Team:ASTWS-China/Description

Description

Welcome to iGEM 2021

OVERVIEW   Plastic pollution is one of the most important and urgent environmental problems today, which led us to carry out this project related to microplastic pollution. In the ASTWS-China 2020 project, we used an enhanced biofilm PETase plastic degradation system, but incomplete degradation and lower efficiency of microplastics caught our attention. Inspired by the new study of super enzymes which can decompose PET to monomers in just a few days [1], we decide to add MHETase to the original system, adopt an enhanced biofilm composite PETase and MHETase dual-enzyme system as well as optimize their expressions. We hope that PET could be efficiently and completely decomposed into small molecules harmless to the environment.   1. Background 1.1 Introduction of micro-/nano-plastics Microplastics or nano-plastics, refer to plastic particles and textile fibers or films with very small particle diameters. Plastic particles with a size of less than 5mm are generally considered to be microplastics. Many microplastics can even reach the micron level or even the nanometer level (Figure 1). Plastic is the main component of marine garbage. The global annual production of plastic exceeds 300 million tons, of which about 10% will enter the ocean. These plastics are chemically stable and difficult to degrade, which will exist in the sea for more than hundreds of years. Figure 1 Tools and costs [2].   Here are some of the origins of microplastic[SS1] :  (1)   Tire wear. (2)   Tennis: Polyethylene terephthalate (PET). (3)   Cigarette butts: Its filter contains cellulose acetate (non-degradable plastic). (4)   Synthetic clothing: acrylic plastic, polyester, polyamide, spandex fibers, etc. (5)   Teabag: contains polypropylene skeleton.   1.2 Damage caused by micro-/nano-plastics 1.2.1 Pollution distribution Microplastic pollution has shown a characteristic of "extensive and concentrated" in the world. It is easy to migrate by water currents, ocean currents and wind, and could be found in the atmosphere, water bodies and soil. At present, large plastic fragments and microplastics are widely distributed along the coast of the Atlantic, Pacific, Indian Ocean and other oceans, while microplastics have even been found in remote polar regions and deep seas. In tap water samples from 14 countries on five continents, 83% was found to contain microplastics[3].And microplastics have also penetrated into basic products such as sea salt, fish, shellfish, sugar, honey, and beer.   Figure 2 Migration pathways and bioaccumulation of microplastics [4].   1.2.2 The hazards of microplastics: (1) May cause fire disasters because most of plastics are combustible. (2) Enter human body and cause health problems. (3) Influence the growth of aquatic organisms. (4) Emit toxicants or absorb heavy metals and organic pollutants. (5) Destroy the natural landscape as time accumulates.   1.3 Current solutions of microplastics 1.3.1 Separation and extraction method of microplastics (1) Density separation: to purify and separate the lightweight and easy-to-fine microplastics from the high-quality, fragile water bodies. (2) Filtration and sieving: to separate microplastics from plastics. (3) Digestion: Chemical digestion and enzymatic digestion: to remove the organic impurities that interfere with the microplastics identification [5].   1.3.2 Current degradation method of microplastics (1) Biophysical settlement: The plankton absorbs the microplastics and migrates. Offshore corals can consume up microplastics; (2) Biochemistry degradation: turn the microplastics particles into bio-CO2 and H2O [3]. The biodegradation of PET is usually used for recycling [6].   2 Introduction of PET & MHET Polyethylene terephthalate (PET) is one of the most common thermoplastics, mainly used in the manufacture of disposable beverage bottles, clothing and carpets. PET has weather resistance, and could exist in the environment for a long time [6]. In the US, less than 30% of PET plastic containers are recycled [7].   Figure 3 PET depolymerization method [8].   MHET is an intermediate product of PET decomposition. PETase can promote the depolymerization of PET into bis(2-hydroxyethyl)-TPA (BHET), mono(2-hydroxyethyl) terephthalate (MHET) and p-benzene Dicarboxylic acid (TPA). MHETase converts MHET into monomers TPA and ethylene glycol (EG) [8]. Scientists have developed a hybrid enzyme by fusing PETase with MHETase, which can increase the digestion speed of plastic by up to six times [1].   Figure 4 Structural analysis of MHETase and PETase [1].   3 Previous iGEM Projects Other iGEM teams also considered a variety of methods to degrade plastics. The following are the previous projects of other enzymes besides the PETase. (1) Transform bacterial strains to antimicrobial peptides through synthetic biology to solve the pollution caused by the plastic part of band-aids and antibiotics. (NWU-CHINA-A, 2019) (2) Embed novel fusion protein into the vesicular membrane of magnetosomes, which is produced by Rhodospirillum rubrum magneticum under special conditions, to specifically bind certain polymers. (BUCT-China, 2019) (3) Use Microbulbifer hydrolyticus strains, which are enhanced by an artificial metabolic pathway, to degrade plastics while synthesizing valuable products such as PHA. (Aachen, 2019) (4) Implement a plastic degradation system right in the dustbin using the LCC enzyme, while using another enzyme, bLIS, to create fragrance in the dustbin to attract tourists to throw plastic waste in the right place. (KEYSTONE, 2020).   Last year, ASTWS-China 2020 engineered a modified E. Coli biodegradation system that produces PETase and strengthens the biofilm system. It was proved that the enhanced biofilm (by overexpressing the OmpR234 gene) could improve the degradation activity of PETase through the proximity effect between the substrate and the enzyme but have a problem of incomplete degradation of PET. Our project is built on the basis of the project of ASTWS-China 2020 and we are committed to further optimization and improvement.   4 Our Project ASTWS-China 2020 has proved that Biofilm is helpful to improve the degradation activity of PETase, but cannot completely degrade PET polymer. Therefore, on this basis, we further designed the enhanced biofilm system and introduced the MHETase enzyme to further degrade plastic products into harmless TPA and EG. We will also verify the degradation efficiency of PETase and MHETase. See more details on our Design  page. It is hoped that this project will have some enlightenment for reducing plastic pollution, energy consumption, and greenhouse gas emissions in the future.   References [1] Brandon C K, Erika E, Mark D A, et al. Characterization and engineering of a two-enzyme system for plastics depolymerization. Proceedings of the National Academy of Sciences, 2020, 117 (41) 25476-25485; DOI: 10.1073/pnas.2006753117. [2] Primpke S, Christiansen SH, Cowger W, et al. Critical Assessment of Analytical Methods for the Harmonized and Cost-Efficient Analysis of Microplastics. Appl Spectrosc. 2020 Sep;74(9):1012-1047. doi: 10.1177/0003702820921465. PMID: 32249594. [3] Wang XX, Qu CF, Wang WY, et al. Research status and prospects of China's marine microplastics pollution. Marine Science, 2018, 133-143. doi:CNKI:SUN:HYKX.0.2018-03-017. [4] CTN NEWS. PM Issues Warning after Microplastic Found in Thailand’s Fish. 2019. Retrieved from https://www.chiangraitimes.com/thailand-national-news/news-asia-thailand/pm-issues-warning-after-microplastic-found-in-thailands-fish/. [5] Gu WK, Yang GF, Liu Y, et al. Research progress in the treatment and detection of microplastics in environmental methods. Chinese Journal of Civil and Environmental Engineering, 2020, 135-143. [6] Xu Cui. Research progress analysis of microplastic pollution and biological evolution. Science and Technology Innovation Herald, 2020, 119-120. [7] Plastics: Material-Specific Data. Retrieved from https://www.epa.gov/facts-and-figures-about-materials-waste-and-recycling/plastics-material-specific-data. [8] Rumiana T. Can eating plastic super enzymes solve the serious plastic problem? 2020. Retrieved from https://www.cas.org/zh-hans/resource/blog/plastic-eating-enzymes.

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