Experimental and mathematical modeling approaches for biocatalytic post-consumer poly(ethylene terephthalate) hydrolysis

Recent advances in the biodegradation of polyethylene terephthalate with cutinase-like enzymes

Polyethylene terephthalate (PET) is a synthetic polymer in the polyester family. It is widely found in objects used daily, including packaging materials (such as bottles and containers), textiles (such as fibers), and even in the automotive and electronics industries. PET is known for its excellent mechanical properties, chemical resistance, and transparency. However, these features (e.g., high hydrophobicity and high molecular weight) also make PET highly resistant to degradation by wild-type microorganisms or physicochemical methods in nature, contributing to the accumulation of plastic waste in the environment. Therefore, accelerated PET recycling is becoming increasingly urgent to address the global environmental problem caused by plastic wastes and prevent plastic pollution. In addition to traditional physical cycling (e.g., pyrolysis, gasification) and chemical cycling (e.g., chemical depolymerization), biodegradation can be used, which involves breaking down organic materials into simpler compounds by microorganisms or PET-degrading enzymes. Lipases and cutinases are the two classes of enzymes that have been studied extensively for this purpose. Biodegradation of PET is an attractive approach for managing PET waste, as it can help reduce environmental pollution and promote a circular economy. During the past few years, great advances have been accomplished in PET biodegradation. In this review, current knowledge on cutinase-like PET hydrolases (such as TfCut2, Cut190, HiC, and LCC) was described in detail, including the structures, ligand-protein interactions, and rational protein engineering for improved PET-degrading performance. In particular, applications of the engineered catalysts were highlighted, such as improving the PET hydrolytic activity by constructing fusion proteins. The review is expected to provide novel insights for the biodegradation of complex polymers.

Enzymatic post-consumer poly(ethylene terephthalate) (PET) depolymerization using commercial enzymes

Poly(ethylene terephthalate) (PET) is a synthetic polymer widely used globally. The high PET resistance to biotic degradation and its improper destination result in the accumulation of this plastic in the environment, largely affecting terrestrial and aquatic animals. This work investigated post-consumer PET (PC-PET) degradation using five commercial hydrolase enzymes (Novozym 51032, CalB, Palatase, Eversa, Lipozyme TL). Humicola insolens cutinase (HiC, Novozym 51032) was the most active among the enzymes studied. Several important reaction parameters (enzyme type, dual enzyme system, enzyme concentration, temperature, ultrasound treatment) were evaluated in PC-PET hydrolysis using HiC. The concentration and the proportion (molar ratio) of hydrolysis products, terephthalic acid (TPA), mono(2-hydroxyethyl) terephthalate (MHET), and bis(2-hydroxyethyl) terephthalate (BHET), were significantly changed depending on the reaction temperature. The TPA released at 70 °C was 3.65-fold higher than at 50 °C. At higher temperatures, the conversion of MHET into TPA was favored. The enzymatic PET hydrolysis by HiC was very sensitive to the enzyme concentration, indicating that it strongly adsorbs on the polymer surface. The concentration of TPA, MHET, and BHET increased as the enzyme concentration increased, and a maximum was achieved using 40-50 vol % of HiC. The presented results add relevant data to optimizing enzyme-based PET recycling technologies.

Determinants for an Efficient Enzymatic Catalysis in Poly(Ethylene Terephthalate) Degradation

The enzymatic degradation of the recalcitrant poly(ethylene terephthalate) (PET) has been an important biotechnological goal. The present review focuses on the state of the art in enzymatic degradation of PET, and the challenges ahead. This review covers (i) enzymes acting on PET, (ii) protein improvements through selection or engineering, (iii) strategies to improve biocatalyst–polymer interaction and monomer yields. Finally, this review discusses critical points on PET degradation, and their related experimental aspects, that include the control of physicochemical parameters. The search for, and engineering of, PET hydrolases, have been widely studied to achieve this, and several examples are discussed here. Many enzymes, from various microbial sources, have been studied and engineered, but recently true PET hydrolases (PETases), active at moderate temperatures, were reported. For a circular economy process, terephtalic acid (TPA) production is critical. Some thermophilic cutinases and engineered PETases have been reported to release terephthalic acid in significant amounts. Some bottlenecks in enzyme performance are discussed, including enzyme activity, thermal stability, substrate accessibility, PET microstructures, high crystallinity, molecular mass, mass transfer, and efficient conversion into reusable fragments.

Comparison of the enzymatic depolymerization of polyethylene terephthalate and AkestraTM using Humicola insolens cutinase

The enzymatic depolymerization of synthetic polyesters has become of great interest in recycling plastics. Most of the research in this area focuses on the depolymerization of polyethylene terephthalate (PET) due to its widespread use in various applications. However, the enzymatic activity on other commercial polyesters is less frequently investigated. Therefore, AkestraTM attracted our attention, which is a copolymer derived from PET with a partially biobased spirocyclic acetal structure. In this study, the activity of Humicola insolens cutinase (HiCut) on PET and AkestraTM films and powder was investigated. HiCut showed higher depolymerization activity on amorphous PET films than on Akestra™ films. However, an outstanding performance was achieved on AkestraTM powder, reaching 38% depolymerization in 235h, while only 12% for PET powder. These results are consistent with the dependence of the enzymes on the crystallinity of the polymer since Akestra™ is amorphous while the PET powder has 14% crystallinity. On the other hand, HiCut docking studies and molecular dynamic simulations (MD) suggested that the PET-derived mono (hydroxyethyl)terephthalate dimer (MHET)2 is a hydrolyzable ligand, producing terephthalic acid (TPA), while the Akestra™-derived TPA-spiroglycol ester is not, which is consistent with the depolymerization products determined experimentally. MD studies also suggest ligand-induced local conformational changes in the active site.

Novel efficient enzymatic synthesis of the key-reaction intermediate of PET depolymerization, mono(2-hydroxyethyl terephthalate) - MHET

Poly(ethylene terephthalate) (PET) is one of the main synthetic plastics produced worldwide. The extensive use of this polymer causes several problems due to its low degradability. In this scenario, biocatalysts dawn as an alternative to enhance PET recycling. The enzymatic hydrolysis of PET results in a mixture of terephthalic acid (TPA), ethylene glycol (EG), mono-(2-hydroxyethyl) terephthalate (MHET) and bis-(2-hydroxyethyl) terephthalate (BHET) as main products. This work developed a new methodology to quantify the hydrolytic activity of biocatalysts, using BHET as a model substrate. The protocol can be used in screening enzymes for PET depolymerization reactions, amongst other applications. The very good fitting (R² = 0.993) between experimental data and the mathematical model confirmed the feasibility of the Michaelis-Menten equation to analyze the effect of BHET concentration (8-200 mmol L^-1) on initial hydrolysis rate catalyzed by Humicola insolens cutinase (HiC). In addition to evaluating the effects of enzyme and substrate concentration on the enzymatic hydrolysis of BHET, a novel and straightforward method for MHET synthesis was developed using an enzyme load of 0.025 g_protein g_BHET^-1 and BHET concentration of 60 mmol L^-1 at 40 °C. MHET was synthesized with high selectivity (97 %) and yield (82 %). The synthesized MHET properties were studied using differential scanning calorimetry (DSC), thermogravimetry (TGA), and proton nuclear magnetic resonance (¹H NMR), observing the high purity of the final product (86.7 %). As MHET is not available commercially, this synthesis using substrate and enzyme from open suppliers adds new perspectives to monitoring PET hydrolysis reactions.

Perspectives on the Role of Enzymatic Biocatalysis for the Degradation of Plastic PET

Plastics are highly durable and widely used materials. Current methodologies of plastic degradation, elimination, and recycling are flawed. In recent years, biodegradation (the usage of microorganisms for material recycling) has grown as a valid alternative to previously used methods. The evolution of bioengineering techniques and the discovery of novel microorganisms and enzymes with degradation ability have been key. One of the most produced plastics is PET, a long chain polymer of terephthalic acid (TPA) and ethylene glycol (EG) repeating monomers. Many enzymes with PET degradation activity have been discovered, characterized, and engineered in the last few years. However, classification and integrated knowledge of these enzymes are not trivial. Therefore, in this work we present a summary of currently known PET degrading enzymes, focusing on their structural and activity characteristics, and summarizing engineering efforts to improve activity. Although several high potential enzymes have been discovered, further efforts to improve activity and thermal stability are necessary.