Microbial Block Copolymerization of 3-Hydroxyalkanoates

Block polyhydroxyalkanoates (PHA) were reported to be resistant against polymer aging that negatively affects polymer properties. Recently, more and more attempts have been directed to make PHA block copolymers. It is possible to produce PHA block copolymers of various kinds using recombinant Pseudomonas putida KT2442. Polyhydroxyalkanoate (PHA) synthesis genes phaPCJ_A.c cloned from Aeromonas caviae were transformed into Pseudomonas putida KTOY06ΔC, a mutant of P.putida KT2442, resulting in the ability of the recombinant P. putida KTOY06ΔC (phaPCJ_A.c) to produce a short-chain-length and medium-chain-length PHA block copolymer consisting of poly-3-hydroxybutyrate (PHB) as one block and random copolymer of 3-hydroxyvalerate (3HV) and 3-hydroxyheptanoate (3HHp) as another block. PHA synthesis genes phbC and orfZ cloned from Ralstonia eutropha H16 were transformed into β-oxidation weakened P. putida KTOY08ΔGC, a mutant of P. putida KT2442. The recombinant P. putida strain termed KTHH06 was able to produce a short-chain-length PHA block copolymer consisting of poly(3-hydroxybutyrate) (P3HB) as one block and poly(4-hydroxybutyrate) (P4HB) as another block. Diblock copolymers PHB-b-PHHx consisting of poly-3-hydroxybutyrate (PHB) block covalently bonded with poly-3-hydroxyhexanoate (PHHx) block were for the first time produced successfully by a recombinant P. putida KT2442 with its β-oxidation cycle deleted to its maximum. These polymers were characterized by nuclear magnetic resonance(NMR), differential scanning calorimetric (DSC) , and thermo- and mechanical analysis. In comparison to a random copolymer with similar ratio and a blend of homopolymers PHB and PHVHHp , PHB and PHHx or P3HB and P4HB with similar ratio, the block copolymer showed improved structural related mechanical properties such as the highest tensile strength and Young’s modulus. Compared with other commercially available PHA including PHB, PHBV, PHBHHx, and P3HB4HB, the block copolymers showed differences in terms of mechanical properties and should draw more attentions from the PHA research community.