New insight into thermo-solvent tolerant lipase produced by Streptomyces sp. A3301 for re-polymerization of poly (DL-lactic acid)

Abstract

Poly (lactic acid) (PLA) is a biodegradable plastic that has received considerable attention since it plays an important role in replacing commercial plastics and resolving the global warming problem. Recently, biological degradation and polymerization studies of PLA have been an active topic. Microbial solvent-tolerant and thermostable lipases are interesting since they have the potential to induce PLA polymerization. The aims of this study are to isolate solvent-tolerant and thermostable lipase-producing actinomycetes and to apply the crude enzyme produced by an isolated strain for PLA polymerization. According to the results, the best solvent-tolerant and thermostable lipase-producing actinomycete was strain A3301, with a lipase activity of 108 U/ml in the production medium. The crude enzyme produced by strain A3301 demonstrated the optimum temperature and thermostability at 60 °C and 45–55 °C, respectively. Moreover, the enzyme showed the ability to tolerate 10–20% (v/v) hexane and toluene. Strain A3301 was identified as Streptomyces sp. by using the 16S rDNA sequence. A thermo-solvent-tolerant lipase from this strain was used in PLA polymerization experiments. The optimal conditions for PLA synthesis were 60 °C for 8 h, using dried lipase at a concentration of 10% (w/v) and a fixed lactic acid concentration of 450 g/L under a nitrogen atmosphere. The highest molecular weight of the PLA product was 525 Da, and the degree of polymerization was approximately 7. Subsequently, the repolymerization process of PLA using this enzyme was investigated. The maximum molecular weight of the PLA product was 577 Da, and the degree of polymerization was approximately 8. © 2020 Elsevier Ltd