Sacrificial Methionine Residues Protect Active-Site Histidines From Oxidation

Histidine is the most common residue in enzymic active sites because of the chemical versatility of its imidazole side chain, but is also highly vulnerable to oxidative damage. Here, we demonstrate that methionine residues in secretory ribonucleases such as bovine RNase A serve as sacrificial antioxidants that protect active-site histidines. In the human homolog RNase 1, five methionine residues cluster near the catalytic histidines His12 and His119, which serve as a base and an acid, respectively, during RNA cleavage. Bioinformatic analyses reveal that these methionines are conserved across the RNase A superfamily and adopt S-aryl motifs with proximal aromatic residues that modulate the electron density at sulfur and thereby tune the susceptibility of methionine to oxidation. Chemoselective labeling of methionine with an oxaziridine probe increased the vulnerability of RNase 1 to oxidation-induced inactivation, implicating Met29 and Met35 as protective residues. Total chemical synthesis of an RNase 1 variant in which these residues were replaced by norleucine, a sulfur-free isostere of methionine, corroborated their protective role. A similar arrangement of methionines is present in unrelated secretory proteins, including serine proteases and calprotectin, suggesting a broadly recurring protective strategy.