Unlocking Ketone Activation for Enantioselective Aldol Reactions by 4‐Oxalocrotonate Tautomerase

4‐Oxalocrotonate tautomerase (4‐OT) is a promising scaffold for asymmetric biocatalysis, but conventional 4‐OT‐catalyzed aldol reactions focus solely on aldehyde substrate activation. Herein, we report the directed evolution of a tandem‐fused 4‐OT enzyme that achieves efficient activation of ketone substrates for enantioselective aldol reactions with substituted benzaldehydes. The engineered biocatalyst yields a range of chiral β‐hydroxy ketone products with up to 99% yield and moderate enantioselectivity, while enabling access to both target enantiomers. This work expands the substrate promiscuity and reaction scope of 4‐OT, laying a foundation for rational design of next‐generation 4‐OT variants for advanced organic synthesis and industrial biocatalysis.