120
4 Polyhydroxyalkanoates
Because of limited petroleum resources, the scientific community is trying to produce biodegradable and environment benign polymers. Polyhydroxyalkanoates
(PHAs) are the most important candidates in terms of their properties and
biodegradability. Since 1980, various companies have produced PHAs at large scale
(summary in Table 2). PHAs are mostly used for packaging and biomedical
applications such as drug delivery and tissue engineering [13, 16]. Some prominent
examples of various PHAs and their biosynthetic routes are discussed as follows:
4.1 Polyhydroxybutyrates (PHB)
Polyhydroxybutyrates (PHBs) are biopolymers that are synthesized by many types
of bacteria. PHB (Fig. 4) is a reserve linear polymer having R-(−)-3-hydroxybutyric
acid as a repeating monomer unit (act as a chiral center), which is attributed to its
optical activity. It was first discovered by French bacteriologist M. Lemoigne of the
Institut Pasteur in 1923 as a reserve material in a bacterial cell, and later, it was
confirmed by Stanier and co-workers that the reserve serves as intracellular food
and energy source to prevent starvation when essential elements are not available.
4.2 Synthesis of PHB
The synthesis of PHB can be achieved in bacterium A. eutrophus (also R. eutropha)
starting from acetyl-CoA using a sequential reaction of three different enzymes:
3-ketothiolase (phbA gene), acetoacetyl-CoA reductase (phbB gene), and PHB
synthase (phbC gene). The biosynthetic pathway of PHB production is illustrated in
Fig. 5. In the first step, 3-ketothiolase promotes the formation of acetoacetyl-CoA
moiety by Claisen condensation of two molecules of acetyl-CoA. Then in the
second step, NADPH-dependent acetoacetyl-CoA reductase catalyzes the stereoselective reduction of acetoacetyl-CoA formed in the first step to R-(−)-3hydroxybutyryl-CoA. In the final stage, the enzyme PHB synthase polymerizes
R-(−)-3-hydroxybutyryl-CoA to form PHB.
4.3 Properties of PHB
PHB is a promising biodegradable plastic and can be an excellent alternative to
petrochemical plastics. This is due to its biocompatibility, biodegradability, and
versatile properties that make it an eco-friendly substitute for synthetic polymers.
M. A. Ali et al.
4 Polyhydroxyalkanoates
Because of limited petroleum resources, the scientific community is trying to produce biodegradable and environment benign polymers. Polyhydroxyalkanoates
(PHAs) are the most important candidates in terms of their properties and
biodegradability. Since 1980, various companies have produced PHAs at large scale
(summary in Table 2). PHAs are mostly used for packaging and biomedical
applications such as drug delivery and tissue engineering [13, 16]. Some prominent
examples of various PHAs and their biosynthetic routes are discussed as follows:
4.1 Polyhydroxybutyrates (PHB)
Polyhydroxybutyrates (PHBs) are biopolymers that are synthesized by many types
of bacteria. PHB (Fig. 4) is a reserve linear polymer having R-(−)-3-hydroxybutyric
acid as a repeating monomer unit (act as a chiral center), which is attributed to its
optical activity. It was first discovered by French bacteriologist M. Lemoigne of the
Institut Pasteur in 1923 as a reserve material in a bacterial cell, and later, it was
confirmed by Stanier and co-workers that the reserve serves as intracellular food
and energy source to prevent starvation when essential elements are not available.
4.2 Synthesis of PHB
The synthesis of PHB can be achieved in bacterium A. eutrophus (also R. eutropha)
starting from acetyl-CoA using a sequential reaction of three different enzymes:
3-ketothiolase (phbA gene), acetoacetyl-CoA reductase (phbB gene), and PHB
synthase (phbC gene). The biosynthetic pathway of PHB production is illustrated in
Fig. 5. In the first step, 3-ketothiolase promotes the formation of acetoacetyl-CoA
moiety by Claisen condensation of two molecules of acetyl-CoA. Then in the
second step, NADPH-dependent acetoacetyl-CoA reductase catalyzes the stereoselective reduction of acetoacetyl-CoA formed in the first step to R-(−)-3hydroxybutyryl-CoA. In the final stage, the enzyme PHB synthase polymerizes
R-(−)-3-hydroxybutyryl-CoA to form PHB.
4.3 Properties of PHB
PHB is a promising biodegradable plastic and can be an excellent alternative to
petrochemical plastics. This is due to its biocompatibility, biodegradability, and
versatile properties that make it an eco-friendly substitute for synthetic polymers.
M. A. Ali et al.