Polyhydroxyalkanoates (PHAs), also referred to as bioplastics or biopolyesters have been biosynthesized through metabolic transformation of various carbon sources and composed of (R)-3-hydroxy fatty acids (Allen et al., 2010). These biopolyesters have been produced by many bacteria and some archaea, recombinant bacterial strains and recombinant eukaryotes. In native PHA producing microorganisms, these polyesters have been produced as intracellular water-insoluble inclusions and functioned as carbon storage polymers and energy reserves. Many PHA biopolyesters have also showed interesting properties, such as biodegradability and a wide array of uses, similar to petroleum based commodity plastics (Sudesh et al., 2000). Properties of PHAs have been defined by the number of carbon atoms in the individual monomer units and the physical structure of monomers followed by their incorporation into biopolymer chains by microbial enzymes (Jain et al., 2010).
Though several pathways for PHA biosynthesis have been suggested, but out of them, only three pathways were well established. Pathway I was the most well characterised and studied pathway for the biosynthesis of short chain length PHAs in Cupriavidus necator, while Pathways II and III were reported to utilized by bacteria such as Pseudomonas aeruginosa for biosynthesis of medium chain length PHAs (Tan et al., 2014). Biosynthesis of the most common short chain length PHA, i.e. polyhydroxybutyrate (PHB), has been carried out by three key enzymes: i) Î²-ketothiolase (encoded by gene PhaA), ii) acetoacetyl-CoA reductase (encoded by gene PhaB) and iii) PHA synthase (encoded by gene PhaC) (Jain et al., 2014a). Acetyl coenzyme-A as a staring molecule has been undergone condensation by PhaA and reduced to precursor molecule (R)-3-hydroxybutyryl-CoA by PhaB and finally polymerised into PHA by PhaC. PHA synthase has been found covalently attached to the core of the PHA granule and considered as an important tag for anchoring proteins on the surface of PHAs. In native producers of PHB, phasins (PhaP) has been contained up to 5% of the intracellular proteins and known as the most abundant granule-associated protein (Mezzina et al., 2014). Pathway II and III have been found to utilize intermediates generated from the fatty acid Î²-oxidation pathway, from related and non-related carbon sources. In these pathways, various Î²-oxidation metabolic intermediate compounds such as alkanes, alkenes and alkanoates have been converted into (R)-3-hydroxyacyl-CoA thioesters. Non-related carbon sources like sucrose, glycerol, glucose or gluconate have been catabolized to acetyl coenzyme-A (Rehm, 2007 and Lu et al., 2009). Irrespective to type of PHA biosynthesized, an associated pathway of PHA degradation catalysed by PHA depolymerase, dehydrogenase, dimer hydrolase and 3-hydroxybutyrate was also reported (Sudesh et al., 2000).
Biosynthesis of PHA has well studied over the past two decades and been concluded that the synthesis of PHA biopolymers has been influenced by several factors such as bacterial strain producing the PHA polymer, the carbon source on which microbial cells have been grown, how that carbon has been metabolized in the cells, the types and role of enzymes directly or indirectly involved (Amara et al., 2011). Commercial fermentation in industries also documented that rate of PHA biosynthesis have …
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