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Reactivity of Inorganic Sulfide Species toward a Heme Protein Model
journal contribution
posted on 2015-01-20, 00:00 authored by Silvina A. Bieza, Fernando Boubeta, Alessandro Feis, Giulietta Smulevich, Darío
A. Estrin, Leonardo Boechi, Sara E. BariThe
reactivity of inorganic sulfide species toward heme peptides was explored
under biorelevant conditions in order to unravel the molecular details
of the reactivity of the endogenous hydrogen sulfide toward heme proteins.
Unlike ferric porphyrinates, which are reduced by inorganic sulfide,
some heme proteins can form stable FeIII–sulfide
adducts. To isolate the protein factors ruling the redox chemistry,
we used as a system model, the undecapeptide microperoxidase (MP11),
a heme peptide derived from cytochrome c proteolysis that retains
the proximal histidine bound to the FeIII atom. Upon addition
of gaseous hydrogen sulfide (H2S) at pH 6.8, the UV–vis
spectra of MP11 closely resembled those of the low-spin ferric hydroxo
complex (only attained at an alkaline pH) and cysteine or alkylthiol
derivatives, suggesting that the FeIII reduction was prevented.
The low-frequency region of the resonance Raman spectrum revealed
the presence of an FeIII–S band at 366 cm–1 and the general features of a low-spin hexacoordinated heme. Anhydrous
sodium sulfide (Na2S) was the source of sulfide of choice
for the kinetic evaluation of the process. Theoretical calculations
showed no distal stabilization mechanisms for bound sulfide species
in MP11, highlighting a key role of the proximal histidine for the
stabilization of the FeIII–S adducts of heme compounds
devoid of distal counterparts, which is significant with regard to
the biochemical reactivity of endogenous hydrogen sulfide.