Mechanistics of pH-Dependent Sulfmyoglobin Formation: Spin Control and His64 Proton Relay

The chemistry of hydrogen sulfide (H₂S) has been directed towards physiologically relevant hemeproteins, including myoglobin, hemoglobin, and other similar proteins. Despite substantial efforts, there remains a need to elucidate the mechanism and identify the species involved in the reaction between oxy-hemeproteins and H₂S. Here, we summarize both our experimental data and computational modeling results revealing the mechanisms by which sulfmyoglobin (sulfMb) and sulfhemoglobin (sulfHb) are formed. Our experimental data at pH 7.4 reveal differences in intensity between sulfMb and sulfHb chromophores in the 620 nm charge transfer region. This behavior could be attributed to the incomplete reaction of tetrameric oxy-Hb with H₂S, where not all heme groups form sulfheme. The data also show that, for the reaction of oxy-myoglobin (oxy-Mb) and H₂S, the 622 nm charge transfer band increases in intensity from a pH of 6.6 to 5.0. This increase is attributed to the presence of the heme pocket distal His64_εδ, which is positively charged, resulting in an elevated yield of sulfMb formation compared to the mono-protonated tautomer, His64ε. Computational hybrid QM/MM methods support the conclusion, indicating that oxy-Mb His64_εδ (pH 5.0) reacts with H₂S in the triplet state, favored by −31.0 kcal/mol over the singlet His64ε (pH 6.6) species. The phenomenon is facilitated by a hydrogen bonding network within the heme pocket, between His64_εδ, heme Fe(II)O₂, and H₂S. The results establish an energetically favored quantitative mechanism to produce sulfMb (−69.1 kcal/mol) from the reactions of oxy-Mb and H₂S. Curiously, the mechanism between met-aquo Mb, H₂O₂, and H₂S shows similar reaction pathways and leads to sulfheme formation (−135.3 kcal/mol). The energetic barrier towards intermediate Cpd-0 is the limiting step in sulfheme formation for both systems. ‬Both mechanisms show that the thiyl radical, HS^•, is the species attacking the β-β double bond of heme pyrrole B, leading to the sulfheme structure.