Acetyl-CoA as precursor
Along with the type of carbon source and the specificity of the PHA synthase, the metabolic pathways play a crucial role in determining the type of PHA that can be produced by a particular microorganism. Most of the P[3HB] producing microorganisms possess pathway which acetyl-CoA is converted into (R)-3-hydroxybutyryl-CoA and subsequently polymerized by the PHA synthase. In some microorganisms, (S)-isomers of 3-hydroxybutyryl-CoA are generated instead of the (R)-isomers. Since the PHA synthase is active only towards the (R)-isomers, additional reaction steps catalysed by enoyl-CoA hydratases are present in microorganisms such as Rhodospirillum rubrum to convert the (S)-isomers into the (R)-isomers [37].
Intermediates in Fatty acid ß-oxidation pathway
Fatty acid ß-oxidation pathway is known to generate substrates that can be polymerised by the PHA synthases of pseudomonads. Pseudomonads that belong to the rRNA-homology group I can synthesise PHAMCL from various alkanes, alkanols, and alkanoates. The monomer composition of the PHAMCL produced is often related to the type of carbon sources. Most of the pseudomonads belonging to this group, except P. oleovorans, can also derive (R)-3-hydroxyacyl-CoA substrates for PHA biosynthesis from unrelated carbon sources such as carbohydrates