Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States.
Shaanxi Key Laboratory of Natural Products & Chemical Biology, College of Chemistry & Pharmacy, Northwest A&F University, Yangling 712100, Shaanxi China.
J Am Chem Soc. 2022 Mar 16;144(10):4478-4486. doi: 10.1021/jacs.1c12010. Epub 2022 Mar 3.
The biosynthesis of blasticidin S has drawn attention due to the participation of the radical -adenosyl methionine (SAM) enzyme BlsE. The original assignment of BlsE as a radical-mediated, redox-neutral decarboxylase is unusual because this reaction appears to serve no biosynthetic purpose and would need to be reversed by a subsequent carboxylation step. Furthermore, with the exception of BlsE, all other radical SAM decarboxylases reported to date are oxidative in nature. Careful analysis of the BlsE reaction, however, demonstrates that BlsE is not a decarboxylase but instead a lyase that catalyzes the dehydration of cytosylglucuronic acid (CGA) to form cytosyl-4'-keto-3'-deoxy-d-glucuronic acid, which can rapidly decarboxylate nonenzymatically . Analysis of substrate isotopologs, fluorinated analogues, as well as computational models based on X-ray crystal structures of the BlsE·SAM (2.09 Å) and BlsE·SAM·CGA (2.62 Å) complexes suggests that BlsE catalysis likely proceeds via direct elimination of water from the CGA C4' α-hydroxyalkyl radical as opposed to 1,2-migration of the C3'-hydroxyl prior to dehydration. Biosynthetic and mechanistic implications of the revised assignment of BlsE are discussed.
由于参与了自由基 - 腺苷甲硫氨酸 (SAM) 酶 BlsE，因此人们对 blasticidin S 的生物合成引起了关注。最初将 BlsE 分配为自由基介导的、氧化还原中性脱羧酶是不寻常的，因为这种反应似乎没有生物合成目的，并且需要通过随后的羧化步骤来逆转。此外，迄今为止报道的所有其他自由基 SAM 脱羧酶除了 BlsE 之外，都是氧化性质的。然而，对 BlsE 反应的仔细分析表明，BlsE 不是脱羧酶，而是一种裂合酶，它催化胞嘧啶葡萄糖醛酸 (CGA) 的脱水形成胞嘧啶-4'-酮-3'-脱氧-d-葡萄糖醛酸，该酶可以快速非酶促脱羧。对底物同量异位标记物、氟化类似物以及基于 X 射线晶体结构的 BlsE·SAM（2.09 Å）和 BlsE·SAM·CGA（2.62 Å）复合物的计算模型的分析表明，BlsE 催化可能通过 CGA C4'α-羟烷基自由基的直接消除水来进行，而不是在脱水之前 C3'-羟基的 1,2-迁移。讨论了重新分配 BlsE 的生物合成和机制意义。
