Polyhydroxyalkanoates: Structure, Properties and Sources - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 28

Table 2.4 Representative PHA synthases used for the synthesis of PHAs consisting
of both SCL and MCL monomers. 14

PHA
synthase Strain Characteristics Monomer composition

d n 2 r 4 n g | 8

Pseudomonas
sp. 61-3

phaC1 Natural 3HB, 3HHx, 3HO, 3HD,

3HDD

phaC1 Combinatorial mutations

in E130, S325, S477,
Q481, Q508

LA, 3HB, 3HHx, 3HO,
3HD, 3HDD

Pseudomonas
sp. MBEL 6-19

phaC2 Natural 3HB, 3HHx, 3HO, 3HD,

3HDD

phaC1 Combinatorial mutations

in E130, S325, S477, Q481

LA, 3HB, 3HHx, 3HO,
3HD, 3HDD

P. aeruginosa phaC1 Combinatorial mutations

in E130, S325, S477,
Q481,

LA, 3HB, 3HHx, 3HO,
3HD, 3HDD

P. putida phaC1 Combinatorial mutations

in E130, S325, S477,
Q481,

LA, 3HB, 3HHx, 3HO,
3HD, 3HDD

P. chlororaphis phaC1 Combinatorial mutations

in E130, S325, S477, Q481

LA, 3HB, 3HHx, 3HO,
3HD, 3HDD

P. resinovorans phaC1 Combinatorial mutations

in E130, S325, S477, Q481

LA, 3HB, 3HHx, 3HO,
3HD, 3HDD

A. caviae phaC Natural 3HB, 3HHx

phaC Combinatorial mutations

in N149, D171, A505

3HB, 3HHx, 3HO

A. hydrophila phaC Natural 3HB, 3HHx

Rhodococcus
aetherivorans

phaC Natural 3HB, 3HHx

.

plastic deformation. This value is obtained from the maximum of the initial
peak in the stress versus strain diagram. While the modulus (stiffness) and
ultimate tensile strength (overall strength) of PHB and related copolymers
are suitable for many commodity-type uses, a major setback in the com-
mercialization of these materials relates to the extremely brittle character of
these polymers and very low flexibility. 59,60 For instance, PHB and PHBV
materials have been shown to fracture at less than 10% elongation. 18
Also, it is hard to process PHB due to its high melting temperature of
approximately 170 1C, which is very near to its degradation temperature.
Therefore, a solution to these drawbacks could be the copolymerization of
3-HB with other monomers that confer less stiffness and tougher properties
(which bestow greater flexibility and lessen breakage) and to reduce the
melting point. PHA copolymers such as poly(hydroxybutyrate-co-3-hydro-
xyvalerate) [P(3HB-co-3HV)], poly(hydroxybutyrate-co-4-hydroxybutyrate)
[P(3HB-co-4HB)], poly(hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-
3HHx)], and poly(hydroxybutyrate-co-3-hydroxyalkanoate) [P(3HB-co-3HA)]
have been examined extensively as candidates with such material prop-
erties. 14,61 Because class I, III, and IV PHA synthases have the ability to
naturally accept 3HBCoA, 3HV-CoA, and 4HB-CoA as substrates,