Mechanistic analysis of carbon-carbon bond formation by deoxypodophyllotoxin synthase
Journal
Proceedings of the National Academy of Sciences of the United States of America
Journal Volume
119
Journal Issue
1
Date Issued
2022-01-04
Author(s)
Tang, Haoyu
Wu, Min-Hao
Lin, Hsiao-Yu
Han, Meng-Ru
Tu, Yueh-Hua
Yang, Zhi-Jie
Chien, Tun-Cheng
Chang, Wei-Chen
Abstract
Deoxypodophyllotoxin contains a core of four fused rings (A to D) with three consecutive chiral centers, the last being created by the attachment of a peripheral trimethoxyphenyl ring (E) to ring C. Previous studies have suggested that the iron(II)- and 2-oxoglutarate-dependent (Fe/2OG) oxygenase, deoxypodophyllotoxin synthase (DPS), catalyzes the oxidative coupling of ring B and ring E to form ring C and complete the tetracyclic core. Despite recent efforts to deploy DPS in the preparation of deoxypodophyllotoxin analogs, the mechanism underlying the regio- and stereoselectivity of this cyclization event has not been elucidated. Herein, we report 1) two structures of DPS in complex with 2OG and (±)-yatein, 2) in vitro analysis of enzymatic reactivity with substrate analogs, and 3) model reactions addressing DPS's catalytic mechanism. The results disfavor a prior proposal of on-pathway benzylic hydroxylation. Rather, the DPS-catalyzed cyclization likely proceeds by hydrogen atom abstraction from C7', oxidation of the benzylic radical to a carbocation, Friedel-Crafts-like ring closure, and rearomatization of ring B by C6 deprotonation. This mechanism adds to the known pathways for transformation of the carbon-centered radical in Fe/2OG enzymes and suggests what types of substrate modification are likely tolerable in DPS-catalyzed production of deoxypodophyllotoxin analogs.
Subjects
C−C coupling; cyclization; natural product; oxygenase; reaction mechanism
Type
journal article