Activities of Key Enzymes in the Biosynthesis of Poly-3-Hydroxybutyrate by Methylosinus trichosporium IMV3011

The activities of the key enzymes involved in the intracellular poly-3-hydroxybutyrate (PHB) synthesis of Methylosinus trichosporium IMV3011 were studied under various cultivation conditions. The enzymes were methane monooxygenase (MMO), β-ketothiolase, acetoacetyl-CoA reductase, PHB synthetase, and PHB depolymerase. Each enzyme had a unique catalytic mechanism. MMO activity decreased continuously with PHB production, but PHB at a high concentration was beneficial for maintaining MMO activity because more NADH was released by PHB depolymerization. The important reaction for entering the PHB cycle was catalyzed by β-ketothiolase. The monomer of β-hydroxybutyrate was synthesized by the catalysis of β-ketothiolase and acetoacetyl-CoA reductase. PHB synthetase played an important role in the PHB synthesis routes. The activity of PHB synthetase increased with PHB production. The changes of enzyme activities involved in PHB synthetase and PHB depolymerase occurred together, which indicated that the polymerization and depolymerization of intracellular PHB occurred simultaneously. The molecular weight of PHB was determined mainly by the combined actions of PHB synthetase and PHB depolymerase. Some important intermediates in the tricarboxylic acid cycle were helpful for PHB production because they increased related enzyme activities in the PHB cycle.

摘要研究了甲烷单加氧酶 (MMO)、β-酮硫解酶、乙酰乙酰 CoA 还原酶、聚 β-羟基丁酸酯 (PHB) 合成酶及 PHB 降解酶在甲烷氧化细菌胞内 PHB 合成途径中的催化作用. 结果表明, MMO 的活性随着胞内 PHB 的积累而持续下降, 而胞内高含量的 PHB 有助于减缓 MMO 活性的降低. β-酮硫解酶控制着反应体系进入 PHB 循环的入口, 在它和乙酰乙酰 CoA 还原酶的共同作用下, 生成聚合单体 β-羟基丁酰, 二者活性在对数生长期达到峰值. PHB 合成酶可将胞内的短链片段聚合, 形成具有稳定高分子量的长链聚酯; 当 PHB 合成酶活性最高时, 胞内 PHB 浓度也随之达到峰值. 同时, PHB 降解酶活性随着 PHB 合成酶活性的增加而增加, 表明在 PHB 循环中, PHB 的合成和降解实际上是同时发生的. PHB 合成酶的活性基本决定了 PHB 的分子量, 但最终取决于 PHB 合成酶和 PHB 降解酶的共同作用.

The key enzymes, methane monooxygenase, ß-ketothiolase, acetoacetyl-CoA reductase, PHB synthetase, and PHB depolymerase, in the intracellular poly-3-hydroxybutyrate (PHB) synthesis of Methylosinus trichosporium IMV3011 all have their own unique catalytic mechanisms.Figure optionsDownload as PowerPoint slide