Unlocking Lactonase Enzymes as Biocatalysts for the Deracemisation of Chiral γ‐Thiolactones

: Lactonases, a class of metalloenzymes that exhibit catalytic promiscuity, have been extensively studied from a biological perspective, yet their application as biocatalysts remains underexplored. In this study, we disclose the biocatalytic activity of lactonase enzymes in the hydrolysis and deracemization of chiral C3-substituted-γ-thiolactones and the asymmetric synthesis of γ-thio-α-substituted-carboxylic acids. The thiolactonase activity of lactonases from different protein superfamilies was investigated. The biocatalyst GcL, from the metallo-β-lactamase-like lactonase family, catalysed the enzymatic kinetic resolution (EKR) of homocysteine (Hcy) thiolactones with excellent enantioselectivity (E-value up to 136), yielding enantioenriched Hcy thiolactones and γ-thio-α-amino-carboxylic acids with high ees. Additionally, the biocatalyst N9 Y71G, a rationally engineered variant of the reconstructed ancestral paraoxonase enzyme N9, catalysed the dynamic kinetic resolution (DKR) of C3-thio-γ-thiolactones, yielding γ-thio-α-thio-carboxylic acids in enantioselective manner with high ees (up to >99%) and yields (up to >99%). Insights on the mechanism and the stereoselectivity of the lactonase biocatalysts were gained through computational and site-directed mutagenesis studies.