Theoretical Study On The Hydrolase-Catalyzed Polyethylene Terephthalate Depolymerization

Plastics have been widely used in our daily life because of their low weight,durability,and low cost.The production of plastics has increased dramatically wordwide since the 20th century,the majority of which entered and accumulated in the environment,posing a serious threat to the global ecosystem.Currently,the disposal of plastic waste mainly relies on landfill,incineration,mechanical and chemical recycling.However,these processes commonly cause resource waste and secondary pollution etc.Based on the goal of low carbon,high efficiency,and environment friendly,enzymatic depolymerization has become one of the most potential methods to deal with plastic pollution.In recent years,experimental methods have achieved great progress in plastic biodepolymerization research.At the same time,the theoretical calculation methods have also been developed rapidly.The theoretical calculation can study the dynamic properties and the depolymerization mechanism of hydrolases at the molecular level and predict the potential beneficial mutation sites.At present,the theoretical calculation techniques,including molecular docking,molecular dynamics simulation,quantum chemistry,and multiscale modelling,are widely used in the research of plastic biodepolymerization.Polyethylene terephthalate(PET)is one of the most widely used polyester plastics.PET waste accounts for about 12%of the total solid waste in the world.Although a variety of hydrolases have been found having the ability to depolymerize PET,their depolymerization efficiency still cannot meet the needs of industrialization.Here,molecular docking,molecular dynamics simulation,quantum chemistry,and multiscale modelling were used to study the catalytic reaction mechanism of six typical PET hydrolases.The main conclusions are as follows:(1)Molecular docking results show that substrate PET tends to bind to the hydrolases in pro-S way,and the depolymerization process in pro-S binding processes lower reaction energy barrier.Therefore,the intermediates produced during depolymerization process have S type chirality.(2)Mechanism studies found that the depolymerization process of PET catalyzed by hydrolases involves two major steps:acylation and deacylation.It can be further divided into four concerted elementary steps:step i is the nucleophilic attack initiated by catalytic traid(Ser-His-Asp);step ii is the cleavage of ester bond;step iii is the nucleophilic attack by the water molecule;step iv is the cleavage of C-O and the formation of products.The energy barrier comparation showed that the rate-determining step and the reaction energy barriers of different PET hydrolases have significant difference,which was in good agreement with the dynamics results measured in the experiment.(3)The key features affecting the catalytic efficiency of hydrolases were determined by means of distortion/interaction energy analysis,non-covalent bond interaction analysis,and key feature analysis.The relationship between key feactures and energy barriers was also built.(4)By comparing the depolymerization efficiency of enzyme to PET with different sizes,it found that the size of PET influences the catalytic efficiency.The molecular mechanism of "sized effect" has also been clarified.The research results of this paper elucidated the depolymerization mechanism of PET catalyzed by hydrolases at the molecular level.In addition,the mutation strategys that can enhance the depolymerization activity of hydrolases,increasing the electronegativity of atoms Ser@O1 and His@N1 and enhancing the accommodate capacity of the active center to water molecule etc.,have also been proposed.This paper provides important theoretical guidance for the rational design of PET hydrolases with higher depolymerization efficiency.