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Studies on the biodegradability of polythioester copolymers and homopolymers by polyhydroxyalkanoate (PHA)-degrading bacteria and PHA depolymerases.

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  • معلومة اضافية
    • نبذة مختصرة :
      The biodegradability of microbial polythioesters (PTEs), a novel class of biopolymers which were discovered recently and can be produced by polyhydroxyalkanoate (PHA)-accumulating bacteria, was studied. Using poly(3-hydroxybutyrate-co-3-mercaptopropionate) [poly(3HB-co-3MP)] as sole carbon source for screening, 22 new bacterial strains were isolated and characterized. Interestingly, none of the PHA-degrading bacteria was able to utilize the homopolymer poly(3MP) as a carbon source for growth or to form clear zones on poly(3MP)-containing agar plates. The extracellular PHA depolymerases from two strains (Schlegelella thermodepolymerans,Pseudomonas indicaK2) were purified to electrophoretic homogeneity and biochemically characterized. The PHA depolymerase ofS. thermodepolymeransexhibited a temperate optimum of about 75°C to 80°C and was stable at 70°C for more than 24 h. Regarding the substrate specificities of the PHA depolymerase ofS. thermodepolymerans, enzyme activities decreased significantly with increasing 3MP content of the copolymer substrates. Interestingly, no activity could be detected with homoPTEs consisting only of 3MP or of 3-mercaptobutyrate. Similar results were obtained with the PHA depolymerases PhaZ2, PhaZ5 and PhaZ7 ofPaucimonas lemoigneiwhich were also investigated. The PHA depolymerase ofPs. indicaK2 did not cleave any of the investigated polymers containing 3MP. Gas chromatography, infrared and1H-NMR spectrometry and matrix-assisted laser desorption/ionization time-of-flight analysis revealed that 3MPs containing oligomers were enriched in the water-insoluble fraction remaining after partial digestion of poly(3HB-co-3MP) by purified poly(3HB) depolymerase ofS. thermodepolymerans. In contrast, 3HB was enriched in the water-soluble fraction, which also contained 3HB-co-3MP dimer obtained by partial digestion of this copolymer by the enzyme. This study clearly indicates that PHA depolymerases are specific for oxoester linkages of PHAs and that the thioester bonds of PTEs cannot be cleaved by this type of enzyme. [ABSTRACT FROM AUTHOR]