Noncanonical Heme Ligands Steer Carbene Transfer Reactivity in an Artificial Metalloenzyme **

Changing the primary metal coordination sphere is a powerful strategy for tuning metalloprotein properties. Here we used amber stop codon suppression with engineered pyrrolysyl‐tRNA synthetases, including two newly evolved enzymes, to replace the proximal histidine in myoglobin with N δ ‐methylhistidine, 5‐thiazoylalanine, 4‐thiazoylalanine and 3‐(3‐thienyl)alanine. In addition to tuning the heme redox potential over a >200 mV range, these noncanonical ligands modulate the protein's carbene transfer activity with ethyl diazoacetate. Variants with increased reduction potential proved superior for cyclopropanation and N–H insertion, whereas variants with reduced E o values gave higher S–H insertion activity. Given the functional importance of histidine in many enzymes, these genetically encoded analogues could be valuable tools for probing mechanism and enabling new chemistries.