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

Unlocking a Sustainable Future for Plastics: A Chemical-Enzymatic Pathway for Efficient Conversion of Mixed Waste to MHET and Energy-Saving PET Recycling

The heterogeneous monomers obtained from plastic waste degradation are unfavorable for PET recondensation and high-value derivative synthesis. Herein, we developed an efficient chemical-enzymatic approach to convert mixed plastic wastes into homogeneous mono-2-hydroxyethyl terephthalate (MHET) without downstream purification, benefiting from three discovered BHETases (KbEst, KbHyd, and BrevEst) in nature. Towards the mixed plastic waste, integrating the chemical K2CO3-driven glycolysis process with the BHETase depolymerization technique resulted in an MHET yield of up to 98.26 % in 40 h. Remarkably, BrevEst accomplished the highest BHET hydrolysis (~87 % efficiency in 12 h) for yielding analytical-grade MHET compared to seven state-of-the-art PET hydrolases (18 %-40 %). In an investigation combining quantum theoretical computations and experimental validations, we established a MHET-initiated PET repolymerization pathway. This shortcut approach with MHET promises to strengthen the valorization of mixed plastics, offering a substantially more efficient and energy-saving route for PET recycling.

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.

Biochemical Characterization and NMR Study of a PET-Hydrolyzing Cutinase from Fusarium solani pisi

In recent years, the drawbacks of plastics have become evident, with plastic pollution becoming a major environmental issue. There is an urgent need to find solutions to efficiently manage plastic waste by using novel recycling methods. Biocatalytic recycling of plastics by using enzyme-catalyzed hydrolysis is one such solution that has gained interest, in particular for recycling poly(ethylene terephthalate) (PET). To provide insights into PET hydrolysis by cutinases, we have here characterized the kinetics of a PET-hydrolyzing cutinase from Fusarium solani pisi (FsC) at different pH values, mapped the interaction between FsC and the PET analogue BHET by using NMR spectroscopy, and monitored product release directly and in real time by using time-resolved NMR experiments. We found that primarily aliphatic side chains around the active site participate in the interaction with BHET and that pH conditions and a mutation around the active site (L182A) can be used to tune the relative amounts of degradation products. Moreover, we propose that the low catalytic performance of FsC on PET is caused by poor substrate binding combined with slow MHET hydrolysis. Overall, our results provide insights into obstacles that preclude efficient PET hydrolysis by FsC and suggest future approaches for overcoming these obstacles and generating efficient PET-hydrolyzing enzymes.