Engineering LmrR protein for L-proline-based asymmetric aldol biocatalysis

L-Proline is a powerful organocatalyst widely applied in asymmetric synthesis due to its secondary amine functionality. However, in proteins, this functional group is locked in peptide bonds, rendering proline catalytically inactive. Natural enzymes that leverage L-proline-based catalysis are exceedingly rare. Here, we engineer the nonenzymatic protein scaffold LmrR into a new-to-nature biocatalyst by exposing its native L-proline residue at the N-terminus to catalyze enantioselective aldol reactions. Through rational design, protein engineering, and reaction optimization, we develop an engineered LmrR variant that achieves up to 99% conversion and >99% enantiomeric excess across a range of aromatic and heteroaromatic aldehyde substrates. Our findings reveal a unique strategy for unlocking dormant catalytic potential in natural amino acids and protein scaffolds, providing an applicable approach to create tailored, L-proline-based enzymes for asymmetric synthesis.