Nonheme Fe Enzyme‐Catalyzed Enantiodivergent Nitrogen Migration: Directed Evolution and Computational Study of Isopenicillin N Synthases for Biocatalytic Synthesis of Arylglycines

We describe the reprogramming and directed evolution of nonheme Fe enzyme isopenicillin N synthase (IPNS) as an efficient biocatalyst for 1,3-nitrogen migration reactions via an unnatural mechanism. Directed evolution of isopenicillin N synthase from Emericella nidulans furnished a quadruple mutant (EniIPNS V185L I187V S102I R279H, IPNS_Nim), enabling the conversion of a range of azanyl esters into N-protected l-arylglycines. IPNS_Nim achieved a TTN of 16 000 and a TOF of 1200 min^-1. This TTN surpassed state-of-the-art small-molecule Fe catalysts by 330-fold and represented the highest TTN value reported for a nonheme Fe enzyme in a new-to-nature reaction. IPNS_Nim and our previously evolved ACCO_Nim (ACCO: 1-aminocyclopropane-1-carboxylic acid oxidase) exhibited complementary enantiopreference, allowing enantioselective synthesis of either l- or d-arylglycines-essential building blocks in clinically important peptide therapeutics. Mechanistic studies revealed a biocatalyst-controlled switch in the rate-determining step (RDS): While the hydrogen atom transfer (HAT) step is the RDS for ACCO_Nim-catalyzed nitrogen migration, it is likely not with IPNS_Nim. Moreover, while ACCO_Nim exhibits almost no enantioselectivity in this HAT step, IPNS_Nim confers excellent enantiocontrol over HAT. Computational studies using density functional theory calculations and molecular dynamics simulations suggested that IPNS and ACCO adopt two different substrate binding modes. Classical MD simulations shed light on important interactions between the substrate and active-site residues that control the substrate binding mode and enantioselectivity.