CONICET | Buscador de Institutos y Recursos Humanos

Several heme proteins are able to bind inorganic sulfide species with variable affinity, unlike the ferric porphyrinates that yield the corresponding ferrous forms. The connection between the binding affinity and structural features of the protein is yet to be understood. In order to unveil molecular details of the reactivity of the endogenous hydrogen sulfide towards heme proteins, the reactivity of inorganic sulfide species towards heme peptides has been explored using theoretical tools and under experimental biorelevant conditions. Ferric microperoxidase 11 (MP11FeIII), the covalent heme undecapeptide derived from cytochrome c proteolysis, has been selected as a heme protein model compound. In the solution experiments, the N-acetylated form of MP11FeIII (NacMP11) was used to prevent the formation of aggregates. While the preservation of the proximal histidine bound to the MP11FeIII has been established, possible interactions with other surrounding amino acids are still ill-defined in the literature. Classics and accelerated molecular dynamics simulations of MP11 with the coordinated SH- revealed that the polypeptide chain showed a great deal of flexibility, although the helical structure remained intact. During the time scale of our simulations, no residues were observed to interact with the coordinated SH- and, hence, this would be even less likely for NacMP11. The simulations also revealed a dynamic hydrogen bonding interaction between the Nd atom of the proximal histidine and the carboxyl group of the backbone of the Thr19, with a slight change on the charge distribution around the heme center. These theoretical results point out at NAcMP11 as a model for the specific evaluation of isolated proximal effects upon the binding of sulfide species. The addition of excess quantities of freshly prepared H2S to NacMP11FeIII at pH 6.8, under anaerobic conditions, brought about the formation of a new complex. The quality of the source of inorganic sulfide is a highly relevant issue, and misleading results can be obtained with certain commercial reagents. The UV-Vis spectrum of the new complex closely resembled those of the low spin ferric-hydroxo complex (only attained at alkaline pH) and the corresponding alkylthiol derivative, suggesting that the FeIII reduction was prevented. Accordingly, the resonance Raman spectrum is characteristic of a low spin hexa-coordinated heme complex and in the low frequency region the presence of a band at 366cm-1 has been observed and tentatively assigned to a n(FeIII-S) stretching mode. The combination of our theoretical and experimental results exposes the capability of the proximal histidine for the stabilization of FeIII-sulfide adducts of heme compounds in the absence of distal interactions. NAcMP11 represents an interesting model for the study of the biochemical reactivity of endogenous hydrogen sulfide towards heme proteins.