Polyhydroxyalkanoates: Structure, Properties and Sources - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 20

In this chapter, recent developments in the field of PHA carbon sources,
their properties and relations between sources, structure and properties are
discussed.

d n 2 r 4 n g | 8

2.2 Sources of PHAs
The biosynthesis of PHA SCL and PHA MCL is performed following different
metabolic pathways (Figure 2.2). 15-17 Thus, PHA SCL are synthesized in three
main steps starting with acetyl-CoA, whereas PHA MCL can be synthesized by
two different pathways. The first pathway consists of the b-oxidation of fatty
acids prior to their incorporation in the polyester chain, the second consists
of the de novo synthesis of fatty acids. 15
Several enzymes are involved in these metabolic pathways. In the case of
PHA SCL , the genes encoding PhaA, PhaB and PhaC are often co-localized and
organized in an operon, whereas in the case of PHA MCL , genes encoding
PhaG and PhaJ are not co-localized, but co-regulated with PhaC. 15
These different metabolic pathways show the importance of different
carbon sources. Indeed, different carbon sources can be used for the
production of PHA SCL , whereas the choice of the carbon source deeply
influences the structure of the resulting PHA MCL , therefore this category of
PHAs is discussed in the section on structure.
Several microorganisms are able to produce PHA SCL , however, the
Cupriavidus genus is by far the most studied for this purpose. Numerous
reasons justify this choice: the genus is rather robust, accepts different

.

Fatty acid
de novo
synthesis

Fatty acid
β-oxidation

Acetyl-CoA

β-Ketothiolase

PhaA

Enoyl-CoA

Acetoacetyl-CoA

(R)-3-hydroxyacyl-ACP

Acetoacetyl-CoA
reductase
PhaB

(R)-specific
Enoyl-CoA hydratase

Transacylase

PhaJ

PhaG

(R)-3-hydroxybutyryl-CoA

(R)-3-hydroxyacyl-CoA

Polyester synthase

Polyester synthase
PhaC

PhaC

PHB

PHA MCL

Figure 2.2 Biosynthetic pathways for PHA synthesis.