Plastic-Degrading Enzymes as Sustainable Solutions for Plastic Waste

1. Introduction

Plastics have been used in almost every sectors of modern world, including packaging, construction, automotive, electronics, agriculture and healthcare [1]. They became ubiquitous commodities, due to their desirable properties such as low cost, light weight and high durability [2]. Synthetic polymers like polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polyethylene terephthalate (PET) and polyurethane (PU) are engineered to meet high standards in performance that natural materials barely match [3]. However, the same properties that confer utility also result in environmental persistence. Most synthetic plastics are highly recalcitrant to natural degradation processes, leading to long-term accumulation in landfills, waterways and ecosystems [4].

Global production of plastics was over 400 million tons in 2020 and is expected to be doubled in 2050 unless major policy and technological interventions are implemented [5]. Alarmingly, only a small proportion of plastics (approximately 9% in the United States) are recycled through current waste management systems [6]. The remainder is either incinerated, contributing to air pollution and greenhouse gas emissions, or discarded in landfills where it may leach harmful additives into soil and groundwater [6]. A significant portion also ends up in terrestrial and marine environments, breaking down into microplastics that pervade water bodies, sediments and even the food we consume [7]. The persistence and ubiquity of microplastics have sparked growing concern among scientists and policymakers due to their potential toxicity and capacity to bioaccumulate in food chains [8].

Traditional strategies in plastic waste management have proven to be inadequate in addressing the scale and complexity of plastic pollution. Mechanical recycling, while, typically works best with clean and single-type plastic waste, and often produces lower-quality recycled materials due to degradation and contamination [9]. Chemical recycling methods such as pyrolysis or solvolysis can depolymerize plastics into monomers or fuels [9]. But they typically require high energy input, specialized infrastructure and often produce toxic by-products [9]. Incineration, though effective at volume reduction, releases harmful emissions and is not considered a sustainable solution [10]. Landfilling, the most common disposal method globally, does not eliminate plastic waste but merely displaces the environmental burden [11].

Given these challenges, there is increasing interest in developing biological approaches to manage plastic waste that are both effective and environmentally benign [12]. In particular, enzyme-mediated depolymerization offers a promising route for breaking down synthetic polymers under mild conditions [13]. Enzymes are naturally occurring biocatalysts capable of accelerating chemical reactions with remarkable specificity and efficiency. They are expressed in a wide range of microorganisms, including bacteria and fungi, many of which have evolved mechanisms to degrade natural polymers such as cellulose, lignin, and cutin [14]. The recent discoveries in microbial enzymes that can also degrade synthetic polymers have opened new avenues for sustainable treatment of plastic waste [15,16].

A landmark discovery in this field was made in 2016 when researchers identified Ideonella sakaiensis, a bacterium capable of using PET as its primary carbon source [17]. This organism secretes PETase, an enzyme that hydrolyzes PET into its constituent monomers, terephthalic acid (TPA) and ethylene glycol (EG), under ambient conditions [17]. This breakthrough demonstrated the feasibility of biological PET degradation and stimulated a surge of research aimed at discovering and optimizing enzymes with similar capabilities [3,12,15,16]. Since then, several classes of plastic-degrading enzymes have been identified, each with distinct substrate preferences and catalytic mechanisms [3,4,12,15,16,18]. These include various polyester hydrolases such as PETase, cutinases, and lipases. They also include polyurethane-degrading enzymes and oxidative enzymes like laccases and peroxidases. Additionally, alkane hydroxylases initiate the breakdown of long-chain hydrocarbons found in polyolefins like PE and PP [18,19].

The mechanisms by which these enzymes function vary depending on the chemical structure of the target plastic [3,18,20]. Enzymes that act on PET and PU typically hydrolyze ester or urethane bonds, while those targeting polyolefins rely on oxidative processes to introduce functional groups that destabilize the carbon backbone [15,18,20]. The effectiveness of enzymatic degradation depends on several factors. These include the crystallinity of plastic, molecular weight and the presence of additives or contaminants [20]. Environmental conditions such as temperature and pH also play a crucial role [20].

In recent years, there has been remarkable progress in improving plastic-degrading enzymes [21,22]. Advances have come through protein engineering, directed evolution and machine learning-based design [21,22]. For example, engineered PETase variants have been developed with better thermal stability and catalytic efficiency [20,21,23,24]. These improvements allow rapid depolymerization of post-consumer PET at higher temperatures [20,21,23,24]. Other studies have focused on building multi-enzyme systems or fusion proteins that replicate the natural metabolic pathways of plastic-degrading microorganisms [16,19]. Additionally, enzyme immobilization techniques, in which enzymes are fixed onto solid supports, have been employed to improve enzyme reusability and operational stability in industrial applications [20,21].

These advancements are beginning to translate into real-world applications [25]. Enzymatic recycling is emerging as a promising alternative to traditional methods [25]. Companies and research consortia are developing pilot-scale processes for enzymatic PET breakdown and repolymerization [25]. Enzyme-based solutions are also being investigated for the treatment of plastic-contaminated environments, including soil and marine ecosystems, as well as for the degradation of microplastics in wastewater treatment facilities [26]. Despite significant technical and economic challenges, enzyme-mediated plastic degradation is rapidly emerging as a critical component of the global strategy to combat plastic pollution [3].

This review provides a comprehensive overview of current knowledge on plastic-degrading enzymes. It covers the types of enzymes involved, their biochemical properties and mechanisms of action, microbial sources and the specific plastic substrates they target. Furthermore, it also highlights recent breakthroughs in enzyme discovery and engineering. In addition, it discusses practical applications and limitations, and explores future directions for research and development. By synthesizing key findings from across the literature, this work aims to shed light on the potential of enzymatic technologies to support a circular plastic economy and contribute meaningfully to environmental sustainability.

2. Plastic–Degrading Enzymes

2.1. Classification of Plastic-Degrading Enzymes

Plastic-degrading enzymes can be broadly categorized based on the types of chemical bonds they act upon and the nature of the plastic substrates they target [20]. Primarily, these enzymes fall into two main categories; hydrolytic enzymes, which cleave hydrolysable bonds like esters or amides, and oxidative enzymes, which degrade inert carbon-carbon backbones through oxidation processes [20]. (see Table 1 for an overview)

2.1.1. Hydrolytic Enzymes

Hydrolytic enzymes target plastics that contain ester or urethane bonds [15,27]. As a result, plastics like PET and certain types of polyurethanes are susceptible to enzymatic hydrolysis [20,27]. Key enzyme classes in this category include;

• Polyester Hydrolases: PETase, cutinases, and lipases cleave ester bonds found in PET and other biodegradable polyesters [20,28].
• Urethane Hydrolases, Proteases, and Ureases: These enzymes degrade PU by targeting urethane (carbamate) bonds [20,29].

2.1.2. Oxidative Enzymes

Oxidative enzymes are sourced from lignin-degrading fungi [20,30]. They initiate the degradation of plastics lacking hydrolysable bonds, such as polyolefins (PE, PP), PS, and PVC [20,30];

• Laccases and peroxidases: Catalyze radical-mediated oxidation reactions, introducing functional groups like carbonyls that facilitate subsequent microbial degradation [20,31].

2.1.3. Specialized and Auxiliary Enzymes

Some enzymes defy simple categorization but play crucial roles in specific degradation pathways:

• Styrene Monooxygenases and Oxygenases: Convert styrene monomers derived from PS into biodegradable intermediates [20,32].
• MHETase: Acts downstream of PETase by hydrolyzing mono(2-hydroxyethyl) terephthalate (MHET) intermediates into PET monomers, thereby enhancing the efficiency of PET degradation [17,20,33].

This classification and the accompanying examples provide a foundation for understanding enzyme-mediated plastic degradation mechanisms. They also help to guide the selection or engineering of enzymes for specific plastic substrates. The following sections examine the biochemical properties and catalytic mechanisms of each major enzyme class in greater detail.

2.2. Mechanisms of Plastic-Degrading Enzymes

2.2.1. PETases and Polyester Hydrolases

PETase, first identified in the bacterium I. sakaiensis in 2016, specifically hydrolyzes PET, a common polyester found in bottles and textiles [17,20]. PETase specifically cleaves ester bonds in PET to yield MHET primarily, alongside minor quantities of TPA [17,20]. The enzyme MHETase, also produced by I. sakaiensis, subsequently hydrolyzes MHET into TPA and EG [17,20]. Structurally, PETases belong to the α/β-hydrolase fold family [34]. They utilize a catalytic serine residue to perform a nucleophilic attack on ester bonds, forming an intermediate complex that is subsequently hydrolyzed into monomers [20,34]. PETases possess an open active-site cleft that facilitates binding to polymer chains [34]. Additionally, they feature specialized surface-binding loops that enhance accessibility to the substrate [34].

Figure 1. Schematic illustration of enzyme-mediated plastic degradations.

Figure 1. Schematic illustration of enzyme-mediated plastic degradations.

Cutinases from organisms such as Fusarium solani and Thermobifida share the catalytic triad mechanism (Ser-His-Asp) with PETases [20,35,36]. They possess an open and shallow active site architecture that is well suited for interacting with polymer substrates [20]. Because of this open active site architecture, cutinases exhibit broad substrate specificity toward synthetic polymers [37]. Thus, they can also depolymerize polycaprolactone (PCL), polylactic acid (PLA), polybutylene succinate (PBS) and polyester-type PU that possess ester linkages [27,38,39,40,41].

Lipases, particularly those derived from Candida antarctica, can hydrolyze aliphatic polyesters such as PCL and PLA, breaking down the ester bonds within their polymer backbones [20,42]. These enzymes also operate through a classical serine hydrolase mechanism [20,42]. They are particularly effective when acting on polymers in an amorphous state or when the substrate is exposed to elevated temperatures [20,42]. Higher temperatures enhance the mobility of polymer chains, thereby improving enzyme accessibility and catalytic efficiency [20].

2.2.2. Polyurethane Esterases

Enzymes secreted by bacterial genera such as Pseudomonas and Comamonas are commonly referred to as polyurethane esterases or polyurethanases [43,44]. These enzymes feature a dual-domain architecture, consisting of a hydrophobic surface-binding domain (SBD) and a catalytic domain [45]. Initially, the SBD facilitates enzyme adhesion to the polymer surface [45]. Subsequently, the catalytic domain hydrolyzes urethane ester linkages, releasing smaller fragments such as diols and oligomeric polyisocyanates [45].

Proteases derived from fungal species such as Aspergillus tubingensis and bacterial species like Bacillus subtilis play a key role in degrading PU [27,46]. These enzymes specifically target peptide-like urethane and urea bonds within the polymer [27,46,47]. Rather than acting alone, these proteases often work in synergy with esterases, which hydrolyze the ester linkages present in PU [20,47]. This combined enzymatic activity enhances overall degradation efficiency [20,47]. As a result, the polymer is broken down into smaller, more accessible intermediates that can be further metabolized or chemically processed [20,47].

2.2.3. Laccases, Peroxidases and Oxidative Enzymes

Oxidative enzymes, predominantly laccases and peroxidases derived from white-rot fungi such as Trametes versicolor and Phanerochaete chrysosporium [48,49]. These enzymes initiate the degradation of polymers that lack hydrolysable bonds, including polyolefins like PE and PP, as well as PS and PVC [48,49]. The enzymes catalyze radical-mediated oxidation reactions, abstracting electrons from polymer backbones to form reactive radical intermediates [27,48,49]. These radicals then introduce oxygen-containing functional groups such as hydroxyl and carbonyl groups into the polymer chains [27,48,49]. This oxidative functionalization reduces polymer chain stability and promotes subsequent biodegradation by secondary microbial enzymes or spontaneous chemical degradation [27,48,49,50]. For instance, manganese peroxidase can effectively degrade PE and facilitate PVC dechlorination by breaking carbon–carbon bonds and releasing hydrochloric acid (HCl), aiding microbial assimilation [48,49,50].

PS, characterized by aromatic side groups, presents unique challenges [51]. Oxidative enzymes such as laccases can generate radicals and initiate chain oxidation [27]. Notably, an esterase from the fungus Lantinus tigrinus directly depolymerizes PS, forming non-toxic intermediates [52]. So, the enzyme demonstrated rare enzymatic activity specifically targeting PS [52]. Additionally, laccase-mediated Fenton-like reactions have been explored to generate hydroxyl radicals that significantly enhance the oxidative degradation of PS and other recalcitrant plastics [53].

2.2.4. Other Notable Enzymes

In addition to the major groups above, there are specialized enzymes and pathways worth noting. Nylon-degrading enzymes such as nylonases (e.g., 6-aminohexanoate-dimer hydrolase) evolved in certain Flavobacterium and Pseudomonas strains to break down nylon oligomers – a famous case of microbes adapting to a synthetic polymer waste [54]. While nylon oligomers are not high-molecular-weight plastics, this example illustrates how enzyme evolution can create entirely new activities toward man-made polymer [54].

Another example is the metabolism of styrene (the monomer of PS) by bacteria like Pseudomonas putida [32,55]. P. putida can’t directly depolymerize PS, but it can consume styrene after pyrolysis or partial degradation [55,56]. It uses a styrene monooxygenase and downstream enzymes to convert styrene into valuable products like polyhydroxyalkanoate (PHA) or other intermediates [57]. This approach, combining partial thermal degradation with microbial enzymatic conversion, is a form of hybrid degradation for plastics like PS [57].

Alkane hydroxylases deserve mention for polyolefins. These bacterial enzymes normally oxidize long-chain alkanes (like those in petroleum); some studies suggest they can slowly oxidize PE chains, since PE is essentially a very long alkane [58,59,60,61]. Strains of Rhodococcus and Pseudomonas have been found to induce alkane hydroxylase and other oxidases when grown on PE films, leading to formation of carbonyl groups on PE [58,59,61]. Although the activity is low, this indicates that common hydrocarbon-degrading enzymes can play a role in plastic degradation if the organism can access the polymer surface [58,59,61]. Often, physical or chemical pretreatment, such as ultraviolet (UV) exposure to create initial cracks and oxidized bonds in PE, is needed to enable such enzymes to attack the polymer [4,62,63].

Finally, recent research has uncovered insect-derived enzymes capable of plastic depolymerization [64]. In 2022, scientists found that the saliva of waxworms (Galleria mellonella larvae) contains two phenol oxidase enzymes that rapidly oxidize and depolymerize PE at room temperature [64]. These enzymes are believed to directly catalyze the oxidation of PE backbone [64,65]. So, they can bypass the typical requirement for high-temperature or UV pretreatment [64]. This reaction breaks the long polymer chains into glycolic acid and other short-chain products within hours [64]. This discovery represents the first known case of enzymes that can efficiently degrade PE under mild conditions [64]. It also introduces a new category of plastic-degrading enzymes derived from insect biochemistry, distinct from those previously identified in microbial systems [64].

In summary, plastic-degrading enzymes utilize a range of catalytic mechanisms, from hydrolysis of ester and urethane bonds to radical-mediated oxidation of carbon–carbon backbones. These diverse biochemical strategies enable the breakdown of both hydrolysable and non-hydrolysable plastics. Hydrolytic enzymes, such as PETases and cutinases, effectively depolymerize polyesters like PET, while oxidative enzymes like laccases and peroxidases initiate the degradation of more inert polymers such as PE, PP and PS. The versatility of these enzymatic approaches highlights their promise as environmentally friendly tools for addressing plastic waste.

3. Microbial Sources of Plastic-Degrading Enzymes

A diverse array of microorganisms, including bacteria, fungi, and symbiotic communities, produce enzymes capable of degrading plastics. This section details some key microbial sources and highlights their abilities to target various plastic types (see Table 2 for an overview).

3.1. Bacteria

Several bacteria isolated from environments rich in plastic waste, such as soil, compost, and marine ecosystems, have demonstrated the ability to degrade plastics. One of the most notable examples is I. sakaiensis [17]. It can degrade low-crystallinity PET in near completion by secreting PETase and MHETase [17]. Many thermophilic actinomycete bacteria, including Thermobifida fusca and Thermomonospora species, secrete cutinases and esterases that can break ester bonds in PET and PU [37,67].

Pseudomonas is a genus particularly rich in degradation capabilities: different Pseudomonas species can utilize PU, PS, and even partially oxidized PE as carbon sources [78]. For instance, P. chlororaphis and P. fluorescens produce extracellular polyurethane esterases (PueA, PueB) that break down polyester-PU [78,79]. Additionally, newly discovered strains like Pseudomonas sp. TDA1 have demonstrated the capability to utilize PU foam as their sole carbon source [43]. Another species, P. putida is known for metabolizing styrene (from PS) and even converting PET-derived terephthalate into valuable bioproducts [80,81,82].

Other notable bacterial genera include Bacillus, known for degrading PU and pre-oxidized PE, and Rhodococcus, which has demonstrated slow mineralization of PS and PE films over extended incubation periods [61,82,83,84,85]. In marine settings, Alcanivorax and Kocuria species have been reported to colonize and degrade floating plastic debris, likely relying on their alkane-degrading enzymes to attack PE [86,87,88].

Overall, bacterial plastic degraders are often found in environments loaded with plastic waste, suggesting adaptive evolution of their enzymatic repertoire.

3.2. Fungi

Fungi, especially saprophytic molds and lignin-degrading fungi, are prolific producers of relevant to plastic biodegradation (laccases, peroxidases, cutinases, etc.) [18]. Ascomycete fungi such as Aspergillus, Penicillium, Fusarium, and Cladosporium have been repeatedly isolated from plastic-contaminated sites and shown to cause degradation of plastics [27]. Aspergillus niger and Penicillium simplicissimum, for example, have demonstrated the ability to colonize and embrittle PE films after pretreatment, likely through secretion of oxidative enzymes and cutinase-like esterases [89]. F. solani produces the classic cutinase enzyme and can depolymerize PET and PU coatings [20]. Basidiomycete fungi, known for their lignin-degrading systems, also play a major role [27]. White-rot fungi like Trametes versicolor, Phanerochaete chrysosporium, and Pleurotus ostreatus are frequently cited for their ability to degrade recalcitrant plastics such as PE and PVC by virtue of their laccases and peroxidases [27]. For PS, fungi have been less studied, but Lantinus species have shown some PS breakdown via esterase activity [90]. Fungal enzymes often have high stability and are secreted in large amounts, making fungi attractive for enzyme production in plastic waste treatment. Notably, fungi can sometimes directly penetrate plastics with their hyphae, aiding in mechanical disintegration in combination with enzymatic action [91].

3.3. Symbiotic and Other Sources

Symbiotic microbial communities within insect guts present another exciting avenue for plastic biodegradation. The gut microbes of waxworms and mealworms enable these larvae to digest plastics like PE and PS, respectively. Waxworms (G. mellonella larvae) secrete enzymes in their saliva and gut capable of oxidizing PE effectively under mild conditions [64]. In mealworms (Tenebrio molitor larvae), gut bacteria including Exiguobacterium and Yokenella have been implicated in degrading PS foam, as the worms can subsist on a PS diet and excrete degraded fragments [92,93,94].

Additionally, ruminants (like cows) have also been suggested as a source: recent studies found that the microflora in cow rumen fluid can degrade PET films, possibly due to a mix of cutinases or lipases present from their plant-based diet digestion [95,96].

Compost and soil consortia (mixed communities) sometimes outperform pure cultures, as different organisms can attack different components of a plastic simultaneously [97]. For example, a mixed fungal consortium of Candida (yeast) and Trichoderma was shown to more effectively break down UV-treated PE than either alone, likely combining Candida’s oxidative metabolism with Trichoderma’s enzymatic arsenal [97]. Metagenomic surveys have indeed found that environments chronically exposed to plastic (e.g. landfill soils) harbor a higher diversity of plastic-degrading enzymes, indicating microbial communities collectively adapt to exploit plastic as a nutrient source [98].

In summary, microbial communities from diverse environments, including soil, marine ecosystems, insect guts and ruminant digestive tracts, serve as both a genetic reservoir of potential plastic-degrading enzymes and active agents for plastic biodegradation. These microbes not only secrete effective depolymerizing enzymes but also metabolize the resulting breakdown products, demonstrating their utility as both tools and templates for sustainable plastic waste treatment.

4. Recent Advances in Plastic-Degrading Enzymes

Over the past decade, research into plastic-degrading enzymes has significantly accelerated, driven by technological advancements in genomics, protein engineering, and synthetic biology [20,24,99]. These efforts have resulted in the discovery of novel enzymes, enhanced catalytic properties of existing enzymes, and innovative deployment strategies for scalable plastic biodegradation [100,101] (see Table 3 for an overview).

4.1. Discovery of New Enzymes and Pathways

Scientists are leveraging genomics and environmental screening to find novel plastic-degrading enzymes [98,102]. A 2021 global study scanned over 200 million DNA sequences from environmental samples and identified 30,000 enzyme homologs with the potential to degrade 10 different plastics [102,103]. The presence and abundance of these enzymes were found to correlate with local plastic pollution levels, suggesting an evolutionary response by microbial communities to exploit plastics as carbon sources [103]. Notably, many of the identified sequences represented entirely new proteins unrelated to known hydrolases, presenting a rich reservoir for future enzyme characterization and development [103].

Among specific discoveries, the phenol oxidases found in the saliva of waxworms (G. mellonella) in 2022 marked a breakthrough as the first enzymes capable of rapidly depolymerizing PE without the need for pretreatment [64]. Similarly, in 2020, researchers isolated a Pseudomonas strain from a waste site that can degrade and even grow on PU, utilizing it as a sole carbon source [104]. They identified enzymes from the strain that break PU into absorbable metabolites [104]. Each new enzyme expands the range of plastics that might be biologically tackled, such as recently discovered PET hydrolases in leaf compost, marine environments, and even in the cow rumen microbiome [95,105].

4.2. Protein Engineering for Enhanced Enzyme Performance

Perhaps the most dramatic progress has come from engineering enzymes like PETases to be faster and more robust. In 2018, scientists solved the crystal structure of I. sakaiensis PETase and noted it was similar to cutinases, providing clues for improvement [106]. By 2020, a team engineered the “double mutant” PETase (inspired by cutinase active-site features), which showed a substantial increase in PET degradation rate [107]. Around the same time, French company Carbios and academic collaborators evolved an enhanced cutinase (from leaf compost bacteria) that achieved 90% depolymerization of PET in just 10 hours [66,101]. This enzyme was 10,000 times more efficient than the wild-type at breaking PET bonds, enabling the complete recycling of PET bottles into new plastic of equal quality [66,101]. Following this, in 2020 researchers at University of Portsmouth linked PETase and MHETase together, creating a “super-enzyme” that could digest PET roughly 6 times faster than PETase alone [108]. Although too large for expression by a single microbe, this chimeric enzyme demonstrated the potential of multi-enzyme systems to accelerate depolymerization [108].

The latest breakthrough came in 2022 with the development of FAST-PETase (Functional, Active, Stable, and Tolerant PETase) [109]. Using machine-learning algorithms to guide mutations, researchers introduced 5 mutations into PETase, improving its thermostability and activity across various pH ranges [109]. FAST-PETase operates effectively at temperatures up to 50 °C (near the glass transition of PET) and remains active in various conditions [109]. Impressively, FAST-PETase degraded post-consumer PET products from 51 different sources within one week [109]. The enzyme could even digest the amorphous portions of a commercial water bottle without any preprocessing [109]. This robustness allowed the team to demonstrate a closed-loop PET recycling [109]. PET trash was enzymatically depolymerized to monomers, purified, and re-polymerized into new PET plastic, completing a full recycle with the help of the engineered enzyme [109]. FAST-PETase represents a significant step toward practical enzymatic recycling at scale, and its design showcases the power of AI-driven protein engineering [110].

Beyond PET, efforts are underway to engineer enzymes for other plastics. For instance, protein engineering of leaf-branch compost cutinase (LCC) has yielded variants capable of degrading both PET and polyethylene furanoate (PEF), a bio-based polyester [111]. Additionally, studies have demonstrated that cutinases can hydrolyze polyamide 6,6 (PA 6,6), indicating potential for broader application in polyamide degradation [112]. These advancements highlight the versatility of engineered cutinases in addressing various plastic waste challenges.​

Enzyme engineering has also targeted thermostability, since plastic depolymerization is often more efficient at higher temperatures where the polymer is softer [100]. Strategies like introducing disulfide bonds or stabilizing mutations have extended enzyme working ranges to 70–75 °C (as with some engineered cutinases), which greatly accelerates PET degradation due to increased chain mobility [66]. Similarly, directed evolution of Aspergillus laccase has produced variants that are more tolerant of the harsh conditions needed to oxidize PE surfaces [113].

4.3. Enzyme Immobilization and Reactor Systems

Another advancement is in how enzymes are deployed. Immobilization of PETases and lipases on solid supports or within biofilms improves enzyme reusability, operational stability, and resistance to denaturation [114]. For instance, immobilizing PETase onto nanostructured cobalt phosphate allowed for high enzyme loading with minimal mass transfer limitations, leading to improved stability and reusability [114]. Similarly, lipases immobilized on magnetic nanoparticles via polydopamine coating exhibited enhanced pH and thermal stability compared to their free counterparts [115]. These immobilized systems are employed in packed-bed and fluidized-bed reactors designed to continuously process plastic waste [116]. There have been pilot demonstrations of enzymatic recycling: for instance, Carbios announced a demonstration plant where tons of PET waste are enzymatically converted to monomers for repolymerization [101]. Immobilization also confers increased thermostability, allowing enzymes to function at higher temperatures and degrade more crystalline plastics [117]. This is particularly important for recycling commercial-grade plastic products that exhibit greater resistance to enzymatic attack [16].

4.4. Metabolic Engineering and Synthetic Biology

Instead of using isolated enzymes, some efforts focus on engineering whole-cell systems due to recent advancements in synthetic biology and metabolic engineering [118]. For example, E. coli or Pseudomonas have been genetically modified to secrete PETases and take up the monomers, effectively creating an “engineered microbial recycler” [119]. One study engineered P. putida to consume terephthalic acid (from PET) and funnel it into the production of biopolymer PHA, thus coupling plastic degradation with bio-upcycling [120].

More advanced strategies involve constructing synthetic consortia wherein one microbial species secretes oxidative enzymes to initiate plastic degradation, while another species carries out depolymerization and downstream bioconversion [121]. Such systems represent a step toward integrated biorefineries for plastic waste, combining biodegradation, resource recovery, and sustainable chemical production in a single platform [121].

5. Applications in Recycling, Bioremediation and Industry

The practical deployment of plastic-degrading enzymes is a rapidly evolving field, with potential applications across recycling, environmental cleanup, and industrial operations [122]. These strategies aim to address the limitations of traditional plastic waste management by enabling selective, sustainable, and scalable alternatives [122].

Figure 2. Schimatic Illustration about Applications of Plastic-Degrading Enzymes.

Figure 2. Schimatic Illustration about Applications of Plastic-Degrading Enzymes.

5.1. Enzymatic Recycling

Enzymatic recycling offers a pathway to circular plastic economy by depolymerizing plastic wastes into their original monomers, which can then be purified and repolymerized into new plastics [123]. PET serves as the leading example. Companies such as Carbios have developed industrial-scale enzymatic recycling plants that utilize PETases and cutinases to break down PET from post-consumer waste—such as bottles and textiles—into TPA and EG [25]. These monomers can then be repolymerized into virgin-grade PET without loss in material quality [25]. Unlike mechanical recycling, which degrades polymer quality, enzymatic recycling yields monomers of equal purity to petrochemical sources [25]. The closed-loop PET recycling demonstrated with FAST-PETase exemplifies this approach [124]. Enzymatic recycling is also being explored for PU, converting them into polyols and other components that can serve as raw material for new polymer synthesis [125].

Addressing mixed plastic waste remains a challenge [100]. One solution involves designing enzyme cocktails tailored to degrade different plastic types either sequentially or simultaneously [100,126]. Another strategy is upcycling, where enzymes convert plastic into chemical intermediates that can be transformed into value-added products [126]. For example, Pseudomonas species can metabolize PET monomers into β-ketoadipate, a precursor for high-performance polymers and biodegradable plastics [32]. These strategies enhance the economic appeal of biocatalytic plastic processing, as well as promote a plastic circular economy.

5.2. Bioremediation and Environmental Cleanup

Using microbes or enzymes to degrade plastics in situ (in the environment) is a more challenging application but one with great appeal [11,46]. Laboratory studies have shown that adding plastic-degrading fungi or bacteria to soil can accelerate the degradation of plastic films, especially if combined with pretreatments like plowing the soil with oxidative chemicals [127]. For marine bioremediation, researchers are investigating bioaugmentation with marine bacteria that have natural plastic-degrading abilities, or deploying enzyme-coated scaffolds that could capture microplastics and degrade them [12]. For example, enzyme-producing bacterial consortia have been tested in composting facilities to degrade compostable plastics like PLA and polybutylene succinate faster than normal [128]. However, for non-compostable plastics like PE and PP, direct bioremediation is slow without pretreatment [129].

Innovative approaches include the use of insects as biodegradation vectors. Waxworms can ingest PE, initiating oxidation and partial breakdown through enzymes in their saliva and digestive tracts [64]. Mealworms, on the other hand, can consume PS foam, aided by gut microbiota such as Exiguobacterium and Yokenella [92,93,94]. While promising, these biotic strategies require careful ecological evaluation before field application.

5.3. Industrial and Commercial Uses

Enzymes are increasingly considered for integration into industrial workflows to assist with plastic processing and waste management. One concept under development involves embedding enzymes or their precursors into plastic materials, creating smart plastics that degrade upon exposure to specific environmental triggers (e.g., heat, moisture, or pH changes) [130,131]. For instance, one study describes the development of a PLA-based plastic with embedded, engineered PLA hydrolase that fully disintegrates under home-compost conditions within 20-24 weeks [130]. These enzyme-activated systems could allow for post-use degradation without compromising product durability during use [131].

Enzymes are also being investigated for use in textile and packaging industries. Cutinases, for example, could be employed to clean PET-based industrial waste (e.g., textile off-cuts or film trimmings) by depolymerizing them directly on-site [132]. In wastewater treatment facilities, enzymatic degradation of microplastic particles using membrane-immobilized enzymes offers a promising avenue for reducing microplastic load in effluents [133]. Nonetheless, this kind of technology is currently underappreciated due to a lack of information and proof, necessitating extensive large-scale study for adoption [133].

While many of these applications remain at the research or pilot stage, they illustrate the potential of enzymatic technologies to provide flexible, efficient, and environmentally friendly solutions across multiple sectors affected by plastic pollution.

6. Conclusion

The ongoing plastic pollution crisis demands transformative solutions, and enzyme-based plastic degradation offers a compelling avenue grounded in nature’s own catalytic machinery. Through years of research, enzymes such as PETase, cutinase, and laccase have emerged as potent tools for targeting a wide array of synthetic polymers. Advances in genomics, metagenomics, and protein engineering have vastly expanded the known repertoire of plastic-degrading enzymes and enhanced their catalytic capabilities under industrially relevant conditions.

Applications in enzymatic recycling, environmental bioremediation, and industrial plastic management are rapidly transitioning from conceptual frameworks to tangible solutions. From the closed-loop recycling of PET using engineered enzymes like FAST-PETase to the potential deployment of enzyme-functionalized filters in wastewater systems, these biotechnologies are redefining how plastic waste can be handled at the molecular level.

However, significant challenges persist. Current enzymes exhibit limited activity against highly crystalline or chemically inert plastics, leading to the need for pretreatment, which could be another limit as cost of process and complexity. Also the stability of enzymes and cost-effective production of them remains a bottleneck. Mixed plastic waste streams further complicate processing, demanding multifaceted enzymatic systems or innovative reactor designs. Moreover, concerns around incomplete degradation and the environmental deployment of engineered biological agents must be addressed through careful risk assessment and policy development.

Despite these limitations, the field continues to progress at an accelerated pace. Indeed, the number of landmark studies in this field has been growing explosively since 2020. Emerging directions include the development of synthetic microbial consortia, chemo-enzymatic hybrid strategies, and the creation of enzyme-sensitive materials designed with end-of-life degradation in mind. Integrating enzymatic solutions with broader systems of circular economy, sustainable manufacturing and environmental stewardship holds immense promise.

In conclusion, the convergence of enzyme discovery, bioengineering, and industrial application is shaping a new frontier in sustainable plastic management. Continued interdisciplinary collaboration and innovation will be essential to scale these technologies and expand their impact. As the field matures, enzymatic plastic degradation could become a cornerstone of global strategies to mitigate plastic pollution and foster a more sustainable material future.