Environmental Engineering Reference
In-Depth Information
Given the number and diversity of PHA synthase genes now available, family shuffling
approaches are being explored as well, and transacylase and hydratase genes are expected to
be targets in the near future [12].
Mathematical modeling, informed by microarray analysis, proteomics, and
metabolomics, is also suggesting new targets for additional metabolic engineering efforts.
Flux analysis, for example, recently elucidated the role of the Entner-Doudoroff pathway in
PHB production in E. coli, while other mathematical models have identified optimal substrate
switching strategies for the production of desired block copolymers [12].
3.1.3. Transgenic plants
. The detailed genetic understanding of PHA biosynthesis
pathways offers hope for the cost-effective production of PHAs in transgenic plants [32, 33].
Initial success in Arabidopsis thaliana suffered from a depletion of suitable substrate for
growth [34], but genetic manipulation led to more effective production in plant plasmids [35].
The construction of transgenic A. thaliana using the PHA syntheses gene of P. aeruginosa
indicates that plant fatty acids can generate a range of (R-)3-hydroxyalkanoate monomers that
can be used to synthesize medium molecular weight PHAs [36]. From a commercial
perspective, the copolymers P(3HB-co-3HV) and P(3HB-co-4HB) are attractive, and recent
reports show promise for the production of both monomers and polymers plants [16, 14].
Development of PHA-producing transgenic switchgrass is also underway, with the goal
of incorporating the synthesis into a more productive, easily-grown plant [37].
In 2001, Metabolix Inc. was awarded a $15 million “Industries of the Future” cost-shared
grant from DOE to help fund the development of a biomass biorefinery based on switchgrass.
The goal of this program is to produce PHAs in plants and, after polymer extraction, use the
residual plant biomass for fuel generation, thereby generating both materials and fuels from a
sustainable resource (http://www.metabolix.com/biotechnologypercent20foundation
/plants.html). Efforts are also underway to produce PHAs/PHBs in photosynthetic bacteria.
3.2. Physical Properties
The molecular weight (MW) of poly(3HB) produced from wild-type (unmodified)
bacteria usually ranges from 104 to 106 grams per mole, with a polydispersity of about 2 [38].
Within the bacteria, the polymers remain amorphous and are found as water-insoluble
inclusions [39, 40]. This fact is surprising, as the polyester only has (R)-configuration
stereochemical sites in the backbone and thus exhibits a perfectly isotactic structure.
Crystallinity is typically 55-80 percent in poly(3HB) isolated from bacteria [7].
3.2.1. Homopolymers
. The physical properties of amorphous, crystalline, and ultrahigh
MW poly(3HB) are tabulated below, showing comparisons with isotactic PP (Table 1) [41, 9,
42]. The primary difficulty with ordinary poly(3HB) is that it is a relatively brittle plastic, as
shown by the low extension to break value in comparison to polypropylene [43].
Films have been prepared from ultra-high molecular-weight poly(3HB) that show
improved mechanical properties when stretched, however, raising both the elongation to
break and tensile strength values [44, 45]. Mechanical properties have been improved further
upon annealing, as well as by the incorporation of structural variations made possible by
genetic engineering, with the result that new PHAs are promising candidates for further
commercial exploitation.
