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Enzymatic hydrolysis of polyethylene terephthalate (PET) has been the subject of extensive previous research that can be grouped into two categories, viz. enzymatic surface modification of polyester fibers and management of PET waste by enzymatic hydrolysis. Different enzymes with rather specific properties are required for these two processes. Enzymatic surface modification is possible with several hydrolases, such as lipases, carboxylesterases, cutinases, and proteases. These enzymes should be designated as PET surface–modifying enzymes and should not degrade the building blocks of PET but should hydrolyze the surface polymer chain so that the intensity of PET is not weakened. Conversely, management of PET waste requires substantial degradation of the building blocks of PET; therefore, only a limited number of cutinases have been recognized as PET hydrolases since the first PET hydrolase was discovered by Müller et al. ( Macromol Rapid Commun 26:1400–1405, 2005 ). Here, we introduce current knowledge on enzymatic degradation of PET with a focus on the key class of enzymes, PET hydrolases, pertaining to the definition of enzymatic requirements for PET hydrolysis, structural analyses of PET hydrolases, and the reaction mechanisms. This review gives a deep insight into the structural basis and dynamics of PET hydrolases based on the recent progress in X-ray crystallography. Based on the knowledge accumulated to date, we discuss the potential for PET hydrolysis applications, such as in designing waste stream management. Electronic supplementary material The online version of this article (10.1007/s00253-019-09717-y) contains supplementary material, which is available to authorized users.

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Polyethylene terephthalate (PET) hydrolase is a challenging target as PET is a commonly used plastic that is extremely resistant to enzymatic attack. Since the discovery of a PET hydrolase from Thermobif ida f usca in 2005, novel PET hydrolases and their availability toward waste PET have been investigated. At present, at least four thermophilic cutinases are known as PET hydrolases that could be used for the management of amorphous PET waste, such as packaging materials. Heat-labile PETase from Ideonella sakaiensis and its homologues from mesophilic bacteria exist in the environment. However, PET can be efficiently hydrolyzed with thermophilic hydrolases. This Review focuses on the current state of PET hydrolases and the potential of their application. Contrary to an amorphous PET, the enzymatic hydrolysis of crystalline PET (particularly PET bottles) remains to be fully elucidated. It cannot be assured whether the biorecycling of general PET would be put into practice in the near future, but the plan is getting closer to the goal. PET hydrolases can be versatile polyesterases as they can hydrolyze not only PET but also other polyesters. Additionally, the thermostability of PET hydrolases is advantageous to their application in terms of reaction speed and durability.

Only two polyethylene glycol terephthalate (PET)-degrading enzymes have been reported, and their mechanism for the biochemical degradation of PET remains unclear. To identify a novel PET-degrading enzyme, a putative cutinase gene (cut190) was cloned from the thermophile Saccharomonospora viridis AHK190 and expressed in Escherichia coli Rosetta-gami B (DE3). Mutational analysis indicated that substitution of Ser226 with Pro and Arg228 with Ser yielded the highest activity and thermostability. The Ca(2+) ion enhanced the enzyme activity and thermostability of the wild-type and mutant Cut190. Circular dichroism suggested that the Ca(2+) changes the tertiary structure of the Cut190 (S226P/R228S), which has optimal activity at 65-75 °C and pH 6.5-8.0 in the presence of 20 % glycerol. The enzyme was stable over a pH range of 5-9 and at temperatures up to 65 °C for 24 h with 40 % activity remaining after incubation for 1 h at 70 °C. The Cut190 (S226P/R228S) efficiently hydrolyzed various aliphatic and aliphatic-co-aromatic polyester films. Furthermore, the enzyme degraded the PET film above 60 °C. Therefore, Cut190 is the novel-reported PET-degrading enzyme with the potential for industrial applications in polyester degradation, monomer recycling, and PET surface modification. Thus, the Cut190 will be a useful tool to elucidate the molecular mechanisms of the PET degradation, Ca(2+) activation, and stabilization.