7.5 Enzymes Responsible for Biodegradation

101

The oxidation of reactions makes a polymer more hydrophilic and susceptible to

enzymatic attack, further leading to the complete mineralization of the polymer.

7.4.1.2

Polyethylene Terephthalate (PET)

There are very few bacterial isolates known to be involved in the degradation of

PET, which generally includes Ideonella sakaiensis [24], Pseudomonas mendocina

[25], and Thermobifida fusca [26] along with some fungal communities which

include fusarium species and Humicola insolens [27]. The enzyme PET hydrolase

(the best-studied enzyme for PET degradation) has relatively lower turnover rates.

The enzymes involved in the PET degradation contain a C-terminal disulfide bond,

which helps in the attachment of organism with the hydrophobic surface.

Mechanism:

PET

PETase

−−−−−→mono(2 −hydroxyethyl)terephthalate

MHETase

−−−−−−−→terephthalic acid + ethylene glycol

Terephthalic acid is then internalized by a TPA transporter protein.

Terephthalic acid

TPA −1, 2 −dioxygenase (TPADO)

1, 2 −dihydroxy −3, 5 −cyclohexadiene −1, 4 −dicarboxylate dehydrogenase (DCDDH)

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→protochatechuic acid

Protochatechuic acid

PCA−3,4−dioxygenase (PCA34)

−−−−−−−−−−−−−−−−−−−−−−→4-carboxy-2-hydroxymuconic acid

(hemiacetal form)

4 −carboxy −2 −hydroxymuconic acid

dehydrogenase

−−−−−−−−−−→2 −pyrone −4, 6 −dicarboxylic acid

Finally, 2-pyrone-4,6-dicarboxylic acid enters TCA cycle.

7.4.1.3

Polystyrene (PS)

This high molecular weight, highly hydrophobic polymer, supports only partial

degradation. To date, no enzyme is reported to degrade polystyrene completely.

However, black-rot fungi Gloeophyllum Striatum and Gloeophyllum trabeum were

reported to employ hydroquinone-driven Fenton reaction to attack polystyrol

moiety. The degradation pathway involved mainly the oxidation of the styrene side

chain:

Styrene

styrene monooxygenase

−−−−−−−−−−−−−−−−→Styrene epoxide

styrene oxide

−−−−−−−−−→

Phenylacetaldehyde →Phenylacetic acid

yields

−−−−−→Phenylacetyl Co −A

Phenyacetyl Co-A enters TCA cycle, where the final products are acetyl Co-A and

succinyl Co-A.

7.5

Enzymes Responsible for Biodegradation

See Table 7.2.