Microbial enzymes for the recycling of recalcitrant petroleum‐based plastics: how far are we?

Abstract

SummaryPetroleum‐based plastics have replaced many natural materials in their former applications. With their excellent properties, they have found widespread uses in almost every area of human life. However, the high recalcitrance of many synthetic plastics results in their long persistence in the environment, and the growing amount of plastic waste ending up in landfills and in the oceans has become a global concern. In recent years, a number of microbial enzymes capable of modifying or degrading recalcitrant synthetic polymers have been identified. They are emerging as candidates for the development of biocatalytic plastic recycling processes, by which valuable raw materials can be recovered in an environmentally sustainable way. This review is focused on microbial biocatalysts involved in the degradation of the synthetic plastics polyethylene, polystyrene, polyurethane and polyethylene terephthalate (PET). Recent progress in the application of polyester hydrolases for the recovery of PET building blocks and challenges for the application of these enzymes in alternative plastic waste recycling processes will be discussed.

Protein Crystallography and Site‐Direct Mutagenesis Analysis of the Poly(ethylene terephthalate) Hydrolase PETase from Ideonella sakaiensis

“…For example, cutinase from Fusarium solani pisi (FsC) is a promising candidate for the enzymatic degradation of PET; another special enzyme from leaf‐branch compost metagenome (LC cutinase, LCC) can break down PET, cutin, and polycaprolactone (PCL); and the hydrolases from Thermobifida fusca (TfH) has prominent activity on polyester hydrolysis . However, even though the levels of activity can be maintained up to 50–70 °C, cutinase has extremely low turnover rates towards a highly aggregated PET substrate . Although protein‐engineering strategies have been applied to enhance the activity of cutinase, the continuous mining of new PET hydrolases with highly efficiency and specificity towards polyesters is an alternative strategy for enzymatic hydrolysis of PET …”

“…However, even though the levels of activity can be maintained up to 50–70 °C, cutinase has extremely low turnover rates towards a highly aggregated PET substrate . Although protein‐engineering strategies have been applied to enhance the activity of cutinase, the continuous mining of new PET hydrolases with highly efficiency and specificity towards polyesters is an alternative strategy for enzymatic hydrolysis of PET …”

Laboratory evolution reveals the metabolic and regulatory basis of ethylene glycol metabolism byPseudomonas putidaKT2440

“…These include bacteria in the gut of the wax worm Plodia interpunctella capable of degrading polyethylene (PE) (Yang et al ., ; Bombelli et al ., ) and the bacterium Ideonella sakaiensis , which can depolymerize poly(ethylene terephthalate) (PET) and grow on the resulting terephthalate component (Yoshida et al ., ). The identification and engineering of plastic‐degrading organisms and enzymes provide a compelling opportunity to increase plastic recycling and thereby reduce plastic pollution through the utilization of plastic waste as carbon source for microbial biotechnology (Cho et al ., ; Wierckx et al ., ; Narancic and O'Connor, ; Wei and Zimmermann, , ; Austin et al ., ).…”

Towards an ecology of soil microplastics

“…To be sure, the carbon-carbon backbones of many common plastic polymers make them more resistant to enzymatic breakdown than many biological materials (Wei & Zimmermann, 2017) and some types of plastic may be virtually unchanged even after decades of burial (Otake, Kobayashi, Asabe, Murakami, & Ono, 1995). However, exposure to the elements, particularly ultraviolet radiation, may sensitize some plastics to microbial colonization and biodegradation (Gong, Yang, Zhuang, & Zeng, 2019;Vimala & Mathew, 2016;Wei & Zimmermann, 2017), and Otake et al (1995) reported that polyethylene films were heavily degraded after years of burial. Numerous soil microbes, both bacteria and fungi, have been implicated in plastic degradation (Kale et al, 2015;Shah et al, 2008;Yoshida et al, 2016).…”