A distal ligand mutes the interaction of hydrogen sulfide with human neuroglobin.

Hydrogen sulfide is a critical signaling molecule, but high concentrations cause cellular toxicity. A four-enzyme pathway in the mitochondrion detoxifies H2 S by converting it to thiosulfate and sulfate. Recent studies have shown that globins like hemoglobin and myoglobin can also oxidize H2 S to thiosulfate and hydropolysulfides. Neuroglobin, a globin enriched in the brain, was reported to bind H2 S tightly and was postulated to play a role in modulating neuronal sensitivity to H2 S in conditions such as stroke. However, the H2 S reactivity of the coordinately saturated heme in neuroglobin is expected a priori to be substantially lower than that of the 5-coordinate hemes present in myoglobin and hemoglobin. To resolve this discrepancy, we explored the role of the distal histidine residue in muting the reactivity of human neuroglobin toward H2 S. Ferric neuroglobin is slowly reduced by H2 S and catalyzes its inefficient oxidative conversion to thiosulfate. Mutation of the distal His64 residue to alanine promotes rapid binding of H2 S and its efficient conversion to oxidized products. X-ray absorption, EPR, and resonance Raman spectroscopy highlight the chemically different reaction options influenced by the distal histidine ligand. This study provides mechanistic insights into how the distal heme ligand in neuroglobin caps its reactivity toward H2 S and identifies by cryo-mass spectrometry a range of sulfide oxidation products with 2-6 catenated sulfur atoms with or without oxygen insertion, which accumulate in the absence of the His64 ligand.