Biomedical Engineering Reference
In-Depth Information
[15], biodegradable plastics still have minimal participation in the market because
of the high cost compared to fuel-based polymers. Therefore, many research
groups are conducting studies to reduce the production costs of biopolyesters by
using low- cost substrates [11, 16 - 19] , large - scale fermentation methods [19 - 22] ,
and metabolic engineering to develop strains with higher productivity and capable
of assimilating renewable carbon sources. In addition, the PHA granule has
various protein-based functions and has attracted interest due to the utilization
of these bionanoparticles in medical and biotechnological applications [23, 24].
2.2
History
Beijerinck [25] observed granules in a microscope inside
Rhizobium
cells. Such
granules were present in the “bacteroides” isolated from nodules and were
described as being extremely refractile globules. Another microbiologist, Lem-
oigne [26], noticed that, when cultures of
Bacillus subtilis
were followed by autolysis
in distilled water, the pH value decreased because of the formation of an unknown
acid. This acid was subsequently identifi ed as monomer of poly-
- hydroxybutyric
acid [27]. In the same period, Stapp [28], analyzing the results of other researchers,
suggested that
Azotobacter chroococcum
inclusions could be easily extracted with
chloroform, and identifi ed this structure as poly-
β
- hydroxybutyrate. In 1958, the
functional P(3HB) pathway was proposed by Macrae and Wilkinson [29]. They
observed that
Bacillus megaterium
stored the polymer especially when the ratio
glucose/nitrogen in the medium was high, and that the subsequent degradation
occurred quickly with the absence of the carbon source. PHA's potential useful-
ness has been recognized since the fi rst half of the 1960s through patents related
to P(3HB) production process [30]; extraction from the producing biomass [31];
plasticization with additives [32]; the use unextracted as a polymer mixed with
other cell material [33]; and pure for absorbable prosthetic devices [34]. In a review
about the regulatory role and energy resource microorganisms, published in 1973
by Dawes and Senior [35], P(3HB) was found to be a microbial resource material
storage as starch and glycogen. In the period between 1974 [36] and 1989 [37],
other hydroxyalkanoates (HAs) have been identifi ed besides 3HB, such as
4 - hydroxybutyrate (4HB), 3 - hydroxyhexanoate (3HH
x
), 3 - hydroxyoctanoate (3HO),
3-hydroxyvalerate (3HV), among others. The identifi cation of copolymer poly(3-
hydroxybutyrate -
co
- 3 - hydroxyvalerate) (P[3HB -
co
-3HV]) has led to a positive impact
on research and commercial interest because the homopolymer (P[3HB]) is brittle
and has a low extension break. This lack of fl exibility limits its range of application
in relation to the copolymer, which has a much lower melting point and is less
crystalline [38]. The industrial production of these polymers began in 1980 by the
UK chemical group Imperial Chemical Industry (ICI) [39]; after that, it started to
be produced by others industries (Table 2.1 ).
In the past few decades, PHA researchers have been living a period of interest
for metabolic engineering [40], and site-directed mutagenesis of the enzymes
β
