Homolytic Cleavage of Both Heme-Bound Hydrogen Peroxide and Hydrogen Sulfide Leads to the Formation of Sulfheme

Abstract

Many heme-containing proteins with a histidine in the distal E7 (HisE7) position can form sulfheme in the presence of hydrogen sulfide (H2S) and a reactive oxygen species such as hydrogen peroxide. For reasons unknown, sulfheme derivatives are formed specifically on solvent-excluded heme pyrrole B. Sulfhemes severely decrease the oxygen-binding affinity in hemoglobin (Hb) and myoglobin (Mb). Here, use of hybrid quantum mechanical/molecular mechanical methods has permitted characterization of the entire process of sulfheme formation in the HisE7 mutant of hemoglobin I (HbI) from Lucina pectinata. This process includes a mechanism for H2S to enter the solvent-excluded active site through a hydrophobic channel to ultimately form a hydrogen bond with H2O2 bound to Fe(III). Proton transfer from H2O2 to His64 to form compound (Cpd) 0, followed by hydrogen transfer from H2S to the Fe(III)-H2O2 complex, results in homolytic cleavage of the O-O and S-H bonds to form a reactive thiyl radical (HS(•)), ferryl heme Cpd II, and a water molecule. Subsequently, the addition of HS(•) to Cpd II, followed by three proton transfer reactions, results in the formation of a three-membered ring ferric sulfheme that avoids migration of the radical to the protein matrix, in contrast to that in other peroxidative reactions. The transformation of this three-membered episulfide ring structure to the five-membered thiochlorin ring structure occurs through a significant potential energy barrier, although both structures are nearly isoenergetic. Both three- and five-membered ring structures reveal longer NB-Fe(III) bonds compared with other pyrrole nitrogen-Fe(III) bonds, which would lead to decreased oxygen binding. Overall, these results are in agreement with a wide range of experimental data and provide fertile ground for further investigations of sulfheme formation in other heme proteins and additional effects of H2S on cell signaling and reactivity.

Hydrogen Sulfide and Persulfides Oxidation by Biologically Relevant Oxidizing Species

“…It has also been proposed that, in the case of myeloperoxidase, the presence of a sulfonium ion linkage between a methionine and the heme ring determines that, in contrast to lactoperoxidase, the sulfheme cannot be formed [106]. Computational simulations suggest that sulfheme formation occurs through a concerted interaction between the distal histidine, H 2 S, and a ferric-bound hydroperoxide to form oxoferryl compound II, H 2 O and HS•; the latter subsequently adds to a specific pyrrole site [113].…”

Charge Transfer and π to π* Transitions in the Visible Spectra of Sulfheme Met Isomeric Structures

“…The results supported the unique function of E7 histidine in sulfheme formation. 25,28 The NMR of several sulfMbCN complexes revealed that there are three isomeric forms of the chlorin structure in sulfMb derivatives named S A Mb, S B Mb, and S C Mb (Figure 1, inset). 20−24 The S A structure is an episulfide formed across the C3−C4 double bond, S B is characterized as a "ring opened episulfide", and S C is a thiochlorin structure.…”

“…As previously reported, we performed a PM6/CHARMM molecular dynamics (MD) simulation for a model of the HbI GlnE7His mutant. 28 This HbI mutant has an active site and experimentally determined properties very similar to myoglobin (Mb). A snapshot from the 1.4 ns (ns) PM6/MM MD simulation was used as a starting model for investigation into the mechanism of sulfheme formation using a hybrid B3PW91/MM potential energy function.…”

“…Calculations were performed in the lowest energy sextet state in agreement with experimental observations. 28 TDDFT Calculations. TDDFT calculations were performed using the geometry optimized sulfheme structures.…”

Why does sulfite reductase employ siroheme?

“…Such environments are known 12 to initiate sulfheme formation reaction in myoglobin and hemoglobin in the presence of H2S. Furthermore, heme radical states are invoked 13 in the sulfheme formation mechanism. In a different mechanism, 14 H2S was shown to react with heme via HS -, i.e.…”