Plastic degrading enzymes and their role
The key polymers (plastics) that are manufactured and are of significance to our economy are: Polyurethane (PUR), Polyethylene (PE), Polyamide (PA), Polyethylene terephthalate (PET), Polystyrene (PS), Polyvinylchloride (PVC) and Polypropylene (PP). Plastic degradation is a change in the properties—tensile strength, color, shape, etc.—of a plastic polymer or polymer-based product under the influence of one or more environmental factors such as heat, light, and chemicals such as acids, alkalis, salts or biological enzymes. Three major mechanisms for degradation of plastics are photodegradation, thennooxidative degradation and biodegradation. The first two types of degradation mechanisms are abiotic and the last one is biotic in nature. Biodegradation of polymers occurs through various mechanisms like solubilization, dissolution, hydrolysis and enzyme- catalyzed degradation and can occur under aerobic or anaerobic conditions. Microbes mediated degradation of plastics is facilitated by secreting enzymes which act on long chain polymer surface converting them into monomers (enzymatic biodegradation).
The microbial degradation of synthetic polymers is an extremely sluggish process, stemming from the high molecular weight of the polymer fiber, the resilient C-C bonds and the exceedingly hydrophobic surface, which is very difficultly attacked by the enzymes. Particularly, the different polymers with varying molecular weights having amorphous and crystalline nature resulting in their
difference in degradability as well as susceptibility to enzymes. Enzymatic hydrolysis of polyesters can be grouped under:
A. enzymatic surface modification (hydrolyze the surface polymer chain) - hydrolases, such as lipases, carboxylesterases, cutinases, and proteases and
B. enzymatic hydrolysis (substantial degradation of the building blocks of PET) - cutinases and some other hydrolases.
First step of enzymatic biodegradation is hydrolysis and addition of a functional group which enhances the hydrophobicity of polymer. Subsequently, the polymers are broken down to monomers and enter the microbes through then- semi-permeable membranes. Even after hydrolysis, if the polymers’ size exceeds the threshold, these are first depolymerized which allows them to penetrate through the cell membrane and are thereafter broken down by intracellular enzymes.
The hydrolytic reaction is first initiated by class of enzymes known as ‘hydrolase’ comprising cutinase, phosphatase, laccase, esterase, lipase, glycosidase and many more (Bano et al. 2017). These enzymes have a characteristic ct/p-hydrolase fold and the catalytic triad is comprised of a serine, a histidine and an aspartate residue (Wei et al. 2014). They can also comprise numerous disulfide bonds produced by cysteine residues, which help in thermal stability and specific binding to polymers such as PET.
Cutinases are involved in (catalyze) hydrolysis of cutin which are aliphatic polyesters and are found in plant cuticle structure. Under the super family of ot/p hydrolases, this category of polyester hydrolases acts upon several polyester plastics (Wei and Zimmennann 2017a, b). Depending on their homology, origin, and structure, plastic-degrading cutinases can be divided into fungal and bacterial. Cutinases from fungal origin such as Thermomycesinsolens, Fusarium and Humicola are useful and show excellent activity in the hydrolysis and surface alterations of polyethylene terephthalate (PET) films and fibers (Zimmennann and Billig 2010), due to their remarkable activity and thermal stability at 70°C near the glass transition temperature of PET (Ronkvist et al. 2009). Bacterial cutinases capable of hydrolyzing PET have been segregated from various Thermobifida species (Then et al. 2015), Thermomonosporacurvata (Wei et al. 2014), Saccharomonosporaviridis (Kawai et al. 2014), Ideonellasakaiensis (Yoshida et al. 2016), as well as the metagenome isolated from plant compost (Sulaiman et al. 2012). The bacterium, Ideonellasakaiensis 201-F6, exhibits rare capability to thrive on PET as a major carbon and energy source and secretes PETase (PET- digesting enzyme) leading to its biodegradation (Yoshida et al. 2016).
Lipases belonging to а/p hydrolases are capable of hydrolyzing aliphatic polyesters or aliphatic- aromatic co-polyesters (Herzog et al. 2006). Lipases from Thermomyceslanuginosus degrade PET and polytrimethylene terephthalate (Ronkvist et al. 2009). Lipases exhibit slow hydrolytic activity against PET in comparison to cutinases, possibly dvte to their lid type assembly, shelling the buried hydrophobic catalytic centre, which restricts the entry of aromatic polymeric substrates to the active site of the enzyme (Zimmermann and Billig 2010). Lipases from T. lamtginosus (Eberl et al. 2009) and Candida antarctica (Camiel et al. 2017) also degrade low- molecular-weight degradation products of PET. The PueB lipase from Pseudomonas chlororaphis as well as from Fusarium solani and Candida ethanolica are repoxted to be involved in metabolism of Polyurethanes (PUR).
Bacteria such as Thermobifidafusca, Bacillus licheniformis and Bacillus subtilis contain enzyme carboxylesterases instigating hydrolysis of PET oligomers and their analogues (Barth et al. 2016). The carboxylesterase TfCa from T. fusca removes water-soluble components from high-crystalline PET fibers (Zimmermann and Billig 2010). Strains of Comantonasacidovorans TB-35 produced a PUR-active esterase enzyme, with hydrophobic PUR surface binding properties essential for PUR degradation. Fungi belonging to the Cladosporiumclado sporioides complex, 249 including the species C. pseudocladosporioides, C. tenuissimum, C. asperulatum, C. montecillanum, Aspergillus fumigatus and Penici Ilium, secreted esterases responsible for the degradation of poly butylene adipate-co-terephthalate (PBAT) (Wallace et al. 2017). Polyesterase acting on aromatic polyesters (primaiily PET) was first reported by Muller et al. (2005) in Thermobifidafusca.
Proteases obtained from Pseudomonas chlororaphis and Pseudomonas ftuorescens degrade polyester polyurethanes (PU) (Matsumiya et al. 2010). In addition to the microbial enzymes, other proteases active against PU include papain, a cysteine protease from papaya, which can hydrolyze amide and urethane bonds and elastase obtained from porcine pancreas (Labow et al. 1996).
6.5 Lignin-modifying enzymes
Laccases, manganese peroxidases and lignin peroxidases involved in the degradation of lignin (Ruiz-Duenas and Martinez 2009) also have role in the biodegradation of polyethylene (Rrueger et al. 2015). A laccase isolated from Trametes versicolor degraded a high-molecular weight PE (PE- HMW) membrane in the presence of 1-hydroxybenzotriazole, which mediated the oxidation of non- phenolic substrates by the enzyme (Fujisawa et al. 2001). APenicillitun derived laccase, potentially involved in PE breakdown, was reported by Sowmya et al. (2015). Amicrobial peroxidase extracted from white rot fungus was shown to act on high molecular weight nylon fibers (Danso et al. 2019).
Limited information is still available on the known variety of enzymes and microbes acting on synthetic polymers. Consequently, effort has been made towards identification of organisms capable of acting on the most dominant polymers. The major constraint lies in the initial breaking of the high molecular weight crystalline structure of the robust polymers. Enzymatic degradation of polymers and its coimnercial implementation are of prime importance and require extensive research and funding that would allow the effective management of plastic thereby reducing enviromnental pollution.