Biomedical Engineering Reference
In-Depth Information
two forms - a soluble form in the cytoplasm (balanced growth) and associated with
P(3HB) granules (unbalanced growth) [5].
Based on the types of monomer incorporated into PHA, various metabolic
pathways have been shown to be involved in the generation of these monomers
[12, 81] .
Biosynthesis of poly(3 - hydroxybutyrate -
co
- 3 - hydroxyvalerate) P(3HB -
co
- 3HV)
requires, besides 3HB - CoA (3 - hydroxybutyryl - CoA), also 3 - hydroxyvaleryl - CoA
(3HV-CoA). The latter is also required if other copolyesters containing 3HV or
poly(3HV) homopolyester are synthesized. 3HV- CoA ([R] - 3 - hydroxyvaleryl - CoA) is
obtained from the condensation of acetyl - CoA and propionyl - CoA into 3 - ketovaleryl -
CoA (Figure 2.2, Pathways II and III) and a subsequent reduction in the condensa-
tion product to 3HV-CoA. The specifi c substrates for P(3HB-
co
- 3HV) production
can be propionic acid [7, 11, 82], valeric acid, heptanoic acid, or nonanoic acid.
Steinbüchel and Lütkte-Eversloh [12] cited many other sources for poly-(3HV)
production. Among them,
n
- pentanol (metabolized from
Paracoccus denitrifricans
),
valine, isoleucine, threonine, and methionine are considered precursor substrates
for 3HV containing PHA.
Fatty acid
de novo
biosynthesis (Figure 2.2, Pathway IV) is the main route during
growth on carbon sources, like gluconate, acetate, or ethanol, that are metabolized
to acetyl-CoA, for the PHA
MCL
synthesis by pseudomonads like
Pseudomonas
putida, P. aeruginosa
,
P. aureofaciens, P. citronellolis,
and
P. mendocina
[81, 83, 84].
From the results of labeling studies, nuclear magnetic resonance spectroscopy,
and gas chromatography mass spectroscopy ([85, 86], cited by [84]), authors con-
cluded that the precursors of PHA
MCL
biosynthesis from simple carbon sources
are predominantly derived from (
R
) - 3 - hydroxyacyl - ACP intermediates occurring
during the fatty acid
de novo
biosynthetic route. Since the constituents of P(3HB)
and PHA represent the
R
confi guration and since PHA
SCL
and PHA
MCL
synthases
are highly homologous, the intermediates in fatty acid metabolism are presumably
converted to (
R
) - 3 - hydroxyacyl - CoA before polymerization (Figure 2.2 , Pathway V).
Nevertheless, some other routes of PHA synthesis are also possible. Other conceiv-
able alternatives are the release of free fatty acids by the activity of a thioesterase
with a thiokinase, subsequently activating these fatty acids to the corresponding
hydroxyacyl-CoA thioesters or chain elongation with 3-ketothiolase, or
β
- oxidation
of synthesized fatty acids.
2.4.2
Plants as Polyhydroxyalkanoates Producers
Another important example for establishing PHA biosynthesis is the production
by plants. With this strategy, the steps necessary to produce the substrates used
in a fermentative process are no longer required, as naturally occurring carbon
dioxide and sunlight serve as carbon and energy sources, respectively [6]. In the
fi rst investigations reported, the plant
Arabidopsis thaliana
, harboring the PHA
genes of
C. necator
, was used to produce P(3HB). An endogenous plant
3-ketothiolase is present in the cytoplasm of this plant as part of the mevalonate
