Polyethylene terephthalate (PET) is a synthetic polymer made from chemicals derived from crude oil. As a result of the high volume of plastic produced in the world today, only 9% of all plastics have been recycled while the rest is either dumped in the natural environment or incinerated, leading to problems like plastic pollution and overflowing landfills. PET and other polyesters have been accumulating in the Great Pacific garbage patch. The island of trash causes disturbances in the North Pacific Subtropical Gyre food webs and poses threats towards marine species through entanglement and consumption. While the majority of PET is currently recycled mechanically, this method often consumes high amounts of energy, releases harmful by-products, and results in a loss of material properties in PET, decreasing the intrinsic value of the polymer. There are also few affordable chemical recycling solutions because of PET’s high resistance to biodegradation, which is due to its structural elements such as its aromatic groups and crystallinity that limit polymer chain movement, generate surface hydrophobicity, and limit its accessibility to ester linkages; making it recalcitrant to catalytic or biological polymerization. In 2016, a bacterium called Ideonella Sakaiensis 201-F6 was found to have the ability to degrade PET at room temperatures via the secretion of two main enzymes: IsPETase and IsMHETase. IsPETase has a penchant for breaking down crystallized PET over other types of polyester using a wide substrate-binding pocket. The second enzyme IsMHETase is a hydrolase that has an α/β-hydrolase domain and a lid domain granting substrate specificity. The degradation process begins when the PET hydrolase (IsPETase) cuts the PET polymer into mono(2-hydroxyethyl) terephthalate acid (MHET), terephthalate (TPA), and byproduct bis(2-hydroxyethyl) terephthalate (BHET). Subsequently, the second enzyme MHET hydrolase (IsMHETase) converts MHET into ethylene glycol and TPA. The end products can then be used in further applications such as making antifreeze and hybrid materials for plastic carrier bags, or simply be broken down by microorganisms into carbon dioxide and water. There are, however, concerns about the large-scale application of microorganisms such as I. Sakaiensis for industrial recycling purposes, as the process is time consuming and often unreliable. Researchers have therefore begun looking at protein engineering as a means to allow enzymes like IsPETase and IsMHETase to be more efficient and degrade PET 100-1000x faster. Some proposed solutions include narrowing the active-site cleft, combining multiple PET-active enzymes, and further lowering the protein’s optimal reaction temperature.
The Ashbury College MSOE Center for BioMolecular Modeling A Protein Story (MAPS) team uses 3D modelling and printing technology to examine structure-function relationships of the enzymes PETase and MHETase involved in the PET degradation process.
