# A Hooker Oxygenase Archetype in Polyketide Biosynthesis Challenging the Baeyer–Villiger Monooxygenase Paradigm

## TL;DR

This study reveals a new enzyme mechanism in natural product biosynthesis that challenges previous assumptions about how certain chemical transformations occur.

## Contribution

The study reclassifies RslO9 as a Hooker oxygenase rather than a Baeyer–Villiger monooxygenase, offering a new framework for understanding related enzymes.

## Key findings

- RslO9 catalyzes hydroxylation and benzilic acid rearrangement, forming a Hooker intermediate.
- The enzyme's mechanism involves alkyl migration, challenging prior assumptions about its function.
- This discovery redefines the role of RslO9 and related enzymes in polyketide biosynthesis.

## Abstract

Aromatic polyketides from Actinobacteria are structurally
complex
bioactive natural products with significant therapeutic potential,
whose biosynthesis involves polyketide chain assembly, keto reduction,
cyclization, and aromatization. This is followed by pathway-specific
enzymatic tailoring steps, occasionally including rare oxidative rearrangements
of the carbon skeleton, as exemplified by the rishirilides. In this
study, we investigate RslO9, a flavin-dependent tailoring key enzyme
of rishirilide biosynthesis, previously hypothesized to facilitate
a lactone-forming Baeyer–Villiger oxidation of the rishirilide
naphthoquinone core and subsequent intramolecular aldol condensation.
Through detailed investigation of RslO9’s mechanism, structural
features, and substrate scope, we unexpectedly found that the naphthoquinone
moiety of the non-natural substrate lapachol undergoes hydroxylation
followed by a benzilic acid rearrangement, producing the Hooker intermediate–a
hallmark of the intricate Hooker oxidation. Our data support a similar
alkyl migration mechanism for RslO9’s native substrate, upending
its prior classification as a Baeyer–Villiger monooxygenase
and challenging the proposed role of related enzymes while also providing
a novel framework for exploring their catalytic roles.