2022 journal article

Mechanistic analysis of carbon-carbon bond formation by deoxypodophyllotoxin synthase

PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 119(1).

By: H. Tang n, M. Wu*, H. Lin*, M. Han*, Y. Tu*, Z. Yang*, T. Chien*, N. Chan*, W. Chang n

co-author countries: Taiwan, Province of China 🇹🇼 United States of America 🇺🇸
author keywords: C-C coupling; cyclization; oxygenase; natural product; reaction mechanism
MeSH headings : Berberidaceae / enzymology; Drugs, Chinese Herbal / chemistry; Ligases / chemistry; Oxidation-Reduction; Plant Proteins / chemistry; Podophyllotoxin / analogs & derivatives; Podophyllotoxin / chemistry
Source: Web Of Science
Added: February 21, 2022

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.