Department of Chemistry & Biochemistry, ontana State University, ozeman, MT. USA. 59717.
Department of Chemistry, Northwestern University, Evanston, IL. USA 60208.
Angew Chem Int Ed Engl. 2021 Feb 23;60(9):4666-4672. doi: 10.1002/anie.202014337. Epub 2020 Dec 1.
Radical S-adenosyl-l-methionine (SAM) enzymes initiate biological radical reactions with the 5'-deoxyadenosyl radical (5'-dAdo•). A [4Fe-4S] cluster reductively cleaves SAM to form the Ω organometallic intermediate in which the 5'-deoxyadenosyl moiety is directly bound to the unique iron of the [4Fe-4S] cluster, with subsequent liberation of 5'-dAdo•. Here we present synthesis of the SAM analog S-adenosyl-l-ethionine (SAE) and show SAE is a mechanistically-equivalent SAM-alternative for HydG, both supporting enzymatic turnover of substrate tyrosine and forming the organometallic intermediate Ω. Photolysis of SAE bound to HydG forms an ethyl radical trapped in the active site. The ethyl radical withstands prolonged storage at 77 K and its EPR signal is only partially lost upon annealing at 100 K, making it significantly less reactive than the methyl radical formed by SAM photolysis. Upon annealing above 77K, the ethyl radical adds to the [4Fe-4S] cluster, generating an ethyl-[4Fe-4S] organometallic species termed Ω.
激进的 S-腺苷-L-甲硫氨酸（SAM）酶与 5'-脱氧腺苷自由基（5'-dAdo•）一起引发生物自由基反应。一个 [4Fe-4S] 簇还原裂解 SAM 形成 Ω 有机金属中间体，其中 5'-脱氧腺苷部分直接与 [4Fe-4S] 簇的独特铁结合，随后释放 5'-dAdo•。在这里，我们提出了 SAM 类似物 S-腺苷-L-蛋氨酸（SAE）的合成，并表明 SAE 是 HydG 的一种在机制上等效的 SAM 替代物，都支持底物酪氨酸的酶促转化，并形成有机金属中间体 Ω。将 SAE 与 HydG 结合的光解形成在活性位点中捕获的乙基自由基。乙基自由基能够在 77 K 下长时间储存，并且在 100 K 退火时其 EPR 信号仅部分丢失，使其比 SAM 光解形成的甲基自由基的反应性显著降低。在 77K 以上退火时，乙基自由基会添加到 [4Fe-4S] 簇中，生成一种称为 Ω 的乙基-[4Fe-4S] 有机金属物种。
