Chemo-enzymatic synthesis of polyhydroxyalkanoate (PHA) incorporating 2-hydroxybutyrate by wild-type class I PHA synthase from Ralstonia eutropha
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A previously established improved two-phase reaction system has been applied to analyze the substrate specificities and polymerization activities of polyhydroxyalkanoate (PHA) synthases. We first analyzed the substrate specificity of propionate coenzyme A (CoA) transferase and found that 2-hydroxybutyrate (2HB) was converted into its CoA derivative. Then, the synthesis of PHA incorporating 2HB was achieved by a wild-type class I PHA synthase from Ralstonia eutropha. The PHA synthase stereoselectively polymerized (R)-2HB, and the maximal molar ratio of 2HB in the polymer was 9 mol%. The yields and the molecular weights of the products were decreased with the increase of the (R)-2HB concentration in the reaction mixture. The weight-average molecular weight of the polymer incorporating 9 mol% 2HB was 1.00 × 105, and a unimodal peak with polydispersity of 3.1 was observed in the GPC chart. Thermal properties of the polymer incorporating 9 mol% 2HB were analyzed by DSC and TG-DTA. Tg, Tm, and Td (10%) were observed at −1.1°C, 158.8°C, and 252.7°C, respectively. In general, major components of PHAs are 3-hydroxyalkanoates, and only engineered class II PHA synthases have been reported as enzymes having the ability to polymerize HA with the hydroxyl group at C2 position. Thus, this is the first report to demonstrate that wild-type class I PHA synthase was able to polymerize 2HB.
KeywordsPolyhydroxyalkanoate (PHA) Improved two-phase reaction system (iTPRS) 2-Hydroxybutyrate (2HB) Ralstonia eutropha Class I PHA synthase
We thank Dr. Hideto Tsuji of Toyohashi University of technology for the useful comments on this paper. We also thank Mr. Eiji Yamada of Hokkaido University for his technical support with NMR measurements and Dr. Tokuo Matsushima and Mr. Tetsuya Toriyabe for their technical support. This work was supported by the 2003 Industrial Technology Research Grant Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, by the Global COE Program (Project No. B01: Catalysis as the Basis for Innovation in Materials Science), and by Grants-in-Aid for Scientific Research (Nos. 21310060 and 21760632) and a research grant from Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. This work was partially supported by the Regional Innovation Cluster Program (Global Type).
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