Adsorption of enzymes with hydrolytic activity on polyethylene terephthalate

Biodegradation of Poly(ε-caprolactone): Microorganisms, Enzymes, and Mechanisms

Poly(ε-caprolactone) (PCL) is a synthetic plastic known for its excellent physicochemical properties and a wide range of applications in packaging, coatings, foaming, and agriculture. In medicine, its versatility allows it to function as a scaffold for drug delivery, sutures, implants, tissue engineering, and 3D printing. In addition to its biocompatibility, PCL's most notable characteristic is its biodegradability. However, this property is affected by temperature, microbial activity, and environmental conditions, which means PCL can sometimes remain in nature for long periods. This review shows that various types of microorganisms can efficiently degrade PCL, including different strains of Pseudomonas spp., Streptomyces spp., Alcaligenes faecalis, and fungi like Aspergillus oryzae, Fusarium spp., Rhizopus delemar, and Thermomyces lanuginosus. These microorganisms produce enzymes such as lipases, esterases, and cutinases that break down PCL into smaller molecules that act as substrates. The review also examines the phylogenetic diversity of organisms capable of biodegrading PCL, the biochemical pathways involved in this process, and specific aspects of the genetic framework responsible for the expression of the enzymes that facilitate degradation. Targeted research on microbial PCL biodegradation and its practical applications could significantly aid in reducing and managing plastic waste on a global ecological scale.

Molecular Details of Polyester Decrystallization via Molecular Simulation

Waste polyesters are a potential feedstock for recycled and upcycled products. These polymers are generally semicrystalline, which presents a challenge for chemical and biological recycling to monomers, and thus the thermodynamic work associated with polyester decrystallization is an important consideration in some depolymerization strategies. Here, we use molecular dynamics simulations to calculate the free energy required to decrystallize a single chain from the crystal surface of five commercially and scientifically important, semiaromatic polyesters (PET, PTT, PBT, PEN, and PEF) in water. Our results indicate the decrystallization work ranges from approximately 15 kcal/mol (PEN) to 8 kcal/mol (PEF) per repeat unit for chains in the middle of a crystal surface. The insight gained into the molecular interactions that form the structural basis of semicrystalline synthetic polyesters can guide the pursuit of more efficient plastic processing, which could include catalyst development, optimizing recycling conditions including pretreatment, enzyme and solvent selections, and design of new materials.

Impact of Ball Milling on the Microstructure of Polyethylene Terephthalate

Polyethylene terephthalate (PET) is a semi-crystalline polymer that finds broad use. Consequently, it contributes to the accumulation of plastics in the environment, warranting PET recycling technologies. Ball milling is a commonly used technique for the micronization of plastics before transformation. It has also recently been reported as an efficient mixing strategy for the enzymatic hydrolysis of plastics in moist-solid mixtures. However, the effect of milling on the microstructure of PET has not been systematically investigated. Thus, the primary objective of this study is to characterize the changes to the PET microstructure caused by various ball milling conditions. PET of different forms was examined, including pre- and post-consumer PET, as well as textiles. The material was treated to a range of milling frequencies and duration, before analysis of particle size, crystallinity by differential scanning calorimetry and powder X-ray diffraction, and morphology by scanning electron microscopy. Interestingly, our results suggest the convergence of crystallinity to ~30 % within 15 minutes of milling at 30 Hz. These results are consistent with an equilibrium between amorphous and crystalline regions of the polymer being established during ball milling. The combined data constitutes a reference guide for PET milling and recycling research.

Atomistic adsorption of PETase onto large-scale PET 3D-models that mimic reality

Polyethylene terephthalate (PET) has been widely used in plastic products, leading to massive PET waste accumulation in ecosystems worldwide. Efforts to find greener processes for dealing with post-consumer PET waste led to the discovery of PET-degrading enzymes such as Ideonella sakaiensis PETase (IsPETase). In silico studies have provided valuable contributions to this field, shedding light on the catalytic mechanisms and substrate interactions in many PET hydrolase enzymes. However, most of these studies have often relied on short PET oligomers, failing to replicate catalytic-relevant interactions and true substrate motions occurring during contact with a PET-degrading enzyme. A comprehensive atomistic study of PET in both its crystalline (cPET) and amorphous (aPET) states, along with investigation of the adsorption of PET-degrading enzymes onto solid PET, would greatly advance our understanding of mechanisms driving PET biodegradation. In this study, we developed large-scale computational models of cPET, comprising thousands of monomers, and conducted molecular dynamics simulations to follow the transformation of cPET into aPET. Next, these models were validated by comparison with experimentally determined data. We then studied the adsorption of IsPETase on the assembled PET models, investigated the main phenomena that differentiate the two adsorption processes, and explored them from a catalytic perspective. The results and computational PET models provided herein are envisioned to aid in the development of innovative strategies for PET waste biodegradation.

3D Printing in Biocatalysis and Biosensing: From General Concepts to Practical Applications

3D printing has matured into a versatile technique that offers researchers many different printing methods and materials with varying properties. Nowadays, 3D printing is deployed within a myriad of different applications, ranging from chemistry to biotechnology -including bioanalytics, biocatalysis or biosensing. Due to its inherent design flexibility (which enables rapid prototyping) and ease of use, 3D printing facilitates the relatively quick and easy creation of new devices with unprecedented functions.. This review article describes how 3D printing can be employed for research in the fields of biochemistry and biotechnology, and specifically for biocatalysis and biosensor applications. We survey different relevant 3D printing techniques, as well as the surface activation and functionalization of 3D-printed materials. Finally, we show how 3D printing is used for the fabrication of reaction ware and enzymatic assays in biocatalysis research, as well as for the generation of biosensors using aptamers, antibodies, and enzymes as recognition elements.

Understanding the Catalytic Efficiency of Two Polyester Degrading Enzymes: An Experimental and Theoretical Investigation

The discovery of novel plastic degrading enzymes commonly relies on comparing features of the primary sequence to those of known plastic degrading enzymes. However, this approach cannot always guarantee success. This is exemplified by the different degradation rates of the two polymers poly(ethylene terephthalate) (PET) and polybutylene succinate (PBS) by two hydrolases: IsPETase from Ideonella sakaiensis and AdCut from Acidovorax delafieldii. Despite the enzymes showing a very high sequence identity of 82%, IsPETase shows significant hydrolysis activity for both polymers, whereas AdCut only shows significant hydrolysis activity for PBS. By solving the structure of AdCut using X-ray crystallography, and using this as the basis for computer simulations, comparisons are made between the differences in the calculated binding geometries and the catalytic results obtained from biochemical experiments. The results reveal that the low activity of AdCut toward PET can be explained by the low sampling of the productive conformation observed in the simulations. While the active site serine in IsPETase can closely encounter the PET carbonyl carbon, in AdCut it cannot: a feature that can be attributed to the shape of the catalytic binding pocket. These results yield an important insight into the design requirements for novel plastic degrading enzymes, as well as showing that computational methods can be used as a valuable tool in understanding the molecular basis for different hydrolysis activities in homologous polyesterase enzymes.

Bridging the Gap in the Structure-Function Paradigm of Enzymatic PET Degradation-Aromatic Residue Driven Balanced Interactions with Catalytic and Anchoring Subsite

Understanding all parameters contributing to enzyme activity is crucial in enzyme catalysis. For enzymatic PET degradation, this involves examining the formation of the enzyme-PET complex. In IsPETase (WT), a PET-degrading enzyme from Ideonella sakaiensis, mutating two non-catalytic residues (DM) significantly enhances activity. Such mutations, depending on their position in the tertiary structure, fine-tune enzyme function. However, detailed molecular insights into these mutations' structure-function relationship for PET degradation are lacking. This study characterizes IsPETase's catalytic ability compared to WT TfCut2 using molecular dynamics simulations and quantum mechanical methods. We explore the conformational landscape of the enzyme-PET complex and quantify residue-wise interaction energy. Notably, aromatic and hydrophobic residues Tyr, Trp, and Ile in the catalytic subsite S1, and aromatic Phe and polar Asn in the anchoring subsite S3, crucially optimize PET binding. These residues enhance PET specificity over non-aromatic plastics. Our findings suggest that the balance between binding at subsite S1 and subsite S3, which is influenced by cooperative mutations, underlies catalytic activity. This balance shows a positive correlation with experimentally obtained kcat/Km values: WT TfCut2 Collapse

Key Words

• Cooperative effect
• Enzyme catalysis
• IsPETase
• Molecular dynamics
• Mutation

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MESH Headings

• Molecular Dynamics Simulation
• Polyethylene Terephthalates/chemistry
• Polyethylene Terephthalates/metabolism
• Catalytic Domain
• Structure-Activity Relationship
• Burkholderiales/enzymology
• Biocatalysis
• Mutation
• Bacterial Proteins/chemistry
• Bacterial Proteins/metabolism
• Bacterial Proteins/genetics

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Grants

• CRG/2019/002891 DST
• SPG/2022/001350 DST
• SIR/2022/000924 DST
• University of Calicut

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Affiliation(s)

Anjima James Department of Applied Chemistry, Cochin University of Science and Technology, Thrikakkara, Kochi, Kerala, 682 022, India
Anjitha Bhasi Department of Applied Chemistry, Cochin University of Science and Technology, Thrikakkara, Kochi, Kerala, 682 022, India
Susmita De Department of Chemistry, University of Calicut, Calicut University P.O., Malappuram, Kerala, 673 635, India

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Collaborators Collapse

From Bulk to Binding: Decoding the Entry of PET into Hydrolase Binding Pockets

Plastic-degrading enzymes facilitate the biocatalytic recycling of poly(ethylene terephthalate) (PET), a significant synthetic polymer, and substantial progress has been made in utilizing PET hydrolases for industrial applications. To fully exploit the potential of these enzymes, a deeper mechanistic understanding followed by targeted protein engineering is essential. Through advanced molecular dynamics simulations and free energy analysis methods, we elucidated the complete pathway from the initial binding of two PET hydrolases-the thermophilic leaf-branch compost cutinase (LCC) and polyester hydrolase 1 (PES-H1)-to an amorphous PET substrate, ultimately leading to a PET chain entering the active site in a hydrolyzable conformation. Our findings indicate that initial PET binding is nonspecific and driven by polar and hydrophobic interactions. We demonstrate that the subsequent entry of PET into the active site can occur via one of three key pathways, identifying barriers related to both PET-PET and PET-enzyme interactions, as well as specific residues highlighted through in silico and in vitro mutagenesis. These insights not only enhance our understanding of the mechanisms underlying PET degradation and facilitate the development of targeted enzyme enhancement strategies but also provide a novel framework applicable to enzyme studies across various disciplines.

Interfacial Reactions in Chemical Recycling and Upcycling of Plastics

Depolymerization of plastics is a leading strategy to combat the escalating global plastic waste crisis through chemical recycling, upcycling, and remediation of micro-/nanoplastics. However, critical processes necessary for polymer chain scission, occurring at the polymer-catalyst or polymer-fluid interfaces, remain largely overlooked. Herein, we spotlight the importance of understanding these interfacial chemical processes as a critical necessity for optimizing kinetics and reactivity in plastics recycling and upcycling, controlling reaction outcomes, product distributions, as well as improving the environmental sustainability of these processes. Several examples are highlighted in heterogeneous processes such as hydrogenation over solid catalysts, reaction of plastics in immiscible media, and biocatalysis. Ultimately, judicious exploitation of interfacial reactivity has practical implications in developing practical, robust, and cost-effective processes to reduce plastic waste and enable a viable post-use circular plastics economy.

Developing tardigrade-inspired material: Track membranes functionalized with Dsup protein for cell-free DNA isolation

When developing functionalized biomaterials, the proteins from extremophilic organisms, in particular unique tardigrade disordered proteins, are of great value. The damage suppressor protein (Dsup), initially discovered in the tardigrade Ramazzottius varieornatus and found to be an efficient DNA protector under oxidative and irradiation stress, has been hypothesized to possess a good potential for the development of the material, which can isolate cell-free DNA. With this in mind, DNA-nonadsorbing polyethylene terephthalate track membranes have been functionalized using the Dsup protein via covalent bonding with glutaraldehyde. The filtration experiments have verified the ability of track membranes with the immobilized Dsup protein to adsorb cell-free DNA, with an accumulation capacity of 70 ± 19 mg m-2. The resulting track membrane-based biomaterial might be used in various devices for filtration and separation of cell-free DNA molecules from biological solutions and environmental samples, and also for their accumulation, storage, and further manipulation.

Non-ionic surfactant PEG: Enhanced cutinase-catalyzed hydrolysis of polyethylene terephthalate

To enhance the enzymatic digestibility of polyethylene terephthalate (PET), which is highly oriented and crystallized, a polyethylene glycol (PEG) surfactant of varying molecular weights was utilized to improve the stability of mutant cutinase from Humicola insolens (HiC) and to increase the accessibility of the enzyme to the substrate. Leveraging the optimal conditions for HiC hydrolysis of PET, the introduction of 1 % w/v PEG significantly increased the yield of PET hydrolysis products. PEG600 was particularly effective, increasing the yield by 64.58 % compared to using HiC alone. Moreover, the mechanisms by which PEG600 and PEG6000 enhance enzyme digestion were extensively examined using circular dichroism and fluorescence spectroscopy. The results from CD and fluorescence analyses indicated that PEG alters the protein conformation, thereby affecting the catalytic effect of the enzyme. Moreover, PEG improved the affinity between HiC and PET by lowering the surface tension of the solution, substantially enhancing PET hydrolysis. This study suggests that PEG holds considerable promise as an enzyme protector, significantly aiding in the hydrophilic modification and degradation of PET in an environmentally friendly and sustainable manner.

Exploring the pH dependence of an improved PETase

Enzymatic recycling of plastic and especially of polyethylene terephthalate (PET) has shown great potential to reduce its negative impact on our society. PET hydrolases (PETases) have been optimized using rational design and machine learning, but the mechanistic details of the PET depolymerization process remain unclear. Belonging to the carboxylic-ester hydrolase family with a canonical Ser-His-Asp catalytic triad, their observed alkaline pH optimum is generally thought to be related to the protonation state of the catalytic His. Here, we explore this aspect in the context of LCCICCG, an optimized PETase, derived from the leaf-branch compost cutinase enzyme. We use NMR to identify the dominant tautomeric structure of the six histidines. Five show surprisingly low pKa values below 4.0, whereas the catalytic H242 in the active enzyme displays a pKa value that varies from 4.9 to 4.7 when temperatures increase from 30°C to 50°C. Whereas the hydrolytic activity of the enzyme toward a soluble substrate can be modeled by the corresponding protonation/deprotonation curve, an important discrepancy is found when the substrate is the solid plastic. This opens the way to further mechanistic understanding of the PETase activity and underscores the importance of studying the enzyme at the liquid-solid interface.

Bottlenecks in biobased approaches to plastic degradation

Plastic waste is an environmental challenge, but also presents a biotechnological opportunity as a unique carbon substrate. With modern biotechnological tools, it is possible to enable both recycling and upcycling. To realize a plastics bioeconomy, significant intrinsic barriers must be overcome using a combination of enzyme, strain, and process engineering. This article highlights advances, challenges, and opportunities for a variety of common plastics.

Enhanced biodegradation activity toward polyethylene by fusion protein of anchor peptide and Streptomyces sp. strain K30 latex clearing protein

Polyethylene is the most commonly used plastic product, and its biodegradation is a worldwide problem. Latex clearing protein derived from Streptomyces sp. strain K30 (LcpK30) has been reported to be able to break the carbon-carbon double bond inside oxidized polyethylene and is an effective biodegradation enzyme for polyethylene. However, the binding of the substrate to the enzyme was difficult due to the hydrophobic nature of polyethylene. Therefore, to further improve the efficiency of LcpK30, the effect of different anchor peptides on the binding capacity of LcpK30 to the substrate was screened in this study. The results of fluorescence confocal microscopy showed that the anchoring peptide LCI had the most significant improvement in effect and was finally selected for further application in a UV-irradiated PE degradation system. The degradation results showed that LCI was able to improve the degradation efficiency of LcpK30 by approximately 1.15 times in the presence of equimolar amounts of protein compared with wild-type. This study further improves the application of LcpK30 in the field of polyethylene degradation by modification.

Depolymerization within a Circular Plastics System

The societal importance of plastics contrasts with the carelessness with which they are disposed. Their superlative properties lead to economic and environmental efficiency, but the linearity of plastics puts the climate, human health, and global ecosystems at risk. Recycling is fundamental to transitioning this linear model into a more sustainable, circular economy. Among recycling technologies, chemical depolymerization offers a route to virgin quality recycled plastics, especially when valorizing complex waste streams poorly served by mechanical methods. However, chemical depolymerization exists in a complex and interlinked system of end-of-life fates, with the complementarity of each approach key to environmental, economic, and societal sustainability. This review explores the recent progress made into the depolymerization of five commercial polymers: poly(ethylene terephthalate), polycarbonates, polyamides, aliphatic polyesters, and polyurethanes. Attention is paid not only to the catalytic technologies used to enhance depolymerization efficiencies but also to the interrelationship with other recycling technologies and to the systemic constraints imposed by a global economy. Novel polymers, designed for chemical depolymerization, are also concisely reviewed in terms of their underlying chemistry and potential for integration with current plastic systems.

Unveiling PET Hydrolase Surface Dynamics through Fluorescence Microscopy

PET hydrolases are an emerging class of enzymes that are being heavily researched for their use in bioprocessing polyethylene terephthalate (PET). While work has been done in studying the binding of PET oligomers to the active site of these enzymes, the dynamics of PET hydrolases binding to a bulk PET surface is an unexplored area. Here, methods were developed for total internal reflection fluorescence (TIRF) microscopy and fluorescence recovery after photobleaching (FRAP) microscopy to study the adsorption and desorption dynamics of these proteins onto a PET surface. TIRF microscopy was employed to measure both on and off rates of two of the most commonly studied PET hydrolases, PHL7 and LCC, on a PET surface. It was found that these proteins have a much slower off rates on the order of 10-3  s-1 , comparable to non-productive binding in enzymes such as cellulose. In combination with FRAP microscopy, a dynamic model is proposed in which adsorption and desorption dominates over lateral diffusion over the surface. The results of this study could have implications for the future engineering of PET hydrolases, either to target them to a PET surface or to modulate interaction with their substrate.

Computational redesign of a hydrolase for nearly complete PET depolymerization at industrially relevant high-solids loading

Biotechnological plastic recycling has emerged as a suitable option for addressing the pollution crisis. A major breakthrough in the biodegradation of poly(ethylene terephthalate) (PET) is achieved by using a LCC variant, which permits 90% conversion at an industrial level. Despite the achievements, its applications have been hampered by the remaining 10% of nonbiodegradable PET. Herein, we address current challenges by employing a computational strategy to engineer a hydrolase from the bacterium HR29. The redesigned variant, TurboPETase, outperforms other well-known PET hydrolases. Nearly complete depolymerization is accomplished in 8 h at a solids loading of 200 g kg-1. Kinetic and structural analysis suggest that the improved performance may be attributed to a more flexible PET-binding groove that facilitates the targeting of more specific attack sites. Collectively, our results constitute a significant advance in understanding and engineering of industrially applicable polyester hydrolases, and provide guidance for further efforts on other polymer types.

Concentration-Dependent Inhibition of Mesophilic PETases on Poly(ethylene terephthalate) Can Be Eliminated by Enzyme Engineering

Enzyme-based depolymerization is a viable approach for recycling of poly(ethylene terephthalate) (PET). PETase from Ideonella sakaiensis (IsPETase) is capable of PET hydrolysis under mild conditions but suffers from concentration-dependent inhibition. In this study, this inhibition is found to be dependent on incubation time, the solution conditions, and PET surface area. Furthermore, this inhibition is evident in other mesophilic PET-degrading enzymes to varying degrees, independent of the level of PET depolymerization activity. The inhibition has no clear structural basis, but moderately thermostable IsPETase variants exhibit reduced inhibition, and the property is completely absent in the highly thermostable HotPETase, previously engineered by directed evolution, which simulations suggest results from reduced flexibility around the active site. This work highlights a limitation in applying natural mesophilic hydrolases for PET hydrolysis and reveals an unexpected positive outcome of engineering these enzymes for enhanced thermostability.

Determinants for an Efficient Enzymatic Catalysis in Poly(Ethylene Terephthalate) Degradation

The enzymatic degradation of the recalcitrant poly(ethylene terephthalate) (PET) has been an important biotechnological goal. The present review focuses on the state of the art in enzymatic degradation of PET, and the challenges ahead. This review covers (i) enzymes acting on PET, (ii) protein improvements through selection or engineering, (iii) strategies to improve biocatalyst–polymer interaction and monomer yields. Finally, this review discusses critical points on PET degradation, and their related experimental aspects, that include the control of physicochemical parameters. The search for, and engineering of, PET hydrolases, have been widely studied to achieve this, and several examples are discussed here. Many enzymes, from various microbial sources, have been studied and engineered, but recently true PET hydrolases (PETases), active at moderate temperatures, were reported. For a circular economy process, terephtalic acid (TPA) production is critical. Some thermophilic cutinases and engineered PETases have been reported to release terephthalic acid in significant amounts. Some bottlenecks in enzyme performance are discussed, including enzyme activity, thermal stability, substrate accessibility, PET microstructures, high crystallinity, molecular mass, mass transfer, and efficient conversion into reusable fragments.

Rational mutagenesis of Thermobifida fusca cutinase to modulate the enzymatic degradation of polyethylene terephthalate

Thermobifida fusca cutinase (TfCut2) is a carboxylesterase (CE) which degrades polyethylene terephthalate (PET) as well as its degradation intermediates [such as oligoethylene terephthalate (OET), or bis-/mono-hydroxyethyl terephthalate (BHET/MHET)] into terephthalic acid (TPA). Comparisons of the surfaces of certain CEs (including TfCut2) were combined with docking and molecular dynamics simulations involving 2HE-(MHET)_3, a three-terephthalate OET, to support the rational design of 22 variants with potential for improved generation of TPA from PET, comprising 15 single mutants (D12L, E47F, G62A, L90A, L90F, H129W, W155F, ΔV164, A173C, H184A, H184S, F209S, F209I, F249A, and F249R), 6 double mutants [H129W/T136S, A173C/A206C, A173C/A210C, G62A/L90F, G62A/F209I, and G62A/F249R], and 1 triple mutant [G62A/F209I/F249R]. Of these, nine displayed no activity, three displayed decreased activity, three displayed comparable activity, and seven displayed increased (~1.3- to ~7.2-fold) activity against solid PET, while all variants displayed activity against BHET. Of the variants that displayed increased activity against PET, four displayed more activity than G62A, the most-active mutant of TfCut2 known till date. Of these four, three displayed even more activity than LCC (G62A/F209I, G62A/F249R, and G62A/F209I/F249R), a CE known to be ~5-fold more active than wild-type TfCut2. These improvements derived from changes in PET binding and not changes in catalytic efficiency.

Reaction Pathways for the Enzymatic Degradation of Poly(Ethylene Terephthalate): What Characterizes an Efficient PET-Hydrolase?

Bioprocessing of polyester waste has emerged as a promising tool in the quest for a cyclic plastic economy. One key step is the enzymatic breakdown of the polymer, and this entails a complicated pathway with substrates, intermediates, and products of variable size and solubility. We have elucidated this pathway for poly(ethylene terephthalate) (PET) and four enzymes. Specifically, we combined different kinetic measurements and a novel stochastic model and found that the ability to hydrolyze internal bonds in the polymer (endo-lytic activity) was a key parameter for overall enzyme performance. Endo-lytic activity promoted the release of soluble PET fragments with two or three aromatic rings, which, in turn, were broken down with remarkable efficiency (k_cat /K_M values of about 10⁵  M^-1 s^-1 ) in the aqueous bulk. This meant that approximatly 70 % of the final, monoaromatic products were formed via soluble di- or tri-aromatic intermediates.

Investigating the effects of cyclic topology on the performance of a plastic degrading enzyme for polyethylene terephthalate degradation

Agitation is a commonly encountered stress for enzymes during all stages of production and application, but investigations that aim to improve their tolerance using topological engineering have yet to be reported. Here, the plastic-degrading enzyme IsPETase was cyclized in a range of topologies including a cyclic monomer, cyclic dimer and catenane using SpyTag/SpyCatcher technologies, and their tolerance towards different stresses including mechanical agitation was investigated. The cyclic dimer and catenane topologies were less susceptible to agitation-induced inactivation resulting in enhancement of polyethylene terephthalate (PET) degradation. While contrary to conventional belief, cyclic topologies did not improve tolerance of IsPETase towards heat or proteolytic treatment, the close proximity of active sites in the dimeric and catenane variants was found to enhance PET conversion into small soluble products. Together, these findings illustrate that it is worthwhile to explore the topology engineering of enzymes used in heterogeneous catalysis as it improves factors that are often overlooked in homogeneous catalysis studies.

Rapid depolymerization of poly(ethylene terephthalate) thin films by a dual-enzyme system and its impact on material properties

Enzymatic hydrolysis holds great promise for plastic waste recycling and upcycling. The interfacial catalysis mode, and the variability of polymer specimen properties under different degradation conditions, add to the complexity and difficulty of understanding polymer cleavage and engineering better biocatalysts. We present a systemic approach to studying the enzyme-catalyzed surface erosion of poly(ethylene terephthalate) (PET) while monitoring/controlling operating conditions in real time with simultaneous detection of mass loss and changes in viscoelastic behavior. PET nanofilms placed on water showed a porous morphology and a thickness-dependent glass transition temperature (Tg) between 40°C and 44°C, which is >20°C lower than the Tg of bulk amorphous PET. Hydrolysis by a dual-enzyme system containing thermostabilized variants of Ideonella sakaiensis PETase and MHETase resulted in a maximum depolymerization of 70% in 1 h at 50°C. We demonstrate that increased accessible surface area, amorphization, and Tg reduction speed up PET degradation while simultaneously lowering the threshold for degradation-induced crystallization.

Sabatier Principle for Rationalizing Enzymatic Hydrolysis of a Synthetic Polyester

Interfacial enzyme reactions are common in Nature and in industrial settings, including the enzymatic deconstruction of poly(ethylene terephthalate) (PET) waste. Kinetic descriptions of PET hydrolases are necessary for both comparative analyses, discussions of structure-function relations and rational optimization of technical processes. We investigated whether the Sabatier principle could be used for this purpose. Specifically, we compared the kinetics of two well-known PET hydrolases, leaf-branch compost cutinase (LCC) and a cutinase from the bacterium Thermobifida fusca (TfC), when adding different concentrations of the surfactant cetyltrimethylammonium bromide (CTAB). We found that CTAB consistently lowered the strength of enzyme-PET interactions, while its effect on enzymatic turnover was strongly biphasic. Thus, at gradually increasing CTAB concentrations, turnover was initially promoted and subsequently suppressed. This correlation with maximal turnover at an intermediate binding strength was in accordance with the Sabatier principle. One consequence of these results was that both enzymes had too strong intrinsic interaction with PET for optimal turnover, especially TfC, which showed a 20-fold improvement of k _cat at the maximum. LCC on the other hand had an intrinsic substrate affinity closer to the Sabatier optimum, and the turnover rate was 5-fold improved at weakened substrate binding. Our results showed that the Sabatier principle may indeed rationalize enzymatic PET degradation and support process optimization. Finally, we suggest that future discovery efforts should consider enzymes with weakened substrate binding because strong adsorption seems to limit their catalytic performance.

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.

An Efficient Protein Evolution Workflow for the Improvement of Bacterial PET Hydrolyzing Enzymes

Enzymatic degradation is a promising green approach to bioremediation and recycling of the polymer poly(ethylene terephthalate) (PET). In the past few years, several PET-hydrolysing enzymes (PHEs) have been discovered, and new variants have been evolved by protein engineering. Here, we report on a straightforward workflow employing semi-rational protein engineering combined to a high-throughput screening of variant libraries for their activity on PET nanoparticles. Using this approach, starting from the double variant W159H/S238F of Ideonella sakaiensis 201-F6 PETase, the W159H/F238A-ΔIsPET variant, possessing a higher hydrolytic activity on PET, was identified. This variant was stabilized by introducing two additional known substitutions (S121E and D186H) generating the TS-ΔIsPET variant. By using 0.1 mg mL⁻¹ of TS-ΔIsPET, ~10.6 mM of degradation products were produced in 2 days from 9 mg mL⁻¹ PET microparticles (~26% depolymerization yield). Indeed, TS-ΔIsPET allowed a massive degradation of PET nanoparticles (>80% depolymerization yield) in 1.5 h using only 20 μg of enzyme mL⁻¹. The rationale underlying the effect on the catalytic parameters due to the F238A substitution was studied by enzymatic investigation and molecular dynamics/docking analysis. The present workflow is a well-suited protocol for the evolution of PHEs to help generate an efficient enzymatic toolbox for polyester degradation.