9.3 Biodegradation of Plastics

135

9.3.6.1

Cutinases (EC 3.1.1.74)

Cutinases can hydrolyze the cutin, which is an aliphatic polyester originated from

plant cuticle. This type of polyester hydrolases is from the superfamily of α/β

hydrolases. They are very much active against several polyester plastics. Lipases

and cutinases both display traid composed of Ser-His-Asp. Owing to its lack of

usual lipase lid structure, the active site of cutinases is exposed to the solvent.

Based on origin, structure, and homology, this cutinase enzyme can be divided

into two groups, i.e. (i) fungal origin and (ii) bacterial origin [29]. Cutinases from

fungus are using for the hydrolysis and structure modification of PET films and

fiber [29]. However, cutinases extracted from Thermomyces insolens performed

higher activity against low crystalline PET due to the thermal stability very close to

the glass transition temperature (70 ^∘C) of PET [29]. Cutinases and its homologues

from bacteria (Thermobifida species) show PET hydrolyzing character. However,

cutinases from Thermomonospora curvata, Saccharomonospora viridis, Ideonella

sakaiensis, and as well as its metagenome isolated from plant compost also show

PET hydrolyzing character [30].

9.3.6.2

Lipases (EC 3.1.1.3)

Lipases are similar to cutinases, from the superfamily of α/β hydrolases, and both

display traid composed of Ser-His-Asp. Microbial lipases have the ability to hydrolyze

aliphatic polyester or aliphatic-aromatic co-polyesters. Lipases from Thermomyces

lanuginosus degraded PET and poly (trimethylene terephthalate) [29, 31]. Lipases

demonstrated lower hydrolytic activity against PET, comparing to cutinases. This

might be due to its lid structure covering the buried hydrophobic catalytic center,

and it prohibits the contact of aromatic polymeric substrates to the active site of the

enzymes [31]. Lipases from T. lanuginosus [31] and Candida antarctica [32] can also

degrade low-molecular-weight PET degradation products. Combination of lipases

from C. antarctica and cutinases from T. insolens improved the production of tereph-

thalic acid resulted from hydrolysis of PET [32].

9.3.6.3

Carboxylesterases (EC 3.1.1.1)

PET oligomers and their analogues can be degraded by carboxylesterases isolated

from Bacillus licheniformis, Bacillus subtilis, and Thermobifida fusca [33]. Car-

boxylesterases Tfca isolated from T. fusca can release water products from

high-crystalline PET fibers. Combination of carboxylesterase with polyester hydro-

lase exhibits inhibitory activity against low-molecular-weight degradation products

of PET because of their higher activity against PET oligomers [33].

9.3.6.4

Proteases

Research revealed that proteases isolated from Pseudomonas chlororaphis and P. flu-

orescens can degrade polyester PU [34]. Proteases such as papain are very active

against PU and may hydrolyze amide and urethane bonds. The porcine pancreatic

elastase can release degradation of products from polyester and polyester PU due to

the breakdown of hydrolyzable ester, urethane, and urea bonds in the soft segment

domains of the polymer.