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...

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Efficient conversion of cellulosic sugars in cellulosic hydrolysates is important for economically viable production of biofuels from lignocellulosic biomass, but the goal remains a critical challenge. The present study reports a new approach for simultaneous fermentation of cellobiose and xylose by using the co-culture consisting of recombinant Saccharomyces cerevisiae specialist strains. The co-culture system can provide competitive advantage of modularity compared to the single culture system and can be tuned to deal with fluctuations in feedstock composition to achieve robust and cost-effective biofuel production. This study characterized fermentation kinetics of the recombinant cellobiose-consuming S. cerevisiae strain EJ2, xylose-consuming S. cerevisiae strain SR8, and their co-culture. The motivation for kinetic modeling was to provide guidance and prediction of using the co-culture system for simultaneous fermentation of mixed sugars with adjustable biomass of each specialist strain under different substrate concentrations. The kinetic model for the co-culture system was developed based on the pure culture models and incorporated the effects of product inhibition, initial substrate concentration and inoculum size. The model simulations were validated by results from independent fermentation experiments under different substrate conditions, and good agreement was found between model predictions and experimental data from batch fermentation of cellobiose, xylose and their mixtures. Additionally, with the guidance of model prediction, simultaneous co-fermentation of 60 g/L cellobiose and 20 g/L xylose was achieved with the initial cell densities of 0.45 g dry cell weight /L for EJ2 and 0.9 g dry cell weight /L SR8. The results demonstrated that the kinetic modeling could be used to guide the design and optimization of yeast co-culture conditions for achieving simultaneous fermentation of cellobiose and xylose with improved ethanol productivity, which is critically important for robust and efficient renewable biofuel production from lignocellulosic biomass.

Recovering rare earth elements (REEs) from waste streams represents a sustainable approach to diversify REE supply while alleviating the environmental burden. However, it remains a critical challenge to selectively separate and concentrate REEs from low-grade waste streams. In this study, we developed a new type of biosorbent by immobilizing Lanmodulin-SpyCatcher (LanM-Spycatcher) on the surface of SpyTag-functionalized magnetic nanoparticles (MNPs) for selective separation and recovery of REEs from waste streams. The biosorbent, referred to as MNP-LanM, had an adsorption activity of 6.01 ± 0.11 μmol-terbium/gsorbent and fast adsorption kinetics. The adsorbed REEs could be desorbed with >90% efficiency. The MNP-LanM selectively adsorbed REEs in the presence of a broad range of non-REEs. The protein storage stability of the MNP-LanM increased by two-fold compared to free LanM-SpyCatcher. The MNP-LanM could be efficiently separated using a magnet and reused with high stability as it retained ∼95% of the initial activity after eight adsorption−desorption cycles. Furthermore, the MNP-LanM selectively adsorbed and concentrated REEs from the leachate of coal fly ash and geothermal brine, resulting in 967-fold increase of REE purity. This study provides a scientific basis for developing innovative biosorptive materials for selective and efficient separation and recovery of REEs from low-grade feedstocks.

Parabens are emerging environmental contaminants with known endocrine-disrupting effects. This study created a novel biocatalyst (named as SDFsC) by expressing the enzyme Fusarium solani pisi cutinase (FsC) on the cell surface of Baker's yeast Sacchromycese cerevisiae and demonstrated successful enzymemediated removal of parabens for the first time. Parabens with different side chain structures had different degradation rates by the SDFsC. The SDFsC preferentially degraded the parabens with relatively long alkyl or aromatic side chains. The structuredependent degradability was in a good agreement with the binding energy between the active site of FsC and different parabens. In real wastewater effluent solution, the SDFsC effectively degraded 800 μg/L of propylparaben, butylparaben, and benzylparaben, either as a single compound or as a mixture, within 48 h. The estrogenic activity of parabens was considerably reduced as the parent parabens were degraded into 4-hydroxybenzoic acid via hydrolysis pathway by the SDFsC. The SDFsC showed superior reusability and maintained 93% of its initial catalytic activity after six rounds of paraben degradation reaction. Results from this study provide scientific basis for developing biocatalysis as a green chemistry alternative for advanced treatment of parabens in sustainable water reclamation.

The extensive production and use of polyethylene terephthalate (PET) have generated an enormous amount of plastic waste, which potentially threatens the environment and humans. Enzyme biocatalysis is a promising green chemistry alternative, relative to the conventional fossil-derived production process, to achieve plastic waste treatment and recycling. In this work, we created a biocatalyst, BIND-PETase, by genetically engineering the curli of an Escherichia coli cell with a functional PETase enzyme for biocatalytic degradation of PET plastics. BIND-PETase could degrade PET to generate degradation products at the concentration level of greater than 3000 μM under various reaction conditions. The effects of key reaction parameters, including pH, temperature, plastic substrate mass load, and surfactant addition were characterized. BIND-PETase was reusable for PET degradation and remained stable with no significant enzyme activity loss when stored at both 4 °C and room temperature for 30 days (Student’s t test, p > 0.05). Notably, BIND-PETase could enable the degradation of PET microplastics in wastewater effluent matrix. Moreover, BIND-PETase could depolymerize highly crystalline postconsumer PET waste materials under ambient conditions with degradation efficiency of 9.1% in 7 days. This study provides a new horizon for developing environmentally friendly biocatalytic approaches to solve the plastic degradation and recycling challenge.

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