Fig. 1

Biosynthesis of PHA in E. coli LSBJ utilizing short-chain-length (SCL) and medium-chain-length (MCL) fatty acids. The absence of fadB and fadJ in E. coli LSBJ in combination with the plasmid-borne recombinant enzymes PhaC1(STQK) and PhaJ4 establishes a linear pathway for the production of PHA polymers from free fatty acids. Extracellular fatty acids are transported across the outer membrane dependent on size; SCL and shorter MCL fatty acids can diffuse across the outer membrane, while longer MCL fatty acids can be transported by the long-chain fatty acid transporter FadL (Lepore et al. 2011). Inner membrane transport and activation is accomplished by the SCL-specific Ato system (AtoEAD) or the MCL-specific acyl-CoA synthetase FadD (Kameda and Nunn 1981; Theodorou et al. 2006). Acyl-CoA substrates are converted into enoyl-CoA by the acyl-CoA dehydrogenase enzyme FadE (Campbell and Cronan 2002), and are unable to proceed further through β-oxidation due to the absence of FadB and FadJ. The enoyl-CoA pool is then converted to (R)-3-hydroxyacyl-CoA by the R-specific enoyl-CoA hydratase PhaJ4 (Tsuge et al. 2003), and finally polymerized by the PHA synthase PhaC1(STQK) (Takase et al. 2003, 2004). This system allows for the biosynthesis of PHA polymers with tightly controlled repeating unit composition, as the number of carbons present in the fatty acid substrate is retained as the total number of carbons in each repeating unit