Characterization of a poly(butylene adipate-co-terephthalate) hydrolase from the mesophilic actinobacteria Rhodococcus fascians

摘要

Poly(butylene adipate-co-terephthalate) (PBAT) possesses excellent film-forming ability and biodegrad-ability. Therefore, it is considered to be a promising mulching film material that eliminates the need for recovery. In the applications that require PBAT degradation in the field after use, it is important to understand the biodegradation mechanism at moderate temperatures. We have previously isolated from the soil the mesophilic actinobacteria Rhodococcus fascians NKCM2511 that biodegraded PBAT under moderate temperature conditions (20-30 °C). In this study, to clarify the mechanism of PBAT degradation by the strain NKCM2511, a DNA fragment carrying the gene pbath_(Rf)responsible for the PBAT degradation activity was cloned. The gene encoded a 216-amino-acid-long protein designated as PBATH_(Rf). Homology modeling revealed that PBATH_(Rf) belongs to the α/β hydrolase fold family, lacking the lid domain covering the active site. PBATH_(Rf) degraded PBAT film at 30 °C at the rate of 0.10 ± 0.03 mg/cm~2/d and was capable of degrading several other aliphatic polyester films. Liquid chromatography revealed that PBATH_(Rf) preferentially cleaved the ester bond between 1,4-butanediol and adipic acid rather than that between 1,4-butanediol and terephthalic acid (T). This characteristic of PBATH_(Rf) may explain the low degradation rate of the aliphatic-aromatic copolyester PBAT, compared to the rate of degradation of aliphatic polyesters without T. In addition, liquid chromatography showed that PBATH_(Rf) released T, mono(2-hydroxyethyl) terephthalic acid, and bis(2-hydroxybutyl) terephthalate from an amorphous poly(ethylene terephthalate) (PET) film. However, no significant change in the PET film surface after the treatment with PBATH_(Rf) was found by scanning electron microscopy. This is the first report of an enzyme from the mesophilic actinobacteria Rhodococcus fascians that can hydrolyze various polyesters, including PBAT, and catalyze hydrolysis on the surface of an amorphous PET film. This study also provides insight into the biodegradation mechanism of PBAT in the actual field as it describes an enzyme from a naturally occurring organism that acts in the medium temperature range.