microplastics, including natural weathering, catalysts-assisted breakdown and biodegradation. Characterization techniques for microplastic breakdown involve scanning electron microscopy, Fourier infrared spectroscopy, X-ray photoelectron spectroscopy, etc. Bioavailability and adsorption capacity of microplastics may change after they are broken down, therefore leading to variety in microplastics toxicity. Further prospectives for should be focused on the determination and toxicity evaluation of microplastics breakdown products, as well as unraveling uncultivable microplastics degraders via cultivation-independent approaches. This work benefits researchers interested in environmental studies, particularly the removal of microplastics from environmental matrix.

Multiple Substrate Binding Mode-Guided Engineering of a Thermophilic PET Hydrolase

Thermophilic polyester hydrolases (PES-H) have recently enabled biocatalytic recycling of the mass-produced synthetic polyester polyethylene terephthalate (PET), which has found widespread use in the packaging and textile industries. The growing demand for efficient PET hydrolases prompted us to solve high-resolution crystal structures of two metagenome-derived enzymes (PES-H1 and PES-H2) and notably also in complex with various PET substrate analogues. Structural analyses and computational modeling using molecular dynamics simulations provided an understanding of how product inhibition and multiple substrate binding modes influence key mechanistic steps of enzymatic PET hydrolysis. Key residues involved in substrate-binding and those identified previously as mutational hotspots in homologous enzymes were subjected to mutagenesis. At 72 °C, the L92F/Q94Y variant of PES-H1 exhibited 2.3-fold and 3.4-fold improved hydrolytic activity against amorphous PET films and pretreated real-world PET waste, respectively. The R204C/S250C variant of PES-H1 had a 6.4 °C higher melting temperature than the wild-type enzyme but retained similar hydrolytic activity. Under optimal reaction conditions, the L92F/Q94Y variant of PES-H1 hydrolyzed low-crystallinity PET materials 2.2-fold more efficiently than LCC ICCG, which was previously the most active PET hydrolase reported in the literature. This property makes the L92F/Q94Y variant of PES-H1 a good candidate for future applications in industrial plastic recycling processes.

Cutinase ACut2 from Blastobotrysraffinosifermentans for the Selective Desymmetrization of the Symmetric Diester Diethyl Adipate to the Monoester Monoethyl Adipate

Monoethyl adipate (MEA) is a highly valuable monoester for activating resistance mechanisms and improving protective effects in pathogen-attacked plants. The cutinase ACut2 from the non-conventional yeast Blastobotrys (Arxula) raffinosifermentans (adeninivorans) was used for its synthesis by the desymmetrization of dicarboxylic acid diester diethyl adipate (DEA). Up to 78% MEA with 19% diacid adipic acid (AA) as by-product could be synthesized by the unpurified ACut2 culture supernatant from the B. raffinosifermentans overexpression strain. By adjusting pH and enzyme concentration, the selectivity of the free ACut2 culture supernatant was increased, yielding 95% MEA with 5% AA. Selectivity of the carrier immobilized ACut2 culture supernatant was also improved by pH adjustment during immobilization, as well as carrier enzyme loading, ultimately yielding 93% MEA with an even lower AA concentration of 3–4%. Thus, optimizations enabled the selective hydrolysis of DEA into MEA with only a minor AA impurity. In the up-scaling, a maximum of 98% chemical and 87.8% isolated MEA yield were obtained by the adsorbed enzyme preparation with a space time yield of 2.6 g L⁻¹ h⁻¹. The high monoester yields establish the ACut2-catalyzed biosynthesis as an alternative to existing methods.

Mechanism-Based Design of Efficient PET Hydrolases

Polyethylene terephthalate (PET) is the most widespread synthetic polyester, having been utilized in textile fibers and packaging materials for beverages and food, contributing considerably to the global solid waste stream and environmental plastic pollution. While enzymatic PET recycling and upcycling have recently emerged as viable disposal methods for a circular plastic economy, only a handful of benchmark enzymes have been thoroughly described and subjected to protein engineering for improved properties over the last 16 years. By analyzing the specific material properties of PET and the reaction mechanisms in the context of interfacial biocatalysis, this Perspective identifies several limitations in current enzymatic PET degradation approaches. Unbalanced enzyme-substrate interactions, limited thermostability, and low catalytic efficiency at elevated reaction temperatures, and inhibition caused by oligomeric degradation intermediates still hamper industrial applications that require high catalytic efficiency. To overcome these limitations, successful protein engineering studies using innovative experimental and computational approaches have been published extensively in recent years in this thriving research field and are summarized and discussed in detail here. The acquired knowledge and experience will be applied in the near future to address plastic waste contributed by other mass-produced polymer types (e.g., polyamides and polyurethanes) that should also be properly disposed by biotechnological approaches.

A minireview on the bioremediative potential of microbial enzymes as solution to emerging microplastic pollution

Accumulating plastics in the biosphere implicates adverse effects, raising serious concern among scientists worldwide. Plastic waste in nature disintegrates into microplastics. Because of their minute appearance, at a scale of <5 mm, microplastics easily penetrate different pristine water bodies and terrestrial niches, posing detrimental effects on flora and fauna. The potential bioremediative application of microbial enzymes is a sustainable solution for the degradation of microplastics. Studies have reported a plethora of bacterial and fungal species that can degrade synthetic plastics by excreting plastic-degrading enzymes. Identified microbial enzymes, such as IsPETase and IsMHETase from Ideonella sakaiensis 201-F6 and Thermobifida fusca cutinase (Tfc), are able to depolymerize plastic polymer chains producing ecologically harmless molecules like carbon dioxide and water. However, thermal stability and pH sensitivity are among the biochemical limitations of the plastic-degrading enzymes that affect their overall catalytic activities. The application of biotechnological approaches improves enzyme action and production. Protein-based engineering yields enzyme variants with higher enzymatic activity and temperature-stable properties, while site-directed mutagenesis using the Escherichia coli model system expresses mutant thermostable enzymes. Furthermore, microalgal chassis is a promising model system for "green" microplastic biodegradation. Hence, the bioremediative properties of microbial enzymes are genuinely encouraging for the biodegradation of synthetic microplastic polymers.

Expression of a Cutinase of Moniliophthora roreri with Polyester and PET-Plastic Residues Degradation Activity

Cutinases are enzymes produced by phytopathogenic fungi like Moniliophthora roreri. The three genome-located cutinase genes of M. roreri were amplified from cDNA of fungi growing in different induction culture media for cutinase production. The mrcut1 gene was expressed in the presence of a cacao cuticle, while the mrcut2 and mrcut3 genes were expressed when an apple cuticle was used as the inducer. The sequences of all genes were obtained and analyzed by bioinformatics tools to determine the presence of signal peptides, introns, glycosylation, and regulatory sequences. Also, the theoretical molecular weight and pI were obtained and experimentally confirmed. Finally, cutinase 1 from M. roreri (MRCUT1) was selected for heterologous expression in Escherichia coli. Successful overexpression of MRCUT1 was observed with the highest enzyme activity of 34,036 U/mg under the assay conditions at 40°C and pH 8. Furthermore, the degradation of different synthetic polyesters was evaluated; after 21 days, 59% of polyethylene succinate (PES), 43% of polycaprolactone (PCL), and 31% of polyethylene terephthalate (PET) from plastic residues were degraded. IMPORTANCE Plastic pollution is exponentially increasing; even the G20 has recognized an urgent need to implement actions to reduce it. In recent years, searching for enzymes that can degrade plastics, especially those based on polyesters such as PET, has been increasing as they can be a green alternative to the actual plastic degradation process. A promising option in recent years refers to biological tools such as enzymes involved in stages of partial and even total degradation of some plastics. In this context, the MRCUT1 enzyme can degrade polyesters contained in plastic residues in a short time. Besides, there is limited knowledge about the biochemical properties of cutinases from M. roreri. Commonly, fungal enzymes are expressed as inclusion bodies in E. coli with reduced activity. Interestingly, the successful expression of one cutinase of M. roreri in E. coli with enhanced activity is described.

Strategies and progress in synthetic textile fiber biodegradability

Abstract

The serious issue of textile waste accumulation has raised attention on biodegradability as a possible route to support sustainable consumption of textile fibers. However, synthetic textile fibers that dominate the market, especially poly(ethylene terephthalate) (PET), resist biological degradation, creating environmental and waste management challenges. Because pure natural fibers, like cotton, both perform well for consumer textiles and generally meet certain standardized biodegradability criteria, inspiration from the mechanisms involved in natural biodegradability are leading to new discoveries and developments in biologically accelerated textile waste remediation for both natural and synthetic fibers. The objective of this review is to present a multidisciplinary perspective on the essential bio-chemo-physical requirements for textile materials to undergo biodegradation, taking into consideration the impact of environmental or waste management process conditions on biodegradability outcomes. Strategies and recent progress in enhancing synthetic textile fiber biodegradability are reviewed, with emphasis on performance and biodegradability behavior of poly(lactic acid) (PLA) as an alternative biobased, biodegradable apparel textile fiber, and on biological strategies for addressing PET waste, including industrial enzymatic hydrolysis to generate recyclable monomers. Notably, while pure PET fibers do not biodegrade within the timeline of any standardized conditions, recent developments with process intensification and engineered enzymes show that higher enzymatic recycling efficiency for PET polymer has been achieved compared to cellulosic materials. Furthermore, combined with alternative waste management practices, such as composting, anaerobic digestion and biocatalyzed industrial reprocessing, the development of synthetic/natural fiber blends and other strategies are creating opportunities for new biodegradable and recyclable textile fibers.

Article Highlights

Current Knowledge on Polyethylene Terephthalate Degradation by Genetically Modified Microorganisms

The global production of polyethylene terephthalate (PET) is estimated to reach 87.16 million metric tons by 2022. After a single use, a remarkable part of PET is accumulated in the natural environment as plastic waste. Due to high hydrophobicity and high molecular weight, PET is hardly biodegraded by wild-type microorganisms. To solve the global problem of uncontrolled pollution by PET, the degradation of plastic by genetically modified microorganisms has become a promising alternative for the plastic circular economy. In recent years many studies have been conducted to improve the microbial capacity for PET degradation. In this review, we summarize the current knowledge about metabolic engineering of microorganisms and protein engineering for increased biodegradation of PET. The focus is on mutations introduced to the enzymes of the hydrolase class-PETase, MHETase and cutinase-which in the last few years have attracted growing interest for the PET degradation processes. The modifications described in this work summarize the results obtained so far on the hydrolysis of polyethylene terephthalate based on the released degradation products of this polymer.

Experimental and mathematical modeling approaches for biocatalytic post-consumer poly(ethylene terephthalate) hydrolysis

The environmental impact arising from poly(ethylene terephthalate) (PET) waste is notable worldwide. Enzymatic PET hydrolysis can provide chemicals that serve as intermediates for value-added product synthesis and savings in the resources. In the present work, some reaction parameters were evaluated on the hydrolysis of post-consumer PET (PC-PET) using a cutinase from Humicola insolens (HiC). The increase in PC-PET specific area leads to an 8.5-fold increase of the initial enzymatic hydrolysis rate (from 0.2 to 1.7 mmol L^-1 h^-1), showing that this parameter plays a crucial role in PET hydrolysis reaction. The effect of HiC concentration was investigated, and the enzymatic PC-PET hydrolysis kinetic parameters were estimated based on three different mathematical models describing heterogeneous biocatalysis. The model that best fits the experimental data (R² = 0.981) indicated 1.68 mg_protein mL^-1 as a maximum value of the enzyme concentration to optimize the reaction rate. The HiC thermal stability was evaluated, considering that it is a key parameter for its efficient use in PET degradation. The enzyme half-life was shown to be 110 h at 70 ºC and pH 7.0, which outperforms most of the known enzymes displaying PET hydrolysis activity. The results evidence that HiC is a very promising biocatalyst for efficient PET depolymerization.

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.

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.

Recent advances in the discovery, characterization, and engineering of poly(ethylene terephthalate) (PET) hydrolases

Poly(ethylene terephthalate) (PET) is a class of polyester plastic composed of terephthalic acid (TPA) and ethylene glycol (EG). The accumulation of large amount of PET waste has resulted in severe environmental and health problems. Microbial polyester hydrolases with the ability to degrade PET provide an economy- and environment-friendly approach for the treatment of PET waste. In recent years, many PET hydrolases have been discovered and characterized from various microorganisms and engineered for better performance under practical application conditions. Here, recent progress in the discovery, characterization, and enzymatic mechanism elucidation of PET hydrolases is firstly reviewed. Then, structure-guided protein engineering of PET hydrolases with increased enzymatic activities, expanded substrate specificity, as well as improved protein stability is summarized. In addition, strategies for efficient expression of recombinant PET hydrolases, including secretory expression and cell-surface display, are briefly introduced. This review is concluded with future perspectives in biodegradation and subsequent biotransformation of PET wastes to produce value-added compounds.

Criteria for Engineering Cutinases: Bioinformatics Analysis of Catalophores

Cutinases are bacterial and fungal enzymes that catalyze the hydrolysis of natural cutin, a three-dimensional inter-esterified polyester with epoxy-hydroxy fatty acids with chain lengths between 16 and 18 carbon atoms. Due to their ability to accept long chain substrates, cutinases are also effective in catalyzing in vitro both the degradation and synthesis of several synthetic polyesters and polyamides. Here, we present a bioinformatics study that intends to correlate the structural features of cutinases with their catalytic properties to provide rational basis for their effective exploitation, particularly in polymer synthesis and biodegradation. The bioinformatics study used the BioGPS method (Global Positioning System in Biological Space) that computed molecular descriptors based on Molecular Interaction Fields (MIFs) described in the GRID force field. The information was used to generate catalophores, spatial representations of the ability of each enzymatic active site to establish hydrophobic and electrostatic interactions. These tools were exploited for comparing cutinases to other serine-hydrolases enzymes, namely lipases, esterases, amidases and proteases, and for highlighting differences and similarities that might guide rational engineering strategies. Structural features of cutinases with their catalytic properties were correlated. The “catalophore” of cutinases indicate shared features with lipases and esterases.

Biodeinking: an eco-friendly alternative for chemicals based recycled fiber processing

Recycling of recovered paper is an inevitable process for saving resources and the environment. Due to strict forest conservation regulations and limitations of the agro-forestry sector, the paper industry is facing the woody fiber crisis for decades. The recycling of waste paper for its utilization as a source of cellulosic fibers for papermaking is a resource-saving and eco-friendly approach and is a need of time. Deinking is an important stage in the recycling of recovered paper. In the conventional deinking process, chemicals have been used for removal of inks and other impurities from waste paper pulp slurry with some certain drawbacks like deinking inefficiency, fiber damage and generation of chemicals and fiber-rich effluent. The application of enzymes for deinking purposes is known as biodeinking and is considered as the potent and environmentally friendly deinking approach. The present write-up provides comprehensive information on various aspects of biodeinking.

Class I hydrophobins pretreatment stimulates PETase for monomers recycling of waste PETs

Poly(ethylene terephthalate) hydrolase (PETase) from Ideonella sakaiensis 201-F6 was expressed and purified from Escherichia coli to hydrolyze poly(ethylene terephthalate) (PET) fibers waste for its monomers recycling. Hydrolysis carried out at pH 8 and 30 °C was found to be the optimal condition based on measured monomer mono(2-hydroxyethyl) terephthalate (MHET) and terephthalic acid (TPA) concentrations after 24 h reaction. The intermediate product bis(2-hydroxyethyl) terephthalate (BHET) was a good substrate for PETase because BHET released from PET hydrolysis was efficiently converted into MHET. Only a trace amount of MHET could be further hydrolyzed to TPA. Class I hydrophobins RolA from Aspergillus oryzae and HGFI from Grifola frondosa were expressed and purified from E. coli to pretreat PET surface for accelerating PETase hydrolysis against PET. The weight loss of hydrolyzed PET increased from approximately 18% to 34% after hydrophobins pretreatment. The releases of TPA and MHET from HGFI-pretreated PET were enhanced 48% and 62%, respectively. The selectivity (TPA/MHET ratio) of the hydrolysis reaction was approximately 0.5.

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.

Environmental Consortium Containing Pseudomonas and Bacillus Species Synergistically Degrades Polyethylene Terephthalate Plastic

While several research groups are utilizing purified enzymes to break down postconsumer PET to the monomers TPA and ethylene glycol to produce new PET products, here, we present a group of five soil bacteria in culture that are able to partially degrade this polymer. To date, mixed Pseudomonas spp. and Bacillus spp. biodegradation of PET has not been described, and this work highlights the possibility of using bacterial consortia to biodegrade or potentially to biorecycle PET plastic waste.

Plastics, such as polyethylene terephthalate (PET) from water bottles, are polluting our oceans, cities, and soils. While a number of Pseudomonas species have been described that degrade aliphatic polyesters, such as polyethylene (PE) and polyurethane (PUR), few from this genus that degrade the semiaromatic polymer PET have been reported. In this study, plastic-degrading bacteria were isolated from petroleum-polluted soils and screened for lipase activity that has been associated with PET degradation. Strains and consortia of bacteria were grown in a liquid carbon-free basal medium (LCFBM) with PET as the sole carbon source. We monitored several key physical and chemical properties, including bacterial growth and modification of the plastic surface, using scanning electron microscopy (SEM) and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) spectroscopy. We detected by-products of hydrolysis of PET using ¹H-nuclear magnetic resonance (¹H NMR) analysis, consistent with the ATR-FTIR data. The full consortium of five strains containing Pseudomonas and Bacillus species grew synergistically in the presence of PET and the cleavage product bis(2-hydroxyethyl) terephthalic acid (BHET) as sole sources of carbon. Secreted enzymes extracted from the full consortium were capable of fully converting BHET to the metabolically usable monomers terephthalic acid (TPA) and ethylene glycol. Draft genomes provided evidence for mixed enzymatic capabilities between the strains for metabolic degradation of TPA and ethylene glycol, the building blocks of PET polymers, indicating cooperation and ability to cross-feed in a limited nutrient environment with PET as the sole carbon source. The use of bacterial consortia for the biodegradation of PET may provide a partial solution to widespread planetary plastic accumulation.

IMPORTANCE While several research groups are utilizing purified enzymes to break down postconsumer PET to the monomers TPA and ethylene glycol to produce new PET products, here, we present a group of five soil bacteria in culture that are able to partially degrade this polymer. To date, mixed Pseudomonas spp. and Bacillus spp. biodegradation of PET has not been described, and this work highlights the possibility of using bacterial consortia to biodegrade or potentially to biorecycle PET plastic waste.

Microbial and Enzymatic Degradation of Synthetic Plastics

Synthetic plastics are pivotal in our current lifestyle and therefore, its accumulation is a major concern for environment and human health. Petroleum-derived (petro-)polymers such as polyethylene (PE), polyethylene terephthalate (PET), polyurethane (PU), polystyrene (PS), polypropylene (PP), and polyvinyl chloride (PVC) are extremely recalcitrant to natural biodegradation pathways. Some microorganisms with the ability to degrade petro-polymers under in vitro conditions have been isolated and characterized. In some cases, the enzymes expressed by these microbes have been cloned and sequenced. The rate of polymer biodegradation depends on several factors including chemical structures, molecular weights, and degrees of crystallinity. Polymers are large molecules having both regular crystals (crystalline region) and irregular groups (amorphous region), where the latter provides polymers with flexibility. Highly crystalline polymers like polyethylene (95%), are rigid with a low capacity to resist impacts. PET-based plastics possess a high degree of crystallinity (30-50%), which is one of the principal reasons for their low rate of microbial degradation, which is projected to take more than 50 years for complete degraded in the natural environment, and hundreds of years if discarded into the oceans, due to their lower temperature and oxygen availability. The enzymatic degradation occurs in two stages: adsorption of enzymes on the polymer surface, followed by hydro-peroxidation/hydrolysis of the bonds. The sources of plastic-degrading enzymes can be found in microorganisms from various environments as well as digestive intestine of some invertebrates. Microbial and enzymatic degradation of waste petro-plastics is a promising strategy for depolymerization of waste petro-plastics into polymer monomers for recycling, or to covert waste plastics into higher value bioproducts, such as biodegradable polymers via mineralization. The objective of this review is to outline the advances made in the microbial degradation of synthetic plastics and, overview the enzymes involved in biodegradation.

Current knowledge on enzymatic PET degradation and its possible application to waste stream management and other fields

Enzymatic hydrolysis of polyethylene terephthalate (PET) has been the subject of extensive previous research that can be grouped into two categories, viz. enzymatic surface modification of polyester fibers and management of PET waste by enzymatic hydrolysis. Different enzymes with rather specific properties are required for these two processes. Enzymatic surface modification is possible with several hydrolases, such as lipases, carboxylesterases, cutinases, and proteases. These enzymes should be designated as PET surface–modifying enzymes and should not degrade the building blocks of PET but should hydrolyze the surface polymer chain so that the intensity of PET is not weakened. Conversely, management of PET waste requires substantial degradation of the building blocks of PET; therefore, only a limited number of cutinases have been recognized as PET hydrolases since the first PET hydrolase was discovered by Müller et al. (Macromol Rapid Commun 26:1400–1405, 2005). Here, we introduce current knowledge on enzymatic degradation of PET with a focus on the key class of enzymes, PET hydrolases, pertaining to the definition of enzymatic requirements for PET hydrolysis, structural analyses of PET hydrolases, and the reaction mechanisms. This review gives a deep insight into the structural basis and dynamics of PET hydrolases based on the recent progress in X-ray crystallography. Based on the knowledge accumulated to date, we discuss the potential for PET hydrolysis applications, such as in designing waste stream management.

Photochemical Transformation of Poly(butylene adipate- co-terephthalate) and Its Effects on Enzymatic Hydrolyzability

Biodegradable polyesters are being increasingly used to replace conventional, nondegradable polymers in agricultural applications such as plastic film for mulching. For many of these applications, poly(butylene adipate- co-terephthalate) (PBAT) is a promising biodegradable material. However, PBAT is also susceptible to photochemical transformations. To better understand how photochemistry affects the biodegradability of PBAT, we irradiated blown, nonstabilized, transparent PBAT films and studied their enzymatic hydrolysis, which is considered the rate-limiting step in polyester biodegradation. In parallel, we characterized the irradiated PBAT films by dynamic mechanical thermal analysis. The rate of enzymatic PBAT hydrolysis decreased when the density of light-induced cross-links within PBAT exceeded a certain threshold. Mass-spectrometric analysis of the enzymatic hydrolysis products of irradiated PBAT films provided evidence for radical-based cross-linking of two terephthalate units that resulted in the formation of benzophenone-like molecules. In a proof-of-principle experiment, we demonstrated that the addition of photostabilizers to PBAT films mitigated the negative effect of UV irradiation on the enzymatic hydrolyzability of PBAT. This work advances the understanding of light-induced changes on the enzyme-mediated hydrolysis of aliphatic-aromatic polyesters and will therefore have important implications for the development of biodegradable plastics.

Biodegradation of Polymeric Mulch Films in Agricultural Soils: Concepts, Knowledge Gaps, and Future Research Directions

The agricultural use of conventional, polyethylene-based mulch films leads to the accumulation of remnant film pieces in agricultural soils with negative impacts for soil productivity and ecology. A viable strategy to overcome this accumulation is to replace conventional with biodegradable mulch films composed of polymers designed to be degraded by soil microorganisms. However, understanding polymer biodegradation in soils remains a significant challenge due to its dependence on polymer properties, soil characteristics, and prevailing environmental conditions. This perspective aims to advance our understanding of the three fundamental steps underlying biodegradation of mulch films in agricultural soils: colonization of the polymer film surfaces by soil microorganisms, depolymerization of the polymer films by extracellular microbial hydrolases, and subsequent microbial assimilation and utilization of the hydrolysis products for energy production and biomass formation. The perspective synthesizes the current conceptual understanding of these steps and highlights existing knowledge gaps. The discussion addresses future research and analytical advancements required to overcome the knowledge gaps and to identify the key polymer properties and soil characteristics governing mulch film biodegradation in agricultural soils.

Targeting microplastic particles in the void of diluted suspensions

Accumulation of microplastic in the environment and food chain will be a grand challenge for our society. Polyurethanes are widely used synthetic polymers in medical (e.g. catheters) and industrial products (especially as foams). Polyurethane is not abundant in nature and only a few microbial strains (fungi and bacteria) and enzymes (polyurethaneases and cutinases) have been reported to efficiently degrade polyurethane. Notably, in nature a long period of time (from 50 to >100 years depending on the literature) is required for degradation of plastics. Material binding peptides (e.g. anchor peptides) bind strongly to polymers such as polypropylene, polyethylene terephthalate, and polyurethane and can target specifically polymers. In this study we report the fusion of the anchor peptide Tachystatin A2 to the bacterial cutinase Tcur1278 which accelerated the degradation of polyester-polyurethane nanoparticles by a factor of 6.6 in comparison to wild-type Tcur1278. Additionally, degradation half-lives of polyester-polyurethane nanoparticles were reduced from 41.8 h to 6.2 h (6.7-fold) in a diluted polyester-polyurethane suspension (0.04% w/v).

Surface engineering of polyester-degrading enzymes to improve efficiency and tune specificity

Certain members of the carboxylesterase superfamily can act at the interface between water and water-insoluble substrates. However, nonnatural bulky polyesters usually are not efficiently hydrolyzed. In the recent years, the potential of enzyme engineering to improve hydrolysis of synthetic polyesters has been demonstrated. Regions on the enzyme surface have been modified by using site-directed mutagenesis in order to tune sorption processes through increased hydrophobicity of the enzyme surface. Such modifications can involve specific amino acid substitutions, addition of binding modules, or truncation of entire domains improving sorption properties and/or dynamics of the enzyme. In this review, we provide a comprehensive overview on different strategies developed in the recent years for enzyme surface engineering to improve the activity of polyester-hydrolyzing enzymes.

Controlled drug release through regulated biodegradation of poly(lactic acid) using inorganic salts

Biodegradation rate of poly(lactic acid) (PLA) has been regulated, both increase and decrease with respect to the biodegradation of pure PLA, by embedding meager amount of inorganic salts in polymer matrix. Biodegradation is performed in enzyme medium on suspension and film and the extent of biodegradation is measured through spectroscopic technique which is also verified by weight loss measurement. Media pH has been controlled using trace amount of inorganic salt which eventually control the biodegradation of PLA. High performance liquid chromatography confirms the hydrolytic degradation of PLA to its monomer/oligomer. Induced pH by metal salts show maximum degradation at alkaline range (with calcium salt) while inhibition is observed in acidic medium (with iron salt). The pH of media changes the conformation of enzyme which in turn regulate the rate of biodegradation. Thermal degradation and increment of modulus indicate improvement in thermo-mechanical properties of PLA in presence of inorganic salts. Functional stability of enzyme with metal salts corresponding to acidic and alkaline pH has been established through a model to explain the conformational changes of the active sites of enzyme at varying pH influencing the rate of hydrolysis leading to regulated biodegradation of PLA. The tuned biodegradation has been applied for the controlled release of drug from the polymer matrix (both sustained and enhanced cumulative release as compared to pure polymer). The cell proliferation and adhesion are influenced by the acidic and basic nature of polymeric material tuned by two different inorganic salts showing better adhesion and proliferation in calcium based composite and, therefore, suggest biological use of these composites in biomedical applications.