Enzyme discovery and engineering for sustainable plastic recycling

Abstract

The drastically increasing amount of plastic waste is causing an environmental crisis that requires innovative technologies for recycling post-consumer plastics to achieve waste valorization while meeting environmental quality goals. Biocatalytic depolymerization mediated by enzymes has emerged as an efficient and sustainable alternative for plastic treatment and recycling. A variety of plasticdegrading enzymes have been discovered from microbial sources. Meanwhile, protein engineering has been exploited to modify and optimize plastic-degrading enzymes. This review highlights the recent trends and up-to-date advances in mining novel plastic-degrading enzymes through state-of-the-art omics-based techniques and improving the enzyme catalytic efficiency and stability via various protein engineering strategies. Future research prospects and challenges are also discussed. Biocatalysis as an Emerging Solution for the Global Plastic Waste ChallengePlastic materials play a revolutionary role in the modern world, although the enormous manufacture and extensive use of plastic commodities inevitably generate an extraordinary amount of post-consumer plastic waste. Around 12 000 million metric tons of plastic waste are predicted to accumulate in landfills and the natural environment by 2050 [1]. Improper handling of plastic waste has caused a grand environmental challenge. The debris of plastic waste, especially microplastics (see Glossary), can impose hazardous effects on various organisms and eventually threaten human well-being [2-5]. In addition, the degradation resistance of plastics further escalates their adverse environmental impacts [6]. Therefore, it is urgent to develop innovative technologies for treatment and recycling of post-consumer plastics, to achieve both waste valorization and environmental protection.Enzymatic biocatalysis has gained increasing attention as an eco-friendly alternative to conventional plastic treatment and recycling methods (Box 1) [7]. To date, various microbial plastic-degrading enzymes have been discovered, representing promising biocatalyst candidates for plastic depolymerization. Considering the ubiquity of plastics in different ecosystems and the tremendous metabolic and genetic diversity of microorganisms, microbial communities in various habitats have likely evolved capabilities in plastic decomposition and utilization. The plastic-degrading enzymes identified so far might only account for a small portion of the enzymes relevant to plastic depolymerization in the environment. Therefore, it is of ever-growing interest to explore diverse environments to discover new plastic-degrading enzymes with desirable properties and functionalities. However, naturally occurring plasticdegrading enzymes are not well suited for synthetic plastic degradation in industrial applications due to poor thermostability and low catalytic activity. Particularly, synthetic plastic materials usually possess distinct physical and chemical properties (e.g., high crystallinity) that render them more resistant to...

New approaches for the characterization of plastic-associated microbial communities and the discovery of plastic-degrading microorganisms and enzymes

“…1 ) [80] . Unlike homology-based screens, functional metagenomic screens confirm desired plastic-degrading phenotypes and do not rely on a priori sequence information, allowing discovery of previously unknown enzyme families [20] . Functional metagenomics provides a powerful high-throughput approach to study gene function.…”

“…This may be a product of overrepresentation of marine and terrestrial (soil) metagenomes in databases like the Integrated Microbial Genomes (IMG) [87] . Plastic-enriched sites such as landfills and PAMCs are understudied and may offer a more promising source of novel PDEs due to adaptive evolution [20] , [58] . Compost has also proven to be a valuable source of PDEs, likely due to the abundance of recalcitrant biopolymeric substances [18] , [83] .…”

“…Involves engineering biocatalytic systems for more efficient plastic processing. Protein engineering can modify naturally occurring PDEs to enhance features like biodegradation rates or thermostability to increase their biotechnological applications [20] . Synthetic biology efforts are emerging with engineered plastic-processing microbes capable of improved plastic biodegradation [23] , or the ‘bio-upcycling’ of PET into new polymers including PHAs [41] .…”

“…Multi-omic community profiling should be paired with rigorous attempts to characterize changes in plastic properties in line with degradation trials for a more comprehensive view of biotic factors mediating polymer changes in different environments. Characterization of PAMCs is valuable beyond understanding environmental impacts, as PAMCs are a promising source for novel PDEs with potential biotechnological applications [20] .

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“…With the exponential increase in DNA sequencing capacity and maturation of computational biology techniques for annotating sequence data, researchers can now query uncultured microbial lineages from diverse environments. This has enabled high throughput metagenomic screens for PDEs [20] . Total community approaches, or “omics” like metagenomics, metatranscriptomics and metaproteomics, have enabled significant advancements in the understanding of plastic-associated microbial communities (PAMCs) and the microbial ecology of plastic [21] .…”