A Standalone β‑Ketoreductase Acts Concomitantly with Biosynthesis of the Antimycin Scaffold

Antimycins
are anticancer compounds produced by a hybrid nonribosomal
peptide synthetase/polyketide synthase (NRPS/PKS) pathway. The biosynthesis
of these compounds is well characterized, with the exception of the
standalone β-ketoreductase enzyme AntM that is proposed to catalyze
the reduction of the C8 carbonyl of the antimycin scaffold. Inactivation
of antM and structural characterization suggested
that rather than functioning as a post-PKS tailoring enzyme, AntM
acts upon the terminal biosynthetic intermediate while it is tethered
to the PKS acyl carrier protein. Mutational analysis identified two
amino acid residues (Tyr185 and Phe223) that are proposed to serve
as checkpoints controlling substrate access to the AntM active site.
Aromatic checkpoint residues are conserved in uncharacterized standalone
β-ketoreductases, indicating that they may also act concomitantly
with synthesis of the scaffold. These data provide novel mechanistic
insights into the functionality of standalone β-ketoreductases
and will enable their reprogramming for combinatorial biosynthesis.