Environmental Engineering Reference
In-Depth Information
oxidation), and lipid biosynthesis, through common intermediates and cofactors such as
acetyl-CoA, other fatty acyl CoAs, NADH, and NADPH.
PHA is synthesized under the direction of a family of enzymes that shows some diversity
among microorganisms but nevertheless possesses several conserved members. β-
ketothiolases harvest acetyl CoA from the TCA cycle, condensing it into acetoacetyl CoA;
transacylases similarly harvest intermediates from lipid biosynthesis, and enoyl CoA
hydratases divert enoyl CoA intermediates from β-oxidation. Where necessary, as with
acetoacetyl CoA, reductases then reduce keto groups to hydroxy groups. Each of these
enzymes thus provides monomers for PHA synthesis by the final enzyme in the pathway,
PHA synthase. The β-ketothiolases, transacylases, hydratases, reductases, and PHA synthases
are encoded by well-understood genes: PhaA, PhaG, PhaJ, PhaB, and PhaC, respectively. The
genetic understanding of this system, as well as its connectedness to other well-understood
metabolic pathways, has made PHA biosynthesis an ideal target for metabolic engineering.
Once the PHA is synthesized, the microbes are harvested, disrupted, and fractionated to
isolate the polyester. To produce PHA materials with desirable physical properties, a variety
of approaches including copolymerization, blending with other polymers, cross-linking, and
the introduction of functional groups have been studied extensively. PHAs can be produced in
a wide variety of molecular configurations; homopolymers, copolymers, and functionalized
polymer chains may all be created by utilizing various microorganisms and fermentation
conditions. A representative sample is shown in Figure 12 [18].
Figure 12. Chemical structures of PHAs and degradation rates in aqueous solutions at 37 °C containing
depolymerases [18].
