Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, PA, 15260, USA.
Angew Chem Int Ed Engl. 2017 Jul 24;56(31):9062-9066. doi: 10.1002/anie.201703932. Epub 2017 Jun 29.
Hapalindole U (4) is a validated biosynthetic precursor to ambiguine alkaloids (Angew. Chem. Int. Ed. 2016, 55, 5780), of which biogenetic origin remains unknown. The recent discovery of AmbU4 (or FamC1) protein encoded in the ambiguine biosynthetic pathway (J. Am. Chem. Soc. 2015, 137, 15366), an isomerocyclase that can rearrange and cyclize geranylated indolenine (2) to a previously unknown 12-epi-hapalindole U (3), raised the question whether 3 is a direct precursor to 4 or an artifact arising from the limited in vitro experiments. Here we report a systematic approach that led to the discovery of an unprecedented calcium-dependent AmbU1-AmbU4 enzymatic complex for the selective formation of 4. This discovery refuted the intermediacy of 3 and bridged the missing links in the early-stage biosynthesis of ambiguines. This work further established the isomerocyclases involved in the biogenesis of hapalindole-type alkaloids as a new family of calcium-dependent enzymes, where the metal ions are shown critical for their enzymatic activities and selectivities.
Hapalindole U（4）是一种经过验证的生物合成前体，用于合成 ambiguine 生物碱（Angew. Chem. Int. Ed. 2016, 55, 5780），但其生物合成起源尚不清楚。最近在 ambiguine 生物合成途径中发现了 AmbU4（或 FamC1）蛋白（J. Am. Chem. Soc. 2015, 137, 15366），这是一种异构环化酶，能够重排并环化香叶基化的 indolenine（2），生成以前未知的 12-epi-hapalindole U（3），这引发了一个问题，即 3 是否是 4 的直接前体，还是由于体外实验的局限性而产生的假象。在这里，我们报告了一种系统的方法，该方法导致发现了一种前所未有的钙依赖性 AmbU1-AmbU4 酶复合物，用于选择性地形成 4。这一发现驳斥了 3 的中间体假说，并在 ambiguine 的早期生物合成中架起了缺失的环节。这项工作进一步确立了参与 hapalindole 型生物碱生物合成的异构环化酶是一类新的钙依赖性酶，其中金属离子对其酶活性和选择性至关重要。
