PHA biopolymers are mainly produced by:
Polyhydroxyalkanoates represent family of biodegradable thermoplastic polyesters accumulated intracellularly as carbon and energy storage granules 0.2 to 0.5 micrometer diameter by numerous bacteria under unbalanced growth with excess carbon but limitation of nitrogen, phosphate, magnesium or oxygen. While recombinant Escherichia coli engineered with phaCAB operon can synthesize PHA, native high producers remain Pseudomonas putida, Pseudomonas oleovorans producing medium-chain-length PHA containing monomers C6 to C14 via beta-oxidation pathway involving PhaJ enoyl-CoA hydratase linking fatty acid degradation to PHA synthesis, and Ralstonia eutropha reclassified as Cupriavidus necator synthesizing short-chain poly-3-hydroxybutyrate homopolymer from acetyl-CoA. Pathway involves condensation of two acetyl-CoA to acetoacetyl-CoA by beta-ketothiolase PhaA, reduction by NADPH-dependent acetoacetyl-CoA reductase PhaB to 3-hydroxybutyryl-CoA, polymerization by PHA synthase PhaC class I. Granule-associated proteins phasins PhaP regulate surface. Accumulated up to 80 percent dry weight, biopolyesters exhibit properties similar to polypropylene and utility for packaging, medical sutures and drug delivery. Molecular analysis reveals PhaP phasins controlling granule size 50 to 500 nanometer and PhaR transcriptional repression relieved by PHA monomer binding, coordinating carbon storage with cell division cycle and stress response for survival during nutrient fluctuations in soil habitats.
Ref: Steinbuchel & Lutke-Eversloh 2003 PHA biosynthesis Appl Microbiol Biotech; Nature Reviews Microbiology 2005 biopolymers.