A Patatin-Like Protein Associated with the Polyhydroxyalkanoate (PHA) Granules of Haloferax mediterranei Acts as an Efficient Depolymerase in the Degradation of Native PHA

摘要

The key enzymes and pathways involved in polyhydroxyalkanoate (PHA) biosynthesis in haloarchaea have been identified in recent years, but the haloarchaeal enzymes for PHA degradation remain unknown. In this study, a patatin-like PHA depolymerase, PhaZh1, was determined to be located on the PHA granules in the haloarchaeon Haloferax mediterranei. PhaZh1 hydrolyzed the native PHA (nPHA) [including native polyhydroxybutyrate (nPHB) and native poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (nPHBV) in this study] granules in vitro with 3-hydroxybutyrate (3HB) monomer as the primary product. The site-directed mutagenesis of PhaZh1 indicated that Gly16, Ser47 (in a classical lipase box, G-X-S47-X-G), and Asp195 of this depolymerase were essential for its activity in nPHA granule hydrolysis. Notably, phaZh1 and bdhA (encoding putative 3HB dehydrogenase) form a gene cluster (HFX_6463 to _6464) in H. mediterranei. The 3HB monomer generated from nPHA degradation by PhaZh1 could be further converted into acetoacetate by BdhA, indicating that PhaZh1-BdhA may constitute the first part of a PHA degradation pathway in vivo. Interestingly, although PhaZh1 showed efficient activity and was most likely the key enzyme in nPHA granule hydrolysis in vitro, the knockout of phaZh1 had no significant effect on the intracellular PHA mobilization, implying the existence of an alternative PHA mobilization pathway(s) that functions effectively within the cells of H. mediterranei. Therefore, identification of this patatin-like depolymerase of haloarchaea may provide a new strategy for producing the high-value-added chiral compound (R)-3HB and may also shed light on the PHA mobilization in haloarchaea.