Secretory production of an engineered cutinase in Bacillus subtilis for efficient biocatalytic depolymerization of polyethylene terephthalate

Comparative evaluation of the extracellular production of a polyethylene terephthalate degrading cutinase by Corynebacterium glutamicum and leaky Escherichia coli in batch and fed-batch processes

BACKGROUND

With a growing global population, the generation of plastic waste and the depletion of fossil resources are major concerns that need to be addressed by developing sustainable and efficient plastic recycling methods. Biocatalytic recycling is emerging as a promising ecological alternative to conventional processes, particularly in the recycling of polyethylene terephthalate (PET). However, cost-effective production of the involved biocatalyst is essential for the transition of enzymatic PET recycling to a widely used industrial technology. Extracellular enzyme production using established organisms such as Escherichia coli or Corynebacterium glutamicum offers a promising way to reduce downstream processing costs.

RESULTS

In this study, we compared extracellular recombinant protein production by classical secretion in C. glutamicum and by membrane leakage in E. coli. A superior extracellular release of the cutinase ICCGDAQI was observed with E. coli in batch and fed-batch processes on a litre-scale. This phenomenon in E. coli, in the absence of a signal peptide, might be associated with membrane-destabilizing catalytic properties of the expressed cutinase. Optimisations regarding induction, expression temperature and duration as well as carbon source significantly enhanced extracellular cutinase activity. In particular, in fed-batch cultivation of E. coli at 30 °C with lactose as carbon source and inducer, a remarkable extracellular activity (137 U mL-1) and cutinase titre (660 mg L-1) were achieved after 48 h. Literature values obtained with other secretory organisms, such as Bacillus subtilis or Komagataella phaffii were clearly outperformed. The extracellular ICCGDAQI produced showed high efficacy in the hydrolysis of PET textile fibres, either chromatographically purified or unpurified as culture supernatant. In less than 18 h, 10 g L-1 substrate was hydrolysed using supernatant containing 3 mg cutinase ICCGDAQI at 70 °C, pH 9 with terephthalic acid yields of up to 97.8%.

CONCLUSION

Extracellular production can reduce the cost of recombinant proteins by simplifying downstream processing. In the case of the PET-hydrolysing cutinase ICCGDAQI, it was even possible to avoid chromatographic purification and still achieve efficient PET hydrolysis. With such production approaches and their further optimisation, enzymatic recycling of PET can contribute to a more efficient and environmentally friendly solution to the industrial recycling of plastics in the future.

Harnessing extremophilic carboxylesterases for applications in polyester depolymerisation and plastic waste recycling

The steady growth in industrial production of synthetic plastics and their limited recycling have resulted in severe environmental pollution and contribute to global warming and oil depletion. Currently, there is an urgent need to develop efficient plastic recycling technologies to prevent further environmental pollution and recover chemical feedstocks for polymer re-synthesis and upcycling in a circular economy. Enzymatic depolymerization of synthetic polyesters by microbial carboxylesterases provides an attractive addition to existing mechanical and chemical recycling technologies due to enzyme specificity, low energy consumption, and mild reaction conditions. Carboxylesterases constitute a diverse group of serine-dependent hydrolases catalysing the cleavage and formation of ester bonds. However, the stability and hydrolytic activity of identified natural esterases towards synthetic polyesters are usually insufficient for applications in industrial polyester recycling. This necessitates further efforts on the discovery of robust enzymes, as well as protein engineering of natural enzymes for enhanced activity and stability. In this essay, we discuss the current knowledge of microbial carboxylesterases that degrade polyesters (polyesterases) with focus on polyethylene terephthalate (PET), which is one of the five major synthetic polymers. Then, we briefly review the recent progress in the discovery and protein engineering of microbial polyesterases, as well as developing enzyme cocktails and secreted protein expression for applications in the depolymerisation of polyester blends and mixed plastics. Future research aimed at the discovery of novel polyesterases from extreme environments and protein engineering for improved performance will aid developing efficient polyester recycling technologies for the circular plastics economy.

Functional tailoring of a PET hydrolytic enzyme expressed in Pichia pastoris

Using enzymes to hydrolyze and recycle poly(ethylene terephthalate) (PET) is an attractive eco-friendly approach to manage the ever-increasing PET wastes, while one major challenge to realize the commercial application of enzyme-based PET degradation is to establish large-scale production methods to produce PET hydrolytic enzyme. To achieve this goal, we exploited the industrial strain Pichia pastoris to express a PET hydrolytic enzyme from Caldimonas taiwanensis termed CtPL-DM. In contrast to the protein expressed in Escherichia coli, CtPL-DM expressed in P. pastoris is inactive in PET degradation. Structural analysis indicates that a putative N-glycosylation site N181 could restrain the conformational change of a substrate-binding Trp and hamper the enzyme action. We thus constructed N181A to remove the N-glycosylation and found that the PET hydrolytic activity of this variant was restored. The performance of N181A was further enhanced via molecular engineering. These results are of valuable in terms of PET hydrolytic enzyme production in industrial strains in the future.

Recent advances and challenges in sustainable management of plastic waste using biodegradation approach

Versatility and desirable attributes of synthetic plastics have greatly contributed towards their wide applications. However, vast accumulation of plastic wastes in environment as a result of their highly recalcitrant nature has given rise to plastic pollution. Existing strategies in alleviating plastic wastes accumulation are inadequate, and there is a pressing need for alternative sustainable approaches in tackling plastic pollution. In this context, plastic biodegradation has emerged as a sustainable and environmental-friendly approach in handling plastic wastes accumulation, due to its milder and less energy-intensive conditions. In recent years, extensive research effort has focused on the identification of microorganisms and enzymes with plastic-degrading abilities. This review aims to provide a timely and holistic view on the current status of plastic biodegradation, focusing on recent breakthroughs and discoveries in this field. Furthermore, current challenges associated to plastic biodegradation are discussed, and the future perspectives for continuous advancement of plastic biodegradation are highlighted.

Conversion of waste cooked rice into maltose by a Bacillus subtilis-derived maltogenic amylase and production of maltobionic acid from the prepared maltose using genetically modified Pseudomonas taetrolens

In this study, we attempted to produce maltobionic acid (MBA) from waste cooked rice (WCR) using maltose as an intermediate. In our previous study, we produced maltose from WCR using a commercial maltogenic amylase (Maltogenase L). However, in the present study, we used wild-type Bacillus subtilis, which inherently produces maltogenic amylase (AmyE), instead of Maltogenase L to produce maltose from WCR. During cultivation of B. subtilis with WCR, maltose was successfully produced by AmyE in the culture medium. To improve maltose production, we constructed a recombinant B. subtilis strain expressing AmyE and used it for maltose production. Following cultivation of the recombinant B. subtilis strain, the maltose production titer (34.6 g/L) increased approximately 3.6-fold that (9.6 g/L) obtained from the cultivation of wild-type B. subtilis. Using Pseudomonas taetrolens, an efficient MBA-producing bacterium, 28.8 g/L of MBA was produced from the prepared maltose (27.6 g/L). The above results indicated that MBA was successfully produced from WCR via a two-step process, which involved the conversion of WCR into maltose by maltogenic amylase-producing B. subtilis and the production of MBA from the WCR-derived maltose by P. taetrolens.