Biocatalytic recycling of polyethylene terephthalate plastic Zimmermann, Wolfgang
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
07/2020, Letnik:
378, Številka:
2176
Journal Article
Recenzirano
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The global production of plastics made from non-renewable fossil feedstocks has grown more than 20-fold since 1964. While more than eight billion tons of plastics have been produced until today, only ...a small fraction is currently collected for recycling and large amounts of plastic waste are ending up in landfills and in the oceans. Pollution caused by accumulating plastic waste in the environment has become worldwide a serious problem. Synthetic polyesters such as polyethylene terephthalate (PET) have widespread use in food packaging materials, beverage bottles, coatings and fibres. Recently, it has been shown that post-consumer PET can be hydrolysed by microbial enzymes at mild reaction conditions in aqueous media. In a circular plastics economy, the resulting monomers can be recovered and re-used to manufacture PET products or other chemicals without depleting fossil feedstocks and damaging the environment. The enzymatic degradation of post-consumer plastics thereby represents an innovative, environmentally benign and sustainable alternative to conventional recycling processes. By the construction of powerful biocatalysts employing protein engineering techniques, a biocatalytic recycling of PET can be further developed towards industrial applications.
This article is part of a discussion meeting issue ‘Science to enable the circular economy’.
Summary
Petroleum‐based plastics have replaced many natural materials in their former applications. With their excellent properties, they have found widespread uses in almost every area of human ...life. However, the high recalcitrance of many synthetic plastics results in their long persistence in the environment, and the growing amount of plastic waste ending up in landfills and in the oceans has become a global concern. In recent years, a number of microbial enzymes capable of modifying or degrading recalcitrant synthetic polymers have been identified. They are emerging as candidates for the development of biocatalytic plastic recycling processes, by which valuable raw materials can be recovered in an environmentally sustainable way. This review is focused on microbial biocatalysts involved in the degradation of the synthetic plastics polyethylene, polystyrene, polyurethane and polyethylene terephthalate (PET). Recent progress in the application of polyester hydrolases for the recovery of PET building blocks and challenges for the application of these enzymes in alternative plastic waste recycling processes will be discussed.
The high recalcitrance of many synthetic plastics results in their long persistence in the environment and globally in growing amounts of plastic waste. Microbial enzymes are emerging as candidates for the development of biocatalytic plastic recycling processes.
Many pathogens must bind to entry receptors on the surfaces of host cells yet avoid any closely-related phagocytic decoy receptors on granulocytes that evolved as a host defense mechanism. The ...discovery of decoy-receptor polymorphisms in human populations now points to an evolutionary process that allows the host to catch up with pathogens.
Many pathogens must bind to entry receptors on the surfaces of host cells yet avoid any closely-related phagocytic decoy receptors on granulocytes that evolved as a host defense mechanism. The discovery of decoy-receptor polymorphisms in human populations now points to an evolutionary process that allows the host to catch up with pathogens.
The evolution of pregnancy-specific glycoprotein (PSG) genes within the CEA gene family of primates correlates with the evolution of hemochorial placentation about 45 Myr ago. Thus, we hypothesized ...that hemochorial placentation with intimate contact between fetal cells and maternal immune cells favors the evolution and expansion of PSGs. With only a few exceptions, all rodents have hemochorial placentas thus the question arises whether Psgs evolved in all rodent genera. In the analysis of 94 rodent species from 4 suborders, we identified Psg genes only in the suborder Myomorpha in three families (characteristic species in brackets), namely Muridae (mouse), Cricetidae (hamster) and Nesomyidae (giant pouched rat). All Psgs are located, as previously described for mouse and rat, in a region of the genome separated from the Cea gene family locus by several megabases, further referred to as the rodent Psg locus. In the suborders Castorimorpha (beaver), Hystricognatha (guinea pig) and Sciuromorpha (squirrel), neither Psg genes nor so called CEA-related cell adhesion molecule (Ceacam) genes were found in the Psg locus. There was even no evidence for the existence of Psgs in any other genomic region. In contrast to the Psg-harboring rodent species, which do not have activating CEACAMs, we were able to identify Ceacam genes encoding activating CEACAMs in all other rodents studied. In the Psg locus, there are genes encoding three structurally distinct CEACAM/PSGs: (i) CEACAMs composed of one N- and one A2-type domain (CEACAM9, CEACAM15), (ii) composed of two N domains (CEACAM11-CEACAM14) and (iii) composed of three to eight N domains and one A2 domain (PSGs). All of them were found to be secreted glycoproteins preferentially expressed by trophoblast cells, thus they should be considered as PSGs. In rodents Psg genes evolved only recently in the suborder Myomorpha shortly upon their most recent common ancestor (MRCA) has coopted the retroviral genes syncytin-A and syncytin-B which enabled the evolution of the three-layered trophoblast. The expansion of Psgs is limited to the Psg locus most likely after a translocation of a CEA-related gene - possibly encoding an ITAM harboring CEACAM. According to the expression pattern two waves of gene amplification occurred, coding for structurally different PSGs.
Pregnancy-specific glycoprotein (PSG) genes belong to the carcinoembryonic antigen (CEA) gene family, within the immunoglobulin gene superfamily. In humans, 10 PSG genes encode closely related ...secreted glycoproteins. They are exclusively expressed in fetal syncytiotrophoblast cells and represent the most abundant fetal proteins in the maternal blood. In recent years, a role in modulation of the maternal immune system possibly to avoid rejection of the semiallogeneic fetus and to facilitate access of trophoblast cells to maternal resources via the blood system has been suggested. Alternatively, they could serve as soluble pathogen decoy receptors like other members of the CEA family. Despite their clearly different domain organization, similar functional properties have also been observed for murine and bat PSG. As these species share a hemochorial type of placentation and a seemingly convergent formation of PSG genes during evolution, we hypothesized that hemochorial placentae support the evolution of PSG gene families.
To strengthen this hypothesis, we have analyzed PSG genes in 57 primate species which exhibit hemochorial or epitheliochorial placentation. In nearly all analyzed apes some 10 PSG genes each could be retrieved from genomic databases, while 6 to 24 PSG genes were found in Old World monkey genomes. Surprisingly, only 1 to 7 PSG genes could be identified in New World monkeys. Interestingly, no PSG genes were found in more distantly related primates with epitheliochorial placentae like lemurs and lorises. The exons encoding the putative receptor-binding domains exhibit strong selection for diversification in most primate PSG as revealed by rapid loss of orthologous relationship during evolution and high ratios of nonsynonymous and synonymous mutations.
The distribution of trophoblast-specific PSGs in primates and their pattern of selection supports the hypothesis that PSG are still evolving to optimize fetal-maternal or putative pathogen interactions in mammals with intimate contact of fetal cells with the immune system of the mother like in hemochorial placentation.
Over 359 million tons of plastics were produced worldwide in 2018, with significant growth expected in the near future, resulting in the global challenge of end-of-life management. The recent ...identification of enzymes that degrade plastics previously considered non-biodegradable opens up opportunities to steer the plastic recycling industry into the realm of biotechnology.
Here, the sequential conversion of post-consumer polyethylene terephthalate (PET) into two types of bioplastics is presented: a medium chain-length polyhydroxyalkanoate (PHA) and a novel bio-based poly(amide urethane) (bio-PU). PET films are hydrolyzed by a thermostable polyester hydrolase yielding highly pure terephthalate and ethylene glycol. The obtained hydrolysate is used directly as a feedstock for a terephthalate-degrading Pseudomonas umsongensis GO16, also evolved to efficiently metabolize ethylene glycol, to produce PHA. The strain is further modified to secrete hydroxyalkanoyloxy-alkanoates (HAAs), which are used as monomers for the chemo-catalytic synthesis of bio-PU. In short, a novel value-chain for PET upcycling is shown that circumvents the costly purification of PET monomers, adding technological flexibility to the global challenge of end-of-life management of plastics.
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•Tandem enzymatic hydrolysis and microbial conversion of PET.•Complete enzymatic hydrolysis of post-consumer PET.•ALE for enhanced ethylene glycol metabolization.•Biochemical synthesis of a novel bio-based poly(amide urethane).
•Factors limiting the degradation of PET nanoparticles by polyester hydrolases were investigated.•The intermediate hydrolysis products MHET and BHET were identified as competitive inhibitors of the ...PET hydrolase TfCut2.•Product inhibition by MHET was identified as a limiting factor in the biocatalytic hydrolysis of PET.
The biocatalytic hydrolysis of post-consumer polyethylene terephthalate (PET) is a promising approach for an environmentally friendly plastic recycling process. The influence of intermediate hydrolysis products on the degradation of PET by TfCut2, a polyester hydrolase from Thermobifida fusca, was analyzed by reversed-phase HPLC. Nanoparticles prepared from PET films as substrate were hydrolyzed by the enzyme in the presence of ethylene glycol, terephthalic acid, bis(2-hydroxyethyl) terephthalate (BHET) and mono-(2-hydroxyethyl) terephthalate (MHET). The initial reaction rates were determined and kinetically analyzed using a model for PET degradation. BHET and MHET were identified as competitive inhibitors with similar binding constants. The reaction efficiency (Ef) for the hydrolysis of MHET was considerably lower compared to BHET indicating MHET as a relevant inhibitor of the enzymatic hydrolysis of PET nanoparticles by TfCut2. The results demonstrate the importance of product inhibition of the hydrolysis of PET by TfCut2 for the further improvement of biocatalytic plastic recycling processes.
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