Production and characterization of polyhydroxyalkanoates by recombinant Methylobacterium extorquens: Combining desirable thermal properties with functionality

Recombinant Methylobacterium extorquens ATCC 55366 harboring phaC2 from Pseudomonas fluorescens GK13 (M. ex-phaC2) was capable of producing blends of polyhydroxyalkanoates. By co-feeding methanol and 5-hexenoic acid (C6), functionalized PHAs (4 ≤ chain length ≤ 6) containing C–C double bonds were created. Bioreactor studies revealed negative impacts of 5-hexenoic acid on biomass and PHA production by reducing overall yields. In contrast, there was a positive relationship between 5-hexenoic acid supply and abundance of polymeric functional groups, i.e., molar portions of monomeric units bearing double bonds increased with increased 5-hexenoic acid supply. Correlation of C6 consumption to methanol addition resulted in a model that allowed for on-line estimation of toxic co-substrate concentration. The functionalized PHAs were ductile and showed signs of side chain crosslinking, resulting in reduced degrees of crystallinity. Incorporation of 3-hydroxyhex-5-enoate and 3-hydroxyhexanoate into the polymeric chains produced desirable thermal properties with enhanced thermal stability and reduced melting temperatures (139–168 °C). Thermal degradation and melting temperatures obtained suggest a comfortable range for melt-processing of these polymers and allow for autoclaving as convenient sterilization process. Consequently, functionalized PHAs produced in this study are candidates for medical applications as part of biocomposite materials. The use of methanol as main substrate for cultivation of recombinant M. extorquens offers the possibility to reduce production costs and develop new process control strategies.

Research highlights
► Recombinant Methylobacterium extorquens produces modifiable and processable polyhydroxyalkanoates.
► The biopolyester properties suggest applications as medical biocomposite materials.
► Methanol (substrate) monitoring offers novel process control strategies.
► 5-Hexenoic acid (co-substrate) impacts strongly on the metabolism of M. extorquens.