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Keywords:

  • polylactic acid;
  • PLA;
  • PHA synthase;
  • propionate CoA transferase;
  • enzyme evolution

Abstract

For the synthesis of polylactic acid (PLA) and its copolymers by one-step fermentation process, heterologous pathways involving Clostridium propionicum propionate CoA transferase (PctCp) and Pseudomonas sp. MBEL 6-19 polyhydroxyalkanoate (PHA) synthase 1 (PhaC1Ps6-19) were introduced into Escherichia coli for the generation of lactyl-CoA endogenously and incorporation of lactyl-CoA into the polymer, respectively. Since the wild-type PhaC1Ps6-19 did not efficiently accept lactyl-CoA as a substrate, site directed mutagenesis as well as saturation mutagenesis were performed to improve the enzyme. The wild-type PctCp was not able to efficiently convert lactate to lactyl-CoA and was found to exert inhibitory effect on cell growth, random mutagenesis by error-prone PCR was carried out. By employing engineered PhaC1Ps6-19 and PctCp, poly(3-hydroxybutyrate-co-lactate), P(3HB-co-LA), containing 20–49 mol% lactate could be produced up to 62 wt% from glucose and 3HB. By controlling the 3HB concentration in the medium, PLA homopolymer and P(3HB-co-LA) containing lactate as a major monomer unit could be synthesized. Also, P(3HB-co-LA) copolymers containing various lactate fractions could be produced from glucose alone by introducing the Cupriavidus necator β-ketothiolase and acetoacetyl-CoA reductase genes. Fed-batch cultures were performed to produce P(3HB-co-LA) copolymers having 9–64 mol% of lactate, and their molecular weights, thermal properties, and melt flow properties were determined. Biotechnol. Bioeng. 2010; 105: 150–160. © 2009 Wiley Periodicals, Inc.