Theoretical Study On Degradation Mechanism Of Polyester Plastics Catalyzed By Hydrolase

Since the start of production in the 1950s,global plastics production has increased exponentially,including polymer types with a variety of additives and fillers.Plastic directly affects microbial communities,exerts toxic effects,destroys host biological health,hormone balance and immune response,and its pollution has become one of the most pressing global environmental problems facing human.Enzymes have many advantages,such as high catalysis,specificity and non-toxicity,especially hydrolase that play an important role in the biodegradation of polyester plastic.More and more researches are focused on the enzymatic degradation of plastic.However,the specific binding process of enzymes and plastic pollutants and the role of amino acids in the degradation process are difficult to directly analyze through experiments.Therefore,it is particularly important to understand the specific reaction degradation mechanism at the molecular level and guide the rapid degradation of biodegradable plastic.The study mainly took Molecule Dynamic(MD)and Quantum Mechanics/Molecular Mechanics(QM/MM)method to study the biodegradation mechanism of polycaprolactone(PCL)by the novel esterase MGS0156 and IsPETase and IsMHETase catalyzed cascade degradation mechanism toward polyethylene terephthalate.It reveals in detail the degradation mechanism and effects of amino acids near the active center,determines the key rate determining step in the reaction process,provides a deep understanding of the enzymatic degradation mechanism of polyester plastics,and lays a solid foundation for predicting the reaction energy barrier as well as designing more efficient enzymes for plastics recycling and sustainability.1.Biodegradation Mechanism of Polycaprolactone by a Novel Hydrolase MGS0156MGS0156 is a kind of hydrolase screened from environmental metagenomes,which can efficiently degrade many kinds of commercial plastics like polycaprolactone,polylactide,etc.Here a combined Molecular Dynamics,Molecular Mechanics Poisson-Boltzmann Surface Area,and Quantum Mechanics/Molecular Mechanism method were used to reveal the enzymatic depolymerization mechanism.By systematically analyzing the binding processes of nine oligomers(from monomer to tetramer),it was found that longer oligomer has relatively stronger binding energy.The degradation process involves two concerted elementary steps:triad-assisted(Ser225、His528、Asp492)nucleophilic attack and C-O bond cleavage.C-O bond cleavage is the rate determining step with an average barrier of 15.7 kcal/mol,which is in consistent with the experimentally determined kcat(1101 s-1,corresponds to 13.3 kcal/mol).The electrostatic influence analysis of twenty amino acids highlights His231 and Asp237 as the potential mutation targets for designing more efficient MGS0156 mutants.2.IsPETase and IsMHETase Catalyzed Cascade Degradation Mechanism toward Polyethylene TerephthalatePolyethylene terephthalate(PET)is one of the most representative petroleum-based synthetic polymers.It has been widely used in bottles,packaging materials and textile industries.It is difficult to degrade in the natural environment and easy to absorb heavy metals and other pollutants,causing serious environmental problems.This paper systematically studied the two enzyme systems in Ideonella sakaiensis,IsPETase and IsMETase,taking Molecular Dynamics and Quantum Mechanics/Molecular Mechanics methods,which can catalyze the conversion of PET to terephthalic acid and ethylene glycol with four elementary steps:(i)Ser-His-Asp initiated nucleophilic attack,(ii)C-O bond cleavage,(iii)nucleophilic attack by water molecule,(iv)IsPETase/IsMHETase deacylation.Statistical results from twenty independent conformations highlight step i and iv is competitive for determining the turnover rate of IsPETase while step iv is rate-determining step for IsMHETase.We discovered a new "double-proton-shuttling"mechanism in 45%of IsPETase’s conformations.With the newly developed strategy,possible features(bonds,angles,dihedral angles,and charges)that influence the enzymatic catalysis were screened and identified.Robust relationship between active site features and activation energies were established.Distortion-interaction,hydrogen-network and NCI analysis highlight the roles of distortion/interaction energy,hydrogen-network and weak interactions in the IsPETase and IsMHETase catalyzed cascade degradation of PET.These results deepen our understanding on the origin of the catalytic power of IsPETase and IsMHETase and may enhance the PET recycling and sustainability.