Histidine modifications of proteins are broadly based on chemical methods triggering N-substitution reactions such as aza-Michael addition at histidine’s moderately nucleophilic imidazole side chain. While recent studies have demonstrated chemoselective, histidine-specific modifications by further exploiting imidazole’s electrophilic reactivity to overcome interference from the more nucleophilic lysine and cysteine, achieving site-specific histidine modifications remains a significant challenge due to the absence of spatial control over chemical processes¹. Herein, through X-ray crystallography and cryo-electron microscopy structural studies, we describe the rational design of a nature-inspired, noncanonical amino-acid-incorporated², human ferritin-based³ copper metalloenzyme that is capable of introducing site-specific post-translational modifications (PTMs) to histidine in peptides and proteins⁴. Specifically, chemoenzymatic aza-Michael additions on single histidine residues were carried out on eight protein substrates ranging from 10 to 607 amino acids, including the insulin peptide hormone. By introducing an insulin-targeting peptide into our metalloenzyme, we further directed modifications to be carried out site-specifically on insulin’s B-chain histidine 5. The success of this biocatalysis platform outlines a novel approach in introducing residue- and site-specific post-translational modifications to peptides and proteins, which may further enable reactions to be carried out in vivo.