Biosynthesis of poly(δ-valerolactone) (PVL) by Halomonas bluephagenesis

Microbial polyhydroxyalkanoates (PHA) are promising for wide applications including food and medical packaging, drug delivery systems, coatings and bone scaffolds. The diverse properties of PHA are dependent on the variety of hydroxy fatty acid monomers. Microbial synthesis of homopolymers has been challenging except for poly(3-hydroxybutyrate) (PHB). In this study, an engineered metabolic pathway in Halomonas bluephagenesis was constructed to produce poly(5-hydroxyvalerate) (P5HV) or poly(δ-valerolactone) (PVL). PHA synthase PhaC_BP-M-CPF4 (PhaC_un) and 4-hydroxybutyrate CoA-transferase (AbfT) were identified as suitable for the artificial metabolic pathway. Deletion on the endogenous phaCAB for poly-3-hydroxybutyrate (PHB) synthesis was crucial to reduce 3-hydroxybutyric acid (3HB) monomer ratio in P(3HB-5HV) copolymer from 36% to 0.2% when expressing abfT and phaC_un on plasmid pWJL55, or from 38% to 18% when expressing abfT and phaC_un on genome. Additionally, deletion of the endogenous acyl-CoA thioesterase gene tesB enhanced 5HV molar ratio from 75% to 81% in P(3HB-5HV), as it removed the CoA moiety from 5HV-CoA. Subsequently, deletion of endogenous fadB gene encoding enoyl-CoA hydratase formed a near PVL homopolymer. The resulted H. bluephagenesis JL03 was grown to 23 g/L dry cell weight containing 58% PVL after 44 h cultivation in a 7-L bioreactor. The PVL exhibited better mechanical properties compared to chemically synthesized PVL, with an elongation at break of 521%, a Young's modulus of 293 MPa, and a higher molecular weight of 149 kDa.