HEME IRON IN THE SCENE OF H2S TARGETS: AFFINITY OF INORGANIC SULFIDES AND A HEME MODEL COMPOUND (PRESENTACION ORAL)

BIEZA, SILVINA ANDREA; BOUBETA, FERNANDO M.; ESTRIN, DARÍO A.; BOECHI, LEONARDO; BARI, SARA E.

Montevideo

Simposio; Thiol Metabolism and Redox Regulation of Cellular Functions; 2015

Iron(III) centers in hemeproteins have been recognized as biochemical targets of hydrogen sulfide (H2S), with unidentified functions so far. Our research is focused on the reactivity of sulfides toward model compounds of hemeproteins, with the aim of understanding the basis of the coordination to heme iron. A minimalist model, the N-acetylmicroperoxidase 11 (FeIIINAcMP11), was selected for the analysis of the binding reaction. FeIIINAcMP11 is a heme undecapeptide derived from cytochrome c that retains a proximal histidine as fifth ligand of the heme iron, and is devoid of distal effects (except for those imposed by surrounding water). We recently demonstrated the formation of a low spin Fe(III) complex, with uncertainty on the protonation state of the bound sulfide, NAcMP11FeIII(S/SH/SH2).1 It is important to address that the anhydrous Na2S was the selected source for reproducible experiments, and that Na2S.9H2O or NaHS.2H2O led to erratic results in our hands. In the present work, we evaluate the affinity (kon/koff) of inorganic sulfide and FeIIINAcMP11. The constants kon and koff have been obtained using stopped flow methodologies, under strict anaerobiosis. The kinetic evolution revealed a fast process, linearly dependent on the concentration of sulfide in the onset of the reaction, with kon = (2.6±0.6)x104 M-1s-1 at pH 6.8. This value is in accordance with the binding of sulfide to certain ferric hemeproteins.2 The dissociation rate, koff = (5.7±0.1) s-1, is close to the value reported for MP8FeIII(CN-).3 The affinity of the complex NAcMP11FeIII(S/SH/SH2) is hence ≈ 4.6x103. This value is in the lower limit of the affinities for sulfide of the hemeproteins reported up to date, where distal mechanisms assist the stabilization of the bound ligand. The FeIIINAcMP11, with no distal stabilization, thus provides an estimate magnitude for the isolated role of the proximal histidine in the coordination of sulfide ferric hemeproteins.1Bieza, S.A.; Boubeta, F.M.; Smulevich, G.; Feiss, A.; Estrin, D.A.; Boechi, L.; Bari, S.E., Inorg. Chem. 2015, 54, 527-533.2Nicoletti, F.P.; Comandini, A.; Bonamore, A.; Boechi, L.; Boubeta, F.M.; Feis, A.; Smulevich, G.; Boffi A. Biochemistry 2010, 49, 2269?2278.3a) Smith, M.C.; McLendon, G. J. Am. Chem. Soc. 1980, 102, 5666-5670; b) M