Metabolic engineering for the production of polylactic acid (PLA) and its copolymers

Abstract

Polylactic acid (PLA) is a promising biomass-derived polymer that possesses material properties suitable for household products, commercial plastics, and biomedical materials. However, PLA is currently synthesized by a two-step process: fermentative production of lactic acid followed by complicated chemical polymerization. In this study, $\It{Escherichia coli}$ strains have been developed for one-step production of PLA homopolymer and its various copolymers composed of lactate and hydroxyalkanoates monomers. $\It{E. coli}$metabolism was rationally modified by systems-level metabolic engineering based on $\It{in silico}$ genome-scale metabolic flux analysis to reduce by-products formation, increase the metabolic flux towards lactyl-CoA, and ultimately to enhance biosynthesis of PLA polymers. For more efficient production of PLA polymers, the metabolic pathways were reengineered so that induction with an expensive inducer for the gene expression and feeding of dicarboxylic acid for the cell growth could be avoided. Also, type II PHA synthases from various $\It{Pseudomonas}$ strains were engineered for the production of PLA polymers, which allowed diversification of natural variants of lactate-polymerizing enzyme. Furthermore, various PLA copolymers including poly(lactate-$\It{co}$-glycolate) could be produced by further metabolic engineering. Thus, the strategy of combined metabolic engineering and enzyme engineering allowed efficient bio-based one-step production of PLA and its copolymers. This strategy should be generally useful for developing other engineered organisms capable of producing various unnatural polymers by direct fermentation from renewable resources.