Unlocking Iminium Catalysis in Artificial Enzymes to Create a Friedel-Crafts Alkylase

We show that the incorporation of the non-canonical amino acid para-aminophenylalanine (pAF) into the non-enzymatic protein scaffold LmrR creates a proficient and stereoselective artificial enzyme (LmrR_pAF) for the vinylogous Friedel-crafts alkylation between alpha, beta-unsaturated aldehydes and indoles. pAF acts as a catalytic residue, activating enal substrates towards conjugate addition via the formation of intermediate iminium ion species, whilst the protein scaffold provides rate acceleration and enantio-induction. Improved LmrR_pAF varants were identified by direted evolution advised by alanine-scanning to obtain a triple mutant that provided higher yields and enantioselectivities for a range of enals and indoles. Analys of Michaelis-Menten kinetics of LmrR-pAF and tevolved mutants reveals that new activities emerge via evolutionary pathways that diverge from one another and specialise catalytic reactivity.