Biomedical Engineering Reference
In-Depth Information
2.4
Biosynthesis and Biodegradability of Poly(3-Hydroxybutyrate) and Other
Polyhydroxyalkanoates
2.4.1
Polyhydroxyalkanoates Biosynthesis on Microorganisms
Various microorganisms can accumulate a large amount of PHA inside their cells
in response to the limitation of an essential nutrient. Many previous works were
concerned with the control of the synthesis of PHA under unbalanced growth
conditions [7] .
Since the discovery of PHA-producing microorganisms by Lemoigne in 1925,
there are over 300 types of microorganisms that accumulate PHA, belonging to
the genus
Alcaligenes
,
Azobacter
,
Pseudomonads
, methylotrophs, and some recom-
binant microorganisms such as
E. coli
[13] . The Gram - negative bacteria
C. necator
has been the most widely used microorganism for the production of P(3HB).
C. necator
was previously categorized as
Hydrogenomonas eutropha, A. eutrophus,
R. eutropha,
and
Wautersia eutropha
[69] .
C. necator
has also been used for the
commercial production of P(3HB) by many industries [18, 34, 40].
Among the substrates required for PHA production, the carbon source has a
primal signifi cance in the case of P(3HB) production, since P(3HB) is composed
only of C, H, and O atoms [70]. Microorganisms have the ability to produce PHA
from various carbon sources including inexpensive and complex waste effl uents.
In the past years, our group has prepared works [11, 71-74] in order to reduce the
production costs of P(3HB) and its copolymers by the use of renewable carbon
sources.
Figure 2.2 shows P(3HB) (a PHA
SCL
- short chain length PHA) production by
C. necator
in two phases: balanced growth and unbalanced growth (Pathways I
and II, respectively); P(3HB-
co
-3HV) production from propionigenic substrates by
C. necator
(Pathways II and III); and PHA
MCL
(medium chain length PHA) produc-
tion from fatty acid
de novo
biosynthetic route and fatty acids
β
- oxidation (Pathways
IV and V, respectively) according to the substrate.
P(3HB) production by
C. necator
occurs in two phases. The fi rst phase comprises
the exponential growth where all nutrients are present (balance growth - Pathway
I in Figure 2.2) and the second phase shows a nutritional limitation of N, P, S,
Mg, or O
2
in the presence of an excessive carbon source (unbalanced
growth - Pathway II in Figure 2.2) [75]. Hence, the metabolism for the biomass
production during balanced growth catabolizes carbohydrates via the Entner-
Doudoroff pathway to pyruvate, which can be converted through dehydrogenation
to acetyl-CoA. During reproductive growth (Pathway I), acetyl-CoA enters the tri-
carboxylic acid (TCA) cycle, releases CoASH, and is terminally oxidized to CO
2
generating energy in the form of ATP, reducing equivalents (NADH, NADPH,
and FADH
2
) and biosynthetic precursors (2-oxoglutarate, oxalacetate) [76]. Direct
amination or transamination of the oxalocetate leads to the synthesis of amino
