Biosynthesis Of Biodegradable Plastic Poly(Lactic Acid-co-3-Hydroxypropionic Acid)

With the declining non-renewable resources such as fossils and the large-scale use and even abuse of petroleum-based plastic products,the environmental problems caused by white pollution have become a hot spot of global concern.Therefore,how to find a new environmentally friendly biodegradable plastic has become a hot research field in recent years.In this study,the engineering E.coli which can be used to prepare polylactic acid in the laboratory was used as the starting strain,and the key enzyme genes in the biosynthesis of 3-hydroxypropionic acid metabolism were integrated into the existing engineering bacteria by metabolic engineering technology.Finally,a non-natural biosynthetic pathway from the sugar substrate to the lactic acid and 3-hydroxypropionic acid copolymer(P(LA-co-3HP))was constructed.Through the structural analysis and characterization of the extracted polymer after fermentation of the engineering bacteria,the successful synthesis of the target polymer P(LA-co-3HP)in the constructed engineering bacteria was confirmed.In the biosynthetic pathway of P(LA-co-3HP)constructed in this study,PHA synthase is a key enzyme for the polymerization of two monomers.The two PHA synthetases I and II used in this study were derived from Chromobacterium sp.USM2 and Pseudomonas sp.MBEL 6-19,and two control engineering strains were constructed.The results of the final fermenter level experiment showed that the target polymer obtained from the type II PHA synthase engineering strain was P(57.1-67.5 mol% LA-co-3HP),and the crude product accounted for 10.04% of the cell dry weight(wt%).The average yield of the copolymer was 0.86 g/L,the weight average molecular weight(Mw): 10800-143600,and the melting temperature(Tm): 145-167 °C.The crude product of the target polymer obtained by the type I PHA synthase engineering strain accounts for 10.49% of the cell dry weight(wt%),the average yield of the copolymer is 0.90 g/L,and the weight average molecular weight(Mw): 45085-160800,the melting temperature(Tm): 140-160 °C.However,the structural analysis of the polymer extracted by fermentation of two control engineering strains showed that the two engineering bacteria synthesized homopolymer of lactic acid and 3-hydroxypropionic acid,also contains the copolymer of lactic acid and 3-hydroxypropionic acid,but the type II PHA polymerase engineered bacteria are more pure than the type I PHA polymerase engineered bacteria.