Biosynthesis of Tasikamides <i>via</i> Pathway Coupling and Diazonium-Mediated Hydrazone Formation
Guang‐Lei Ma, Hartono Candra, Li Pang, Juan Xiong, Yichen Ding, Hòa Thị Trần, Zhen Jie Low, Hong Ye, Min Liu, Jie Zheng, Mingliang Fang, Bin Cao, Zhao‐Xun Liang
Abstract
Naturally occurring hydrazones are rare despite the ubiquitous usage of synthetic hydrazones in the preparation of organic compounds and functional materials. In this study, we discovered a family of novel microbial metabolites (tasikamides) that share a unique cyclic pentapeptide scaffold. Surprisingly, tasikamides A–C (1–3) contain a hydrazone group (C═N─N) that joins the cyclic peptide scaffold to an alkyl 5-hydroxylanthranilate (AHA) moiety. We discovered that the biosynthesis of 1–3 requires two discrete gene clusters, with one encoding a nonribosomal peptide synthetase (NRPS) pathway for assembling the cyclic peptide scaffold and another encoding the AHA-synthesizing pathway. The AHA gene cluster encodes three ancillary enzymes that catalyze the diazotization of AHA to yield an aryl diazonium species (diazo-AHA). The electrophilic diazo-AHA undergoes nonenzymatic Japp–Klingemann coupling with a β-keto aldehyde-containing cyclic peptide precursor to furnish the hydrazone group and yield 1–3. The studies together unraveled a novel mechanism whereby specialized metabolites are formed by the coupling of two biosynthetic pathways via an unprecedented in vivo Japp–Klingemann reaction. The findings raise the prospect of exploiting the arylamine-diazotizing enzymes (AAD) for the in vivo synthesis of aryl compounds and modification of biological macromolecules.