Modification of Polyhydroxyalkanoates (PHAs) - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 167

COOH
OH

COOH
OH

CO O H
OH

d n 2 r 4 n g | 1

OH

OH OH COOH

CO O H

COOH

(a) (b)

Figure 7.13 Schematic folding structure of (a) linear PHA, (b) branched PHA.

Several key factors in enzymatic catalysis include their activity, stability,
substrate concentration and temperature. Optimization of these factors will
steer the enzyme towards the desired activity. Media pH was shown to affect
the ionizable groups at the active site of the enzyme. 138 Gangoiti et al. 139
reported that P(3HO) depolymerase showed its maximum activity at pH 9.5
and a temperature of 40 1C.

7.4.2.2 Lipases
Lipases are subclass of esterases, capable of hydrolyzing esters, fats and
lipids in aqueous media, hence they are described as hydrolases belonging
to class EC 3.1.1.3. Lipases are one of the most versatile enzymes because
they can catalyze many different reactions such as esterification, inter-
esterification, hydrolysis, alcoholysis, peroxidation, aminolysis, and epox-
idation. 140,141 Most lipases have similarities in their amino acid sequence
including within the catalytic region, His-X-Y-Gly-Z-Ser-W-Gly or Y-Gly-His-
Ser-W-Gly where W, X, Y and Z refer to unspecified amino acid residues. 142
These enzymes are stereospecific towards ester bonds thereby eliminating
any undesirable by-products of the reaction. 143 Lipase has been used
extensively as a biocatalyst in industry because of its high thermal
stability, versatile pH range and it can be used repeatedly if immobilized
(e.g. Novozyme 435). 144
In PHA modification, Mukai et al. 136 reported that lipases originated from
eukaryotes have broad specificities with the abilities to erode P(3HP),
P(4HB), P(5HV) and P(6HH) films compared to prokaryote lipases, which
could barely degrade all the polymers except P(3HP). This shows that lipases
from prokaryotes have high substrate specificities for the hydrolysis of PHA.
Jaeger et al. 135 indicated that Pseudomonas alcaligenes, Pseudomonas fluor-
escens, Pseudomonas aeruginosa, Burkholderia lemoignei and Bacillus subtilis
lipases show specificities towards o-hydroxyalkanoic acid over a-hydro-
xyalkanoic acid. The absence of alkyl side chains in the polymer backbone

.