In the first pathway that creates 3HAs for mcl-PHA synthesis,aliphatic carbon sources are degraded via b-oxidation to produce3-hydroxyacyl-CoA. This b-oxidation intermediate is in the (S) form, so it is unusable by PHA synthase. To make it usable, enoyl-CoA is converted by an (R)-specific enoyl-CoA hydratase, often termedPhaJ, to produce polymerizable (R)-3- hydroxyacyl-CoA. In the second pathway, mcl-PHAs are synthesized by fatty acid biosynthesis from unrelated substrates such as glucose, gluconate or ethanol. These substrates are converted into fatty acids, which PHA-polymerase uses to synthesize mcl-PHA polymers (Madison and Huisman, 1999).
The third kind of pathway is a chain elongation reaction, in which acetyl-CoA moieties are condensed to 3-hydroxyacyl-CoA (Witholt and Kessler, 1999).