Abstract
Biodegradation of synthetic polymers, in particular polyethylene terephthalate (PET), is of great importance, since environmental pollution with PET and other plastics has become a severe global problem. Here, we report on the polyester degrading ability of a novel carboxylic ester hydrolase identified in the genome of the marine hydrocarbonoclastic bacterium Pseudomonas aestusnigri VGXO14T. The enzyme, designated PE-H, belongs to the type IIa family of PET hydrolytic enzymes as indicated by amino acid sequence homology. It was produced in Escherichia coli, purified and its crystal structure was solved at 1.09 Å resolution representing the first structure of a type IIa PET hydrolytic enzyme. The structure shows a typical α/β-hydrolase fold and high structural homology to known polyester hydrolases. PET hydrolysis was detected at 30°C with amorphous PET film (PETa), but not with PET film from a commercial PET bottle (PETb). A rational mutagenesis study to improve the PET degrading potential of PE-H yielded variant PE-H (Y250S) which showed improved activity, ultimately also allowing the hydrolysis of PETb. The crystal structure of this variant solved at 1.35 Å resolution allowed to rationalize the improvement of enzymatic activity. A PET oligomer binding model was proposed by molecular docking computations. Our results indicate a significant potential of the marine bacterium P. aestusnigri for PET degradation.
Highlights
The modern society depends on the production and use of synthetic polymers which are uniformly present in both, basic and high-tech applications
We observed hydrolytic activity indicated by formation of clear halos upon growth of P. aestusnigri on agar plates containing Impranil DLN, an anionic aliphatic polyesterpolyurethane used for surface coating of textiles (Figure 1A)
Additional applications appear feasible for PEH as it has been demonstrated that the PE-H homologous enzyme PpelaLip originating from the closely related genus Pseudomonas pelagia can be used for the biodegradation of different synthetic polyesters and may be applicable for waste water treatment (Haernvall et al, 2017)
Summary
The modern society depends on the production and use of synthetic polymers which are uniformly present in both, basic and high-tech applications. The low production costs for plastic made from fossil feedstock and the high durability of the material are major advantages but have become a burden for the global ecosystem. Plastic waste is produced at a much faster rate than it is recycled (Moharir and Kumar, 2019); it is disposed in landfills at a large extend where it can take. Plastic waste accumulates in the environment to a large extent with very slow biodegradation to occur (Lebreton et al, 2018). The most abundant polyester plastic, present for example in packaging waste, is polyethylene terephthalate (PET) (Adrados et al, 2012). In the European Union, five million metric tons of this polyester were used for the production of plastics in 2017 (PlasticsEurope, 2018)
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