Thiolysis of Poly(3-hydroxybutyrate) with Polyhydroxyalkanoate Synthase from Ralstonia eutropha

Poly(3-hydroxybutyrate) (PHB) is synthesized from 3-hydroxybutyryl-CoA by polyhydroxyalkanoate synthase and hydrolyzed by PHB depolymerase. In this study, we focused on the reverse reaction of polyhydroxyalkanoate synthase, and propose the possibility that PHB can be degraded through a novel process, that is thiolysis of PHB with CoASH. Polyhydroxyalkanoate synthase of Ralstonia eutropha was incubated with ¹⁴C-labeled PHB and CoASH. The reaction mixture was fractionated by HPLC and then analyzed with a scintillation counter. The analysis revealed 3-hydroxybutyryl-CoA to be a product of the reaction. When NADP⁺ and acetoacetyl-CoA reductase were added to the reaction mixture, an increase in absorbance at 340 nm was observed. Native PHB inclusion bodies from R. eutropha also showed thiolytic activity. This is the first indication that polyhydroxyalkanoate synthase catalyzes both the synthesis and degradation of PHB, and that native PHB inclusion bodies has thiolytic activity.