Explorations of Polyethylene Terephthalate (PET) Hydrolase for addressing PET Plastic Pollution

Abstract

Since their initial synthesis in 1907, plastic organic polymers derived from petrochemicals have become ubiquitous and indispensable due to their diverse applications. Polyethylene terephthalate (PET) is the fourth most produced plastic worldwide accounting for a significant share of synthetic textiles, in the form of polyester, single‐use bottles, and other packaging. PET is composed of monomeric units of ethylene terephthalate (C10H8O4), and is a strong, impact‐resistant, semi‐rigid, lightweight, colorless, and semi‐crystalline resin. Although PET can be recycled, it, alongside other plastics, has increasingly become an environmental concern because of its relative indestructible nature. It is estimated that over 8 million tons of plastic waste enter the ocean each year, some of which has accumulated into a “Great Pacific Garbage Patch” covering an area of 1.6 million km2. Plastics and microplastics have been implicated in the deaths of a diversity of marine organisms from whales to zooplankton, and have even been found in human stool samples. As a consequence, the European parliament has instituted a ban on single use plastics by 2021. Fortunately, in 2016, Yoshida et al isolated and characterized a unique bacterium, Ideonella sakaiensis, which degrades PET and assimilates its products using novel enzymes. More recently, the structure of one of these enzymes, PET hydrolase, was determined. PET hydrolase catalyzes the breakdown of PET into mono(2‐hydroxyethyl) terephthalic acid (MHET) which in turn is degraded by MHETase into the raw ingredients for PET production, terephthalic acid (TPA) and ethylene glycol (EG). Structurally, and like other hydrolases, PETase forms a characteristic a/b fold having a core of six a‐helices and eight b‐strands. It has a broad active cleft three times the width of homologous hydrolases, resulting from the single substitution of a typically conserved Phe residue by Ser. The active site is characterized by the conserved catalytic triad of Asp206, His237, and Ser160. The Asp and His residues, and His and Ser residues, are stabilized and activated by hydrogen bonds, respectively. The latter bonding primes the Ser residue for nucleophilic attack and cleavage of the ester linkage in the bound PET substrate. Additionally, the backbone amino groups of Met161 and Tyr87 together form an oxyanion hole which coordinates the carbonyl group of the substrate's ester linkage through hydrogen bonding. A bioengineered PET hydrolase with substitutions W159H/S238F, immediately adjacent to the distal active site residues, was found to narrow the active site cleft, and increase enzymatic activity compared to the wild‐type protein, though further optimization is warranted. The CAPS 2018–19 MSOE Center for BioMolecular Modeling SMART Team modeled PET hydrolase (PDB: 6EQE) using JMol in concert with the exploration of additional mutations that could result in increased PET degradation efficiency, and with the ultimate hopes that we will soon develop a chemical recycling solution for combating the buildup of plastics in the environment.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.