Polyhydroxyalkanoates: Structure, Properties and Sources - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 27

acids, and even mercaptoalkanoic acid, that are known as constituents
of these PHAs. 10 The production of PHA SCL ,PHA MCL or PHA LCL is directly
linked to the microorganism host and the specificities of the enzymes (PHA
synthases). PHAs can be synthesized as homopolymers or copolymers or
even their blends, depending on bacterial strains employed or growth sub-
strates, especially the PHA synthase that the strain contains. 52
For instance, Ralstonia eutropha is well established as one of the best
known bacteria among PHA SCL -producing microorganisms and has several
advantages for ecient PHA production as a recombinant host strain. 53,54
R. eutropha can intrinsically synthesize PHAs from various renewable
inexpensive carbon sources, sugars and plant oils. However R. eutropha
produces only PHA SCL .
Depending on the subunit compositions and substrate specificities of the
enzymes, PHA synthases are generally classified into four groups: class I, II,
III, and IV 14 . Class I and II PHA synthases are composed of one subunit
enzyme, PhaC. Class I PHA synthases such as Ralstonia eutropha and
Alcaligenes latus, have molecular weights (Mw) between 61 and 68 kDa.
According to their in vivo substrate specificity, type I PHA synthases prefer-
entially utilize coenzyme A thioesters of various 3HA SCL comprising three to
five carbon atoms, 55 while class II PHA synthases mainly from pseudomonas
display substrate specificity towards 3HA MCL . Some class II PHA synthases
from Pseudomonas sp. 61-3 and Pseudomonas sp. 6-19 accept both SCL- and
MCL monomers, with weak activity towards SCL monomers. Class III PHA
synthases are composed of two different subunits, PhaC and PhaE.
These subunits have marginal sequence homology to class I and II
PHA synthases; the PhaC subunit displays only 20-30% homology, for ex-
ample. Class III PHA synthases are highly specific for 3HA SCL , but do accept
3HA MCL when expressed in some pseudomonas. Class IV PHA synthases
are composed of two different subunits, PhaC and PhaR, which are
usually found in Bacillus strains producing PHB. Various HA-CoAs are the
substrates of the PHA synthases and undergo polymerization to PHA poly-
mer granules in the cells. 14,56-58 Table 2.4 shows representative PHA syn-
thases used for the synthesis of PHA consisting of both SCL and MCL
monomers. 14
In spite of PHB being considered an environmental friendly polymer with
similar material properties to polypropylene (PP), it has not been used on a
large scale to replace conventional polymers because it presents some
drawbacks in its mechanical properties. Considering polymer mechanical
properties, it is important to consider three basic properties when com-
paring the usefulness of a polymer for a given commodity application. The
elongation at failure is a measurement of toughness and reflects the total
deformation that the polymer can withstand before fracture. The Young's
modulus is a measure of stiffness that considers the slope of the stress re-
sponse to deformation at very low strains. Steeper slopes at low deform-
ations are indicative of stiffer materials. Finally, the ultimate tensile strength
is a measure of the maximum strength of the material prior to the onset of

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