Why project?
Plastic is a polymeric material that has seen wide application in beverage bottles, clothing fiber, electronic components, and other products. PET is the most used plastic with a detrimental impact on the environment. An estimation in 2019 suggested that of the 359 million tons of plastics produced annually worldwide, 150–200 million tons accumulate in landfills or natural environments[1]. The recycling of plastic bottles plays a vital role in environmental protection, yet only 9% of PET material is recycled[2].
Plastic as a packaging material discarded after use brings white pollution and other negative influence to the city. Scattered waste plastic wrappers in a city affect its appearance and deface its scenic spot, causing visual displeasure. More serious is most people's lack of awareness of the long-term potential harms of plastic packaging including excessive land occupation, air and water pollution, and fire hazards. Recent studies show that microplastics are present in human lung, liver, spleen, and kidney tissue samples[3]. Many plastic industrial and household products contain chemicals that interfere with hormones, which erode people's health chronically. To conclude, PET recycling is imperative for the long-term well-being of humankind.
PET recycling
Existing methods for recycling plastics fall into three main directions: physical recycling, chemical recycling, and biological recycling.
1.Physical recycling
In physical recycling the used PET bottles are collected and delivered to the recycling plant, where labels and caps are removed. The bottles are sorted by color and shredded. The material is washed, dried and decontaminated, then melted at 270℃ and granulated. The resulting product, called “regranulation”, is mixed with new granulate and melted, then fed into injection molding machines to produce “preforms” for new PET bottles. The preforms are transported to the filling plant, where they are heated and blown into PET bottles. Once cleaned and labelled, the bottles are ready for refilling and sale[4]. Physical recycling is low cost. However, the disadvantage of physical recycling is that it cannot form a real closed-loop recycling which means the quality of remanufactured products is one level lower than that of raw materials.
2.Chemical recycling
In chemical recycling, waste plastics are depolymerized into chemical intermediates (monomers or oligomers), which can then be re-polymerized. PET may be depolymerized using a range of chemical agents and processing conditions, the most important methods of which are metanalysis, hydrolysis, and glycolysis[4]. Chemical recycling is closed-loop recycling. However, chemical recovery is prohibitively costly and requires extreme conditions, such as strong acid and alkali environment, and even cause secondary pollution.
3.Biological recycling
Biological recycling is the use of enzymes to hydrolyze PET into monomers. Compared with physical and chemical recycling, it is an environmentally friendly method and can achieve closed-loop recycling. However, some factors, such as polymer chains' flexibility and crystallinity and the surface's hydrophobicity, also limit enzymatic degradation[5].
What is our inspiration?
A 2019 research came up with a mutant enzyme, LCC, that hydrolyzes 90% of PET in plastic bottles in just 10 hours, which is more efficient than any previous PET hydrolase. More importantly, the resulting monomers have the same properties as the monomers found in petrochemical materials. This finding greatly facilitated the recycling of plastic bottles. The researchers tested the ability of these mutant enzymes to degrade plastic and found that they were more efficient than the natural enzyme at cutting PET's chemical bonds. They also used the broken down materials produced by the mutant enzyme to make new PET and found that the material was as strong as PET that had already been recycled [6].
In conclusion, the enzyme with the highest degradation efficiency has been found. Therefore, we hope to use synthetic biology to further improve its activity.
Our Goals?
Our goal is to enhance the activity of mLCC by two approaches: constructing a fusion protein of mLCC and hydrophobins and using the technique of Bacillus subtilis surface display. Biodegrading PET takes two processes: adsorption and degradation.
The fusion protein serves the purpose of enhancing the adsorption efficiency since the surface of PET film is hydrophobic while that of mLCC is hydrophilic. An mLCC-linker-hydrophobin fusion protein will enhance the efficiency due to its unique amphiphilicity and self-assembly of hydrophobins. The second approach, Bacillus subtilis surface display, combines mLCC with the anchor protein which forms the fusion protein and immobilizes mLCC on the Bacillus subtilis cell surface to obtain a recyclable whole-cell biocatalyst, which can reduce costs and make the mLCC more efficient degrading PET.
Supplement
[1] Achilias, D. S. , & Karayannidis, G. P. . (2004). The chemical recycling of pet in the framework of sustainable development. Water, Air, and Soil Pollution: Focus, 4(4/5), 385-396.
[2] Tournier, V. , Topham, C. M. , Gilles, A. , David, B. , & Marty, A. . (2020). An engineered pet depolymerase to break down and recycle plastic bottles. Nature, 580(7802), 216-219.
[3] https://cebp.aacrjournals.org/content/7/5/419
[4] Rm, A. , Ffa, C. , Sw, B. , As, A. , Jk, B. , & Abad, E. . Towards a circular economy for plastic packaging wastes – the environmental potential of chemical recycling. Resources, Conservation and Recycling, 162.
[5](2020). Biodegradation of waste pet. EMBO Reports, 21(2).
[6] Tournier, V. , Topham, C. M. , Gilles, A. , David, B. , & Marty, A. . (2020). An engineered pet depolymerase to break down and recycle plastic bottles. Nature, 580(7802), 216-219.
