Redirecting a Native Ene-Reductase Toward Desaturation With Reverse Enantioselectivity.

Chiral enones are valuable motifs in synthetic intermediates and bioactive molecules, driving significant interest in biocatalysis. Although recent enzymatic desaturation strategies for substituted cyclohexenones provide efficient and highly enantioselective synthetic routes, none offer complementary stereoselectivity. To address this gap, we introduce a stereocomplementary biocatalytic system based on an old yellow enzyme (OYE), XenA from Pseudomonas putida. Although XenA natively catalyzes the reduction of electron-deficient alkenes and exhibits negligible desaturation activity, protein engineering redirected its catalytic function toward desaturation, ultimately yielding a variant that accommodates a range of cyclohexanones with 85%-99% ee and 32%-98% yield. Remarkably, the optimal variant (XenA_4) possesses 46 mutations and exhibits an 11°C increase in melting temperature over the wild type. Mechanistic studies revealed that the unique dimeric structure of the enzyme is pivotal in controlling stereoselectivity by modulating the substrate-binding orientation.