11 carbons. Being a polyester, PHA is formed when the carboxyl group of one
monomer established an ester bond with the adjacent monomer’s hydroxyl group
inside the microorganisms. The term biopolymer also clarifies that PHA can be
degraded with the help of microorganisms (Lee 1996). The unique part is PHA was
proven to be biodegraded even in the marine system (Thellen et al. 2008). Its
biodegradability in the marine system has made PHA outshine other bio-based
plastics such as polylactide (PLA) and polybutylene succinate (PBS). This is
because marine microbes possess PHA depolymerase (Mukai et al. 1993; Volova
et al. 2011) so it can degrade PHA, whereas other types of biopolymers might take
quite a long time to be degraded. Such unique property of PHA is therefore expected
to solve the worrying plastic pollution in the marine system.
3.2
Biosynthesis of PHA
Poly[(R)-3-hydroxybutyrate)], P(3HB), was the first member of the PHA family to
be discovered. It was discovered in Bacillus megaterium by Lemoigne in 1926
(Lemoigne 1926). Following this discovery, many other researchers also have
reported the occurrence of P(3HB) in other microorganisms (Stapp 1924; Forsyth
et al. 1958); hence, it hints the role of PHA as a carbon and energy source for bacteria
(Doudoroff and Stanier 1959).
PHA is accumulated in a bacterium when it is exposed to a stressful condition
mainly to support their survival by acting as an alternative energy source (Doudoroff
and Stanier 1959). For example, under nitrogen-limiting condition, TCA cycle in a
bacterium will be affected and elevates the level of acetyl-coenzyme A (acetyl-CoA),
and the increased amount of acetyl-CoA will be then channeled for the synthesis of
PHA monomer (Wang et al. 2009). Therefore, under unconducive environment
bacteria can produce PHA to continue its survival.
In earlier days, 3HB was thought to be the only type of hydroxyalkanoate (HA) to
occur in microorganisms. This well-perceived fact was proved to be wrong with
Wallen and Rohwedder’s finding on the occurrence of 3-hydroxyvalerate (3HV) and
3-hydroxyhexanoate (3HHx) in domestic sewage sludge in 1974 (Wallen and
Rohwedder 1974). Their finding is indeed a watershed breakthrough because it
has intrigued many researchers to look for other new HA units that as for to date
there are nearly 160 monomer units (Choi et al. 2020) that have been reported. PHAs
are grouped into three groups based on the number of carbon atoms in the monomer.
Short-chain-length PHA (scl-PHA) contains 3–5 carbon atoms; medium-chainlength PHA (mcl-PHA) contains 6–14 carbon atoms; and long-chain-length PHA
(lcl-PHA) contains more than 14 carbon atoms (Anderson and Dawes 1990;
Steinbüchel 1992). A lot of bacteria are known to be the natural PHA
producers (Taguchi et al. 2005), and via genetic approaches, many other bacteria
are now able to synthesize PHA (Fidler and Dennis 1992); thus, a myriad of bacteria
have the capability to produce PHA under specific cultivation condition. Among the
numerous PHA producers, Ralstonia eutropha (also known as Cupriavidus necator)
is the best-studied microorganism (Reinecke and Steinbüchel 2009) and is able to
3 Development of Polyhydroxyalkanoate (PHA) and Its Copolymers as a Possible. . .
61
monomer established an ester bond with the adjacent monomer’s hydroxyl group
inside the microorganisms. The term biopolymer also clarifies that PHA can be
degraded with the help of microorganisms (Lee 1996). The unique part is PHA was
proven to be biodegraded even in the marine system (Thellen et al. 2008). Its
biodegradability in the marine system has made PHA outshine other bio-based
plastics such as polylactide (PLA) and polybutylene succinate (PBS). This is
because marine microbes possess PHA depolymerase (Mukai et al. 1993; Volova
et al. 2011) so it can degrade PHA, whereas other types of biopolymers might take
quite a long time to be degraded. Such unique property of PHA is therefore expected
to solve the worrying plastic pollution in the marine system.
3.2
Biosynthesis of PHA
Poly[(R)-3-hydroxybutyrate)], P(3HB), was the first member of the PHA family to
be discovered. It was discovered in Bacillus megaterium by Lemoigne in 1926
(Lemoigne 1926). Following this discovery, many other researchers also have
reported the occurrence of P(3HB) in other microorganisms (Stapp 1924; Forsyth
et al. 1958); hence, it hints the role of PHA as a carbon and energy source for bacteria
(Doudoroff and Stanier 1959).
PHA is accumulated in a bacterium when it is exposed to a stressful condition
mainly to support their survival by acting as an alternative energy source (Doudoroff
and Stanier 1959). For example, under nitrogen-limiting condition, TCA cycle in a
bacterium will be affected and elevates the level of acetyl-coenzyme A (acetyl-CoA),
and the increased amount of acetyl-CoA will be then channeled for the synthesis of
PHA monomer (Wang et al. 2009). Therefore, under unconducive environment
bacteria can produce PHA to continue its survival.
In earlier days, 3HB was thought to be the only type of hydroxyalkanoate (HA) to
occur in microorganisms. This well-perceived fact was proved to be wrong with
Wallen and Rohwedder’s finding on the occurrence of 3-hydroxyvalerate (3HV) and
3-hydroxyhexanoate (3HHx) in domestic sewage sludge in 1974 (Wallen and
Rohwedder 1974). Their finding is indeed a watershed breakthrough because it
has intrigued many researchers to look for other new HA units that as for to date
there are nearly 160 monomer units (Choi et al. 2020) that have been reported. PHAs
are grouped into three groups based on the number of carbon atoms in the monomer.
Short-chain-length PHA (scl-PHA) contains 3–5 carbon atoms; medium-chainlength PHA (mcl-PHA) contains 6–14 carbon atoms; and long-chain-length PHA
(lcl-PHA) contains more than 14 carbon atoms (Anderson and Dawes 1990;
Steinbüchel 1992). A lot of bacteria are known to be the natural PHA
producers (Taguchi et al. 2005), and via genetic approaches, many other bacteria
are now able to synthesize PHA (Fidler and Dennis 1992); thus, a myriad of bacteria
have the capability to produce PHA under specific cultivation condition. Among the
numerous PHA producers, Ralstonia eutropha (also known as Cupriavidus necator)
is the best-studied microorganism (Reinecke and Steinbüchel 2009) and is able to
3 Development of Polyhydroxyalkanoate (PHA) and Its Copolymers as a Possible. . .
61