Scheme. PET degradation and Co-immobilized enzyme 4M-MG8@Fe3O4NPs recycle.

Recent years, the enormous waste plastic has become a severe environmental issue with the wide abuse of plastic products. Waste plastic showed a great negative impact on the ecological environment[1] because its degradation time is more than 100 years, even 500 years[2]. In addition, plastic waste may put a threat to human health[3][4]. For instance, it has been reported that microplastics smaller than 5mm can accumulate in the human body in different ways. Polyethylene terephthalate (PET) accounts for 12% of global solid waste[2]. The traditional methods of PET waste disposal are incineration and landfills[5]. However, a large number of toxic gases such as dioxins[6], and carbon monoxide produced by incineration will cause damage to the atmosphere. Meanwhile, landfill treatment will occupy land resources and cause severe contamination of groundwater[7].

Fortunately, biodegradation of PET plastic has been discovered. Therefore, circular carbon economy for PET could theoretically be achieved through rapid enzymatic depolymerization followed by repolymerization or conversion into other products[8]. Enzymatic degradation of PET has become a hit for its environmental friendliness and sustainability[9].

It has been reported various PET enzymes have excellent properties for the biodegradation of PET plastics[10].In 2016, Yoshida et al. derived PET hydrolase (PETase) from Ideonella sakaiensis[11], which can effectively degrade PET into bis (2-hydroxyethyl) terephthalic (BHET), mono (2-hydroxyethyl) terephthalate (MHET), TPA, and EG. At moderate temperature (30–40◦C), due to its unique PET substrate binding pocket[12]. IsPETase has immediately become one of the most promising PET plastic degradation enzymes. However, this poor thermal stability of wild-type IsPETase limited its application for PET degradation[13]. Until now, the improvement of thermal stability and biodegradation capacity is the main research direction of PETase. And many kinds of engineering enzyme are based on IsPETase[14][15].

We proposed solar-driven enzymatic PET degradation with DuraPETase-4M[16], a high-efficiency mutant of PETase, and MG8[17], a new PET hydrolase from the human saliva metagenome, by using Fe3O4 nanoparticles (NPs) as a solar-to-thermal convertor. Solar irradiation could elevate the temperature of Fe3O4 NPs for the effective PET degradation[18][19]. Also, by covalently binding to Fe3O4 NPs, enzymes can easily be recycled, which will make a contribution to cost reduction. DuraPETase-4M is mainly good at the degradation of amorphous PET[16],while MG8 shows excellent catalytic activity of crystalline PET[17].By combining them, PET can be more thoroughly degraded.


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