Polyethylene terephthalate (PET) is a commonly used plastic due to its durability and molecular stability, but its high molecular weight and hydrophobicity also make it one of the hardest plastics to degrade. Although the PET recycling rate has increased from 7% to 30% in the last few years, a majority of the plastic is still dumped in landfills and continues to pollute the environment . Most plastics are made from petroleum, oil or natural gas, and a variety of chemicals that are toxic to humans (including ethylene glycol). Phthalates and Bisphenol-A (BPA) are the most common types of chemicals used to make plastic materials. Phthalates have been known to cause diseases related to cancer or hormonal imbalances. BPA is often known to leak from bottles and migrate into liquids and foods it comes into contact with. Within the landfills, rainwater can trickle through the trash, creating toxic leachate. The leachate carries microorganisms and toxic chemicals into drinking water sources . Instead of approaching the problem by reducing the amount of input coming in, most firms are simply increasing the amount of bioplastic. These “biobottles” are only 30% biodegradable, meaning that there is still an influx of non-biodegradable plastic coming into our environment . Currently, the processes to recycle these plastics are churning out energy, water, and greenhouse gases, creating a process that is more wasteful than sustainable.
This year, the UC Davis IGEM team aspires to use synthetic biology techniques to create a microbial strain that has the capacity to metabolize PET and degrade it to non-toxic compounds. To achieve this outcome, we utilize targeted bioengineering and directed evolution techniques on the E. coli MG1655 strain, currently one of the most well-studied organisms. More specifically, the goals of our project are:
1. Targeted bioengineering of E. coli MG1655 to utilize PET as a carbon source. We seek to this by introducing a couple different modules to the strain. The first module encodes a cutinase gene which has been found to degrade PET into ethylene glycol and terephthalic acid. The second encodes the first two enzymes in a pathway that has been found to feed ethylene glycol into the TCA cycle.
2. Rational protein engineering to increase the enzymatic activity of cutinase to degrade PET.
3. Directed evolution of the ethylene glycol degradation pathway to increase its efficiency of ethylene glycol metabolism in E. coli cultures. Reintroduction of the glycolaldehyde reductase and glycolaldehyde dehydrogenase enzymes in various constructs to MG1655 and E-15 EG3 to increase the ethylene glycol consumption beyond E-15 EG3’s original capabilities.