Current advances in microbial cell factories for lactate-based polyesters driven by lactate-polymerizing enzymes: Towards the further creation of new LA-based polyesters

Polylactic acid (PLA), a representative bio-based polyester, has been commonly synthesized via a multi-step by chemical process. The current modes of generating PLA involve microbial fermentation of starting material, lactic acid (LA), followed by chemical ring-opening polymerization. Recently, one-pot complete bioprocess for LA-based polyesters has been established as a microbial cell factory (MCF). The concept is a process conversion from the usual chemical factory to the MCF. This new challenge was triggered by discovery of an engineered LA-polymerizing enzyme (LPE). The LPE was found as one of the members of an extensive mutant library that has been created through the long-term evolutionary engineering study of natural biopolyester-synthesizing enzymes. Needless to say, a strategic method of getting the beneficial mutation in the enzyme is of the utmost importance and an essential step towards accomplishing the desired purpose, the acquisition of the LA-polymerizing activity in this case. In this review, the structures and properties of LPE-catalyzed polymerization products will be discussed as well as backgrounds on establishment of the MCFs for synthesis of LA-based polyesters. Also, experimental strategies for enrichment of the LA fraction will be proposed to further advance the prototype of MCF based on the related metabolic pathways.