Directed evolution of an enantiocomplementary S_NArase reveals divergent catalytic features

Enzymes that catalyze non-natural C-C bond-forming reactions are powerful tools in asymmetric synthesis, yet reprogramming their active sites to invert stereochemical outcome remains challenging. Building on our recently engineered S_NArase, S_NAr1.3, which performs enantioselective nucleophilic aromatic substitutions with carbon nucleophiles, we now report the evolution of an enantiocomplementary biocatalyst (eS_NAr1.3) that displays enhanced activity and expanded substrate scope. Structural and computational analyses uncover both conserved and divergent features between S_NAr1.3 and eS_NAr1.3. Despite retaining similar electrophile binding poses and a conserved catalytic arginine, the halide-binding pocket of S_NAr1.3 has been abandoned in eS_NAr1.3. Instead, His23 has emerged as a key motif that works with Arg124 to accurately position the nucleophilic substrate. Calculations reveal that Arg124 also plays a crucial role in facilitating halide release during catalysis. Our study demonstrates how evolution can reshape enzyme mechanisms in unforeseen ways, highlighting the importance of exploring diverse trajectories to access new functions.