9.3 Biodegradation of Plastics

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Most plastics are generally polyesters that are normally catalyzed by microbial

enzymes such as cutinases or esterases. A particular set of enzymes is involved

in the degradation of different plastic materials. There are a number of enzymes

found to be involved in plastics degradation process. In the PLA degradation

processes, protease enzyme mainly targets PLA and depolymerase is responsible

for the first degradation of long-chain polymer. After that, serine proteases, i.e.

protease K and trypsin, further degraded it to low-molecular-weight compounds.

Research revealed that the proteases (the PLA degrading enzyme) may found in

Amycolatopsis, Saccharothrix, and Pseudonocardia, etc. However, these proteases

can degrade PLA only [19].

On the other hand, cutinases have the ability to degrade PCL, PLA, and PET, etc.

and similarly, lipases may degrade PCL, PLA, and polybutylene succinate (PBS), etc.

It is noticeable that a variety of microbial enzymes can degrade plastics. However, a

single plastic material can be degraded by different enzymes. Furthermore, a wide

variety of plastics may be degraded by plenty of bacteria. Besides bacteria, fungi

also use and adhere to plastic materials by decreasing hydrophobicity and forming

a number of chemical bonds. These chemical bonds include carboxyl, carbonyl, and

functional groups of ester. Some fungus such as Penicillium funiculosum, A. fumiga-

tus, and Pseudomonas fluorescens can degrade 10 or more types of plastics. However,

more than 30 species of microorganisms are reported to degrade PE, PU, and PHB.

On the other hand, PCL and PLA can be degraded by over 20 types of microbes [19].

9.3.6

Enzymes Involved in the Plastic Biodegradation

Each living cell, including the microorganisms, contains diverse enzymes that

vary with different species or strains of same species. Thus, the process of plastic

biodegradation involves different enzymes. Several studies report on the use of

enzyme extracted from microorganisms in the process of degradation of plastics.

Table 9.1 displays the examples including that of lignin-degrading enzymes such as

laccase, manganese-depending peroxide and hydrolyase such as urease, protease,

and lipase.

Until now, only 79 established microbial enzymes have been recognized to act as

degrading agent for plastics. Very few researchers have studied the potential mecha-

nism of this degrading activity of the microbial enzymes. Most of them suggest that

the bond cleavage step during hydrolysis process is the main mechanism for degra-

dation of polymers by microbial enzymes. Some studies also indicated mechanism of

affection of microbial enzymes to the polymer surfaces, and mechanism of entrance

of large molecules of polymers to the active site of enzymes [27].

However, increase in chain flexibility of polymers might increase the rate of

hydrolysis of PBAT by lipase form Rhizopus oryzae, and cutinase form Fusarium

solani. Enzyme with higher available active sites has higher hydrolysis tendency

against PBAT [27]. A research demonstrated that combination of cutinases

and a polymer binding segment might heighten the hydrolysis the polyester

poly(1,4-butylene adipate), which recognized as better binding between enzymes

and polymers [28].