# Switching Metazoan Fatty Acid Synthase Between Reducing and Nonreducing Elongation Mode via Programming of the Ketoreductase Domain

## TL;DR

Researchers modified fatty acid synthase to switch between fully reducing and nonreducing modes, enabling the production of complex pyrone compounds.

## Contribution

A helix motif from PKS ketoreductase domains was introduced into fatty acid synthase to control elongation mode and produce pyrones.

## Key findings

- Modified FAS variants produced intermediates that cyclized into pyrone products.
- A new amino acid involved in intermediate protonation was identified in the KR domain.
- The study provides insights into programming FASs for iterative biosynthesis.

## Abstract

Polyketides constitute a large class of natural products with important biological activities and applications such as antibiotics, antitumor agents, pesticides, and pigments. Their biosynthesis is catalyzed by polyketide synthases (PKSs) which are multidomain enzymes evolutionarily related to fatty acid synthases (FASs). Despite their close homology in structure and the chemistry they perform, FASs and PKSs differ fundamentally in their catalytic programming: FASs run fully reducing elongation reactions to yield saturated fatty acids, while iterative PKSs execute reductions just in selected cycles, generating complex oxidized compounds. In this study, we aimed at engineering the metazoan FAS in its ketoreduction (KR) domain to switch from fully reducing to a nonreducing mode during chain elongation. Guided by recent insights into KR programming, we incorporated a helix into metazoan FAS, which is found in KRs from iterative PKSs and type II FASs with chain length programming. These FAS variants initially catalyze complete fatty acid cycles but lose the ability of β‐keto reduction in later elongation rounds, producing intermediates that spontaneously cyclize to pyrone products. Finally, our study provides valuable insight into the mechanism of KR catalysis identifying another amino acid next to the active tyrosine which is capable for intermediate protonation.

Striving to shed light on programming of iterative polyketide synthases (PKSs), we introduced a helix motif found in iterative PKS ketoreductase domains into an evolutionary related fatty acid synthase. With this, we were able to impose programming, switching between a reducing and nonreducing elongation mode. We ultimately enabled the production of acyl substituted pyrones and established a basis for bottom‐up programming of fatty acid synthases (FASs)/PKSs.© 2026 WILEY‐VCH GmbH