Enzymatic Degradation of Polyethylene and Polyethylene Terephthalate: A Mini Review

ABSTRACT


INTRODUCTION
Plastics are artificial materials made from synthetic organic polymers, namely fossil hydrocarbon derivatives (Khairul Anuar et al., 2022).Today, most plastic production is manufactured from non-renewable petrochemicals derived from fossil fuels, natural gas, and coal.Plastics have an important role in all modern societies due to attributes such as durability, resistance, transparency, lightweight, low price, high stability, and compact structural characteristics (Ahmaditabatabaei et al., 2021;Siracusa et al., 2020).Plastics are a vital entity for many domestic and industrial sectors, including electronics, construction, transportation, health care, agriculture, and packaging materials (food and other industries), and moreover, they account for 70% of the market for consumer products (Benyathiar et al., 2022;Bobori et al., 2022;Siracusa et al., 2020).Because of increasing demand and use in many sectors and their commercial importance, the annual production of plastics is increasing, and it is expected to double by 2035 (about 800 Mt) and reach around 1600 Mt by 2050.However, plastics are one of the most vital materials in the modern world (Zhang et al., 2022), but plastic waste is one of the most serious problems in modern society (Zichittella et al., 2022).The increased and strong demand for plastic production has a negative impact on the environment because of the degradation problems in the environment.Only 9% of the generated plastic waste from the 6300 million tons produced between 1950 and 2015 was recycled (Kawai et al., 2022).Annually, global plastic waste was generated at approximately 141 million tons in 2015 (Benyathiar et al., 2022), about 300 million tons in 2020 (Ahmaditabatabaei et al., 2021), and it is expected to triple by 2060 (Kim et al., 2021).Daily plastic waste generation per person is different according to the countries (e.g., Germany with 0.48, the United States with 0.34, the United Kingdom with 0.21, France with 0.19, Italy with 0.13, China with 0.12, Belgium with 0.08, and India with 0.01 Kg/person/day in 2010) (Montanari, 2020).Despite the advantages of plastic use, plastic pollution is one of the most important environmental issues in the world (Kim et al., 2020).Annually, between 4.8 and 12.7 million tons of plastic waste are dispersed in the ocean (Lionetto et al., 2021).Polyethylene (PE) and Polyethylene Terephthalate (PETE/PET) are petroleum-based polymers that are widely used in many applications (films, packaging bottles, manufacture, medicine, textiles, engineering, etc.).These wastes do not degrade easily when they are released into the earth's environment.Although there are several degradation strategies, such as mechanical, chemical, and biological strategies, to reduce the accumulation of plastic waste, Because of the low energy consumption, no need for hazardous chemical materials and expensive machinery, and mild process, biological degradation is a more viable environmental degradation strategy than mechanical and chemical methods (Budhiraja et al., 2022).In this method, microorganisms are the main agents, and they are able to produce some enzymes that degrade plastics and plastic waste in normal conditions.Several enzymes have been isolated from the group of microorganisms, including Bacteria, fungi, and Algae, that break down the plastic polymers into monomers.These microorganisms are able to degrade plastic in the various ecosystems (soil, sea water, compost, and activated sludge) through the production of enzymes https://journals.e-palli.com/home/index.php/ajecAm.J. Environ.Clim.2(3) 41-50, 2023 (Mohanan et al., 2020).These enzymes (e.g., laccase, manganase, peroxidase, alkane hydroxylase, lipase, and cutinase) are able to degrade PE, while cutinase, lipase, carboxylesterase, esterase, PETase, IsPETase, and polyesterases are capable of degrading PET.The production of enzymes by microorganisms has been reported in many studies; this study categorized enzymes and microorganisms.This study was conducted as a review of the literature.We used the scientific web, such as PubMed and another source, and focused on enzymatic degradation topic searches.

MATERIALS AND METHODS
We conducted a literature survey by using the keywords "Polyethylene or PE" and " Polyethylene Terephthalate or PET " "enzymatic degradation of plastics or PE or PET", and "biodegradation of PE or PET" in the web of PubMed and Elsevier.The data were collected from published literature in which the biofragmentation or biodegradation of PE and PET is indicated.The following topics were searched to select and find the literature: Topic search = (Polyethylene or PE" and "Polyethylene Terephthalate or PET " "enzymatic degradation of plastics" or "PE", or "PET", and "biodegradation of plastics" or "PE" or "PET".This search was last updated on August 4, 2023, and returned hundreds of records.Then, we applied the filtering option based on text availability and publication date.The free full text and five years were selected in the filtering phase.Based on the topic search, more than 175 articles were found.These articles were sent to EndNote for more processing.Moreover, TS were done on another web site as well.The following criteria were used for the selection of literature: At least one of the TS should be discussed; The characteristics of enzymes that can degrade polymers in plastics should be reported; The microorganisms that can release enzymes to degrade PE and PET should be reported; The Production of the specific enzyme by the microbes should be identified; Enzymatic degradation should be reported under biological degradation.We applied the above strategies and selected only 26 articles for full review; the summary of our procedure is shown in Figure 1.

RESULTS AND DISCUSSION
Types of Plastics Frequently Used (PE and PET) Some of the most commonly used plastics for varied purposes are described below, along with their main uses in different industries.Table 1 shows the types and amounts of commonly used plastics.

Polyethylene (PE)
Polyethylene (PE) is one of the most common types of plastics and one of the commercial materials that is widely used in applications like films and bottles for packaging applications (Czarnecka-Komorowska et al., 2021).PE is the main polymer of the Polyolefins group, with more than 100 million tons of production per year, which is 34% of the total plastics market (Table 2).This is extensively used due to its good mechanical and chemical resistance, low density, cost-effectiveness, easy of processing, low toxicity, and good electric insulation (Rezvani et al., 2020).It can also be transformed into different shapes (Kaushal et al., 2021).It is a polymer of ethylene monomers and belongs to the class of thermoplastics.Every year, these plastics are produced in millions of tons and are used in many different areas and industries (Kaushal et al., 2021).
Polyethylene is classified mainly into three types: lowdensity PE (LDPE), with a density ranging between 0.910 and 0.940 g/cm3, linear low-density PE (LLDPE), with a density ranging between 0.910 and 0.920 g/cm 3 , and high-density PE, with a density ranging between 0.941 and 0.967 g/cm 3 (Table 2) (Ugbolue, 2017).Polyethylene Terephthalate (PETE/PET) Polyethylene Terephthalate is one of the major synthetic petro-and thermoplastics that is used worldwide in many applications.It is produced in large amounts globally, amounting to 82 million tons per year (Kawai et al., 2022) (Table 2).It is most commonly used in the manufacturing of single-use disposable drink bottles and in many different applications (Kaushal et al., 2021;Montanari, 2020).Due to PET's excellent mechanical properties and biocompatibility, the demand for PET-based products (textile, food, packaging, medicine, and engineering) is very high in very large amounts globally (Damayanti et al., 2021;Flores-Rojas et al., 2022;Panowicz et al., 2021).In 2015, the total production of PET packaging was estimated at 18.8 million tons (Damayanti et al., 2021).It is the most common thermoplastic polymer belonging to the polyester family and containing the alternating ethylene glycolate and terephthalate subunits, linked via the ester functional group in their main chain (Kaushal et al., 2021;Montanari, 2020;Shamas et al., 2020).PET is widely used in the production of fibers for clothing, the textile industry, the production of single-use bottles for liquids and food packaging products, the packaging industry, the manufacturing of thermoforming methods, and in combination with glass fibers for engineering resins ( Kaushal et al., 2021;Montanari, 2020).More than 60% of the PET production is used for synthetic fibers and 30% for bottle production (Montanari, 2020).It is one of the main reasons for the pollution of plastics caused by the one-time use of plastic products (Kaushal et al., 2021).(Kaushal et al., 2021).Several enzymes have an important role in the plastic degradation produced by a group of microorganisms that are both prokaryotic and eukaryotic.According to the release of enzymes by the microbial cells, there are two kinds of enzymes, extracellular and intracellular, which breakdown polymer chains and release new products like CO 2 , H 2 O, CH 4 , and N 2 (Kaushal et al., 2021).Due to low energy consumption, no need for hazardous chemical materials and expensive machinery, and mild process conditions, the biodegradation method is a good option for plastic degradation compared with mechanical and chemical degradation methods (Soong et al., 2022).The degradation processes of PE and PET are done by Bacteria, Fungi, algae, and other microorganisms under biological processes in various ecosystems, including oceans, soil, farmland, animal manure, compost, landfills, sewage, etc. (Rezvani et al., 2020;Zhang et al., 2023).Biodegradation occurs in two types: oxido-biodegradation and Hydrobiodegradation.The first one consists of two stages: abiotic oxidation and biotic degradation.In abiotic oxidation, the carbon backbone of Plastics is oxidized into small parts by thermal and UV radiation.In biotic degradation, microbes colonize the surface of polymers and cause change by releasing some enzymes (Zeenat et al., 2021).The degradation process can be done while microorganisms exist according to the following conditions: enzymes should be released, the released enzyme should adhere to the surface of the polymers, enzymes can cleave polymer chains, and polymers should be changed into end products such as CO 2 , H 2 O, CH 4 , and N 2 (Figure 2).(Shamas et al., 2020;Zeenat et al., 2021;Soong et al., 2022).

Enzymatic Degradation of PE
The enzymes secreted by microorganisms can reduce the number of carbonyl groups, break them into carboxylic acids, and hydrolyze the polymer carbon chains into fragments, or bio-fragmentation, including long-chain aliphatic compounds such as alkanes and alkenes (Zhang et al., 2022).Alkane compounds such as petroleum derivations are hydrolyzed by a key enzyme, namely Alkane hydroxylase (AHs) enzymes (Zhang et al., 2023).Several enzymes (e.g., laccase, manganese peroxidase, and alkane hydroxylase) are released by microorganisms and act on the linear chain of carbons, which are joined together by hydrogen bonds and compound PE structures.This structure is usually semi-crystalline and resistant to biodegradation.PE polymers have different densities and physical structures due to different manufacturing processes and the arrangement of the linear chains (Monahan et al. 2020).Cutinase, lipase, PETase, and esterase are the most common enzymes for plastic degradation.These enzymes are generated by several microorganisms that can degrade PE and PET polymers in a similar manner (Kaushal et al., 2021) in the soil, sea water, compost, and activated sludge (Monahan et al., 2020).Several enzymes capable of degrading PE are oxidoreductases.These are the main enzymes that degrade plastic materials in the environment (Kaushal et al., 2021).Some well-known enzymes, such as Laccase, manganese peroxidase, alkane hydroxylase, and soybean peroxidase, oxidize polyethylene (PE) (Kaushal et al., 2021;Zhang et al., 2022).The terminal carbon in PE can be oxidized by most PE-degrading enzymes.For example, Laccase and manganese peroxidase, by hydroxylation reaction through either terminal or subterminal, can do terminal oxidation in the main component of polyethylene, and Alk B can degrade n-alkanes (Zhang et al., 2022).

Laccases
Laccases, a group of enzymes belonging to the blue copper oxidases, are monomeric glycoproteins, and they use oxygen as an electron acceptor to oxidize compounds, phenolic and non-phenolic (Temporiti et al., 2022).These enzymes were first identified in the plant species Rhus vernicifera in 1883, after many species of fungi belonging to Ascomycetes, Basidiomycetes, and Deuteromycetes were discovered.Laccases have the ability to oxidize a wide range of phenols, and they are produced by fungi and bacteria.in the higher plants have been reported as glycoproteins (Zhang et al., 2022).The LDPE was degraded by laccase isolated from R. ruber C208 strains in experimental conditions for 30 days of incubation at 30 °C with 1.5-2.5% weight loss (Zhang et al., 2022).The bacteria, like Klebsiella pneumoniae, are gram-negative and release lipase, peroxidase, and laccase that play important roles in PE degradation (Zeenat et al., 2021).Because these enzymes have roles in lignin degradation, the synthesis of dihydroxy naphthalene melanin, and the removal of toxic phenols produced, some writers appreciated their properties as protection against environmental stress.Temporiti et al. (2022) wrote "Thanks to their characteristics, laccases are used in a number of industrial applications such as delignification, pulp bleaching, and bioremediation processes removing toxic compounds through oxidative enzymatic coupling".

Alkane
Alkane hydroxylases (AHs) are key enzymes in the degradation of alkane compounds such as petroleum derivations (Zhang et al., 2023).The number and types of Alkane hydroxylases vary greatly in different microorganisms, like bacteria (Rezvani et al., 2020).Alkane has a similar structure to that of PE.These are mono-oxygenase enzymes that are candidates for PE degradation.AlkB is one alkane hydroxylase for PE degradation, and it was first isolated in alkane-consuming Pseudomonas species (Zhang et al., 2022).

Manganase
Manganase was identified as a lignin-degrading enzyme in the fungus Phanerochaete chrysosporium, and it enhanced the degradation of PE (Zhang et al., 2022).In experimental conditions for 12 days of incubation at 37 °C, it decreased the Mw (Zhang et al., 2022).

Peroxidases
Peroxidases are a class of enzymes that catalyze the oxidation of organic and inorganic compounds and the reduction of hydrogen peroxide (Temporiti et al., 2022).Manganese peroxidases, lignin peroxidases, versatile peroxidases, and dye decolorizing peroxidases are the main extracellular fungal peroxidases (Temporiti et al., 2022).A group of microorganisms, including Phanerochaete chrysosporium, Trametes versicolor, Pleurotus spp., Phlebia radiata, Bjerkandera adusta, Ceriporiopsis subvermispora, Dichomitus squalens, and gram-negative bacteria like Klebsiella pneumoniae, produce peroxidases (Temporiti et al., 2022;Zeenat et al., 2021).They have a high redox potential to oxidize substrates, so they are used in a large number of applications.There are some other enzymes that degrade both PE and PET, including cutinases, lipases, and esterases, discussed in the next section under enzymatic degradation of PET.

Cutinases
Cutinases are extracellular enzymes that are able to degrade plastics and catalyze the breaking of ester bonds (Khairul Anuar et al., 2022).These enzymes are produced by groups of microorganisms like fungi and bacteria.These microorganisms are either saprophytic or phytopathogenic (Maurya et al., 2020).Cutin, an insoluble aliphatic polyester excreted from the plant cuticle, is hydrolyzed by cutinases (Soong et al., 2022).Unnatural substrates that are synthetic polyesters, such as PET, are degraded by cutinases as well (Khairul Anuar et al., 2022).Various polyesters are hydrolyzed by cutinases at temperatures between 40 and 70 °C and pH 7-9 (Soong et al., 2022).Cutinases belong to the hydrolase superfamily, and they have an α/β fold and a central sheet composed of five parallel strands covered by two or three helices on either side of the sheet (Maurya et al., 2020;Temporiti et al., 2022).High-molecular-weight compounds such as cutin and other related synthetic compounds are accommodated by the active site of cutinase (Maurya et al., 2020).Cutinases are multifunctional enzymes that are used in the textile, detergent, and food industries as industrial biocatalysts due to their ability to catalyze hydrolysis reactions with many industrial applications for polymer and fiber degradation (Khairul Anuar et al., 2022;Temporiti et al., 2022).Cutinase enzyme that can degrade PET structure was generated by thermophilic actinomycetes bacteria (e.g., Thermobifida fusca, T. alba, T. cellulosilytica, T. curvata, and Sacchromonspora viridis) and fungi (e.g., Humicola insolens, Fusarium solani pisi, Fusarium oxysporum, Penicillium citrinium citrinium, Saccharospora viridis, Aspergillus fumigatus, and Aspergillus nidulans) (Khairul Anuar et al., 2022;Temporiti et al., 2022;Soong et al., 2022).

Carboxylesterase
In both prokaryotic and eukaryotic microorganisms, Carboxylesterase has been reported.Because of its open active site and distinctive binding pocket, this enzyme has broad substrate specificity (Khairul Anuar et al., 2022).Carboxylesterase is released by thermophilic T. fusca belonging to the actinomycete, and it is able to hydrolyze PET fibers at 50 °C and pH 8.0.This enzyme was identified from Pseudomonas aestusnigri's genome, a mesophilic marine bacterium (Khairul Anuar et al., 2022).

Esterase
Esterase is present in nearly all living organisms, and it can cleave ester bonds in PET monomers that are linked by ester bonds (Maurya et al., 2020;Khairul Anuar et al., 2022).Bacillus and Nocardia were the first microorganisms to discover PET esterase.P-nitrobenzylesterase hydrolyzes PET into ATP and mono(2-hydroxyethyl) (MHET) TPA using bis (benzoyloxyethyl) terephthalate in optimum conditions at 37 •C and pH 7.0 (Khairul Anuar et al., 2022).Kawai et al. (2014) reported polyesterase capable of hydrolyzing PET in the presence of Ca ions from Saccharomonospora viridis.Esterase from Thermobifida halotolerans was reported by Ribitsch et al. (2012) to degrade PET into TA and MHET as well (Maurya et al., 2020).

IsPETase
IsPETase belongs to the α/β hydrolase superfamily and was isolated from a plastic bottle recycling factory in Japan.This enzyme is isolated from a mesophilic bacterium, Ideonella sakaiensis, and is active at low temperatures (20 °C-40 °C).This is a unique characteristic of this enzyme for PET degradation.This is because this enzyme has a broader open active site that increases the enzyme's specificity for bulkier substrates like PET (Khairul Anuar et al., 2022;Blázquez-Sánchez et al., 2022).

CONCLUSIONS
PE and PET are the most important plastics used around the globe.Production and use of these synthetic materials are increasing, as is waste generation.The accumulation of plastic waste in landfills, seas, and oceans is an extremely challenging environment, as some reports indicate that 400 metric tons of plastic waste are generated and 174 metric tons enter natural systems.Many studies have been conducted to present waste management and recycling methods, but traditional methods of recycling PE and PET waste are still hazardous and unfriendly to the environment due to their effects on ecosystems.Currently, biodegradation is a good strategy that has some advantages, such as decreasing the use of chemically hazardous elements, reducing the use of expansive machinery, supporting environmental principles, etc.The enzymes that are able to degrade PE and PET are produced by a group of microorganisms like bacteria, fungi, algae, and even insects.This method could be an effective strategy toward a green recycling scheme for plastic waste.Under a biological process, enzymes such as cutinase, lipase, PETase, IsPETase, esterase, and other enzymes can break down polymers of PE and PET wastes into valuable products like CO2, H2O, and CH4.Therefore, the scientific society's attention has increased on the subject.

Figure 1 :
Figure 1: Procedure of the review of the article

Table 2 :
The characteristics of plastics(PE, PETE)

Enzymatic Degradation of PE and PET Enzymes
are biocatalysts that accelerate the process of chemical reaction to change substrate into a valuable product

Table 3 :
Enzymes produced microbes and involved in the enzymatic degradation of PE and PET