PDF(Pubmed)
Abstract：
N-Acetylnorloline synthase (LolO) is one of several iron(II)- and 2-oxoglutarate-dependent (Fe/2OG) oxygenases that catalyze sequential reactions of different types in the biosynthesis of valuable natural products. LolO hydroxylates C2 of 1-exo-acetamidopyrrolizidine before coupling the C2-bonded oxygen to C7 to form the tricyclic loline core. Each reaction requires cleavage of a C-H bond by an oxoiron(IV) (ferryl) intermediate; however, different carbons are targeted, and the carbon radicals have different fates. Prior studies indicated that the substrate-cofactor disposition (SCD) controls the site of H· abstraction and can affect the reaction outcome. These indications led us to determine whether a change in SCD from the first to the second LolO reaction might contribute to the observed reactivity switch. Whereas the single ferryl complex in the C2 hydroxylation reaction was previously shown to have typical Mössbauer parameters, one of two ferryl complexes to accumulate during the oxacyclization reaction has the highest isomer shift seen to date for such a complex and abstracts H· from C7 ∼ 20 times faster than does the first ferryl complex in its previously reported off-pathway hydroxylation of C7. The detectable hydroxylation of C7 in competition with cyclization by the second ferryl complex is not enhanced in 2H2O solvent, suggesting that the C2 hydroxyl is deprotonated prior to C7-H cleavage. These observations are consistent with the coordination of the C2 oxygen to the ferryl complex, which may reorient its oxo ligand, the substrate, or both to positions more favorable for C7-H cleavage and oxacyclization.
摘要：
N-乙酰去甲胆碱合酶（LolO）是几种铁（II）和2-酮戊二酸依赖性（Fe/2OG）加氧酶之一，可在有价值的天然产物的生物合成中催化不同类型的顺序反应。在将C2键合的氧与C7偶联以形成三环洛林核之前，LolO羟基化1-外-乙酰胺并吡咯并氮啶的C2。每个反应都需要通过氧代铁（IV）（铁基）中间体裂解C-H键；但是，不同的碳是目标，和碳自由基有不同的命运。先前的研究表明，底物辅因子处置（SCD）控制H·提取的位点，并可能影响反应结果。这些迹象使我们确定SCD从第一到第二LolO反应的变化是否可能有助于观察到的反应性转换。尽管以前显示C2羟基化反应中的单个铁基络合物具有典型的穆斯堡尔参数，在氧化环化反应过程中积累的两个铁基复合物之一具有迄今为止这种复合物的最高异构体位移，并且从C7中提取H·比先前报道的C7的非途径羟基化中的第一个铁基复合物快20倍。在2H2O溶剂中，与第二铁基络合物的环化竞争中C7的可检测羟基化没有增强，表明C2羟基在C7-H裂解之前被去质子化。这些观察结果与C2氧与铁配合物的配位一致，可以重新定向其氧代配体，基板，或两个位置都更有利于C7-H裂解和氧化环化。
