Unveiling the PET plastics degradation potential of the thermostable EstS1 esterase through integrated biochemical, structural, and morphological analyses

Enzymatic polyethylene terephthalate (PET) plastic degradation is a promising approach to combat the exploding plastic pollution. EstS1, a pH-tolerant, thermostable esterase, has been previously recognized for its degradation potential against phthalate diester plasticizers. The present study elucidates the exceptional potential of this enzyme to degrade crystalline PET plastic and its primary intermediate, bis(2-hydroxyethyl)terephthalate (BHET), into terephthalate. Kinetic analyses revealed that EstS1 degrades 75% of BHET in 1h, liberating mono(2-hydroxyethyl) terephthalate (MHET) and terephthalate as end products. The co-crystal structure of wild-type EstS1 with BHET exhibited the electron density of BHET, MHET, and ethylene glycol, including MHET bound at the active site, in a canonical tetrahedral intermediate conformation. The complex structure of BHET with the Ser154Ala mutant of EstS1 further accommodated two BHET molecules, one interacting directly with the catalytic triad and the oxyanion hole. MD simulation analysis revealed highly stable interactions of BHET at the active site of EstS1. Moreover, SEM imaging displayed significant degradation of the crystalline PET plastic film by EstS1 esterase over a period of 15 days, both under controlled and soil-based fluctuating environmental conditions, highlighting its versatility to varying environmental conditions. XPS analysis discovered the increase in -C-O-, -C-N-, -N-H-, and -N=O- bonds at the surface of EstS1-treated PET film, indicating effective degradation. Consequently, this comprehensive kinetic, structural, and morphology-based analysis of the PET-degrading potential of EstS1 esterase encourages further enzyme engineering studies to exploit the dual potential of EstS1 esterase to degrade both plastic and plasticizers.