Abstract
Escherichia coli NADPH-sulfite reductase can be dissociated into an oligomeric flavoprotein and a monomeric hemoprotein (HP) subunit in 4 M urea. HP catalyzes stoichiometric 6-electron reductions of SO32- (to S2-) and of NO2-, as well as 2-electron reduction of NH2OH, with reduced methyl viologen (MV+) as reductant. While Vmax values are highest with the nitrogenous substrates, Km for SO32- is 2 to 3 orders of magnitude less than the Km for NO2- or NH2OH. EPR spectroscopic and chemical analyses show that HP contains one siroheme and one Fe4S4 center per polypeptide. The heme is in the high spin Fe3+ state in HP as isolated. Near-quantitative reduction of the Fe4S4 center to a state yielding a g = 1.94 type of EPR spectrum by S2O42- and/or MV+ could be achieved if HP was converted to either the CN- or CO complex or treated with 80% dimethyl sulfoxide. HP binds one SO32- or CN- per peptide. Binding of these ligands, as well as CO, appears to be mutually exclusive and to involve the heme. The heme Fe3+/Fe2+ potential is shifted from -340 mV in the free HP to -155 mV in the HP-CN- complex. The potential of the Fe4S4 center is approximately 70 mV more negative in the CN- as opposed to the CO-ligated HP (-420 mV), a result which indicates the presence of heme-Fe4S4-ligand interaction in the HP complexes.
Highlights
NH20H,with reduced methyl viologen ( M V + ) as reduc- which catalyze transfer of electrons from NADPH to thesite tant, While V, values are highest with the nitrogenous substrates, K, for s0s2-is 2 to 3 orders of magnitude less than theK, for NO2- or NH20H
EPR spectroscopic and chemical analyses show that HP contains one siroheme and one Fe4S4center per polypeptide
Of 50:- reduction, which resides on the p subunits (Siegel et al, 1974; Faeder et al, 1974).The four,8 polypeptides bind a total of 3 to 4 siroheme and 14 to 17 non-heme Fe/acid-labile
Summary
Of 50:- reduction, which resides on the p subunits (Siegel et al, 1974; Faeder et al, 1974).The four ,8 polypeptides bind a total of 3 to 4 siroheme and 14 to 17 non-heme Fe/acid-labile. Dimethyl sulfoxide.HP binds one SO3’- or CN- per 54,600, contained siroheme, non-heme Fe, and labile S” in peptide. The heme Fe3+/Fe2+potential is shifted from -340 mV thetic groups per polypeptide was not well established by in the free HP to -155 mV in theHP-CN- complex.The Siegel and Davis (1974))and it catalyzed reduction of SOa2-. NADPH-sulfite reductase; Fe2Sz,Fe&, and Fe& binuclear, trinuclear, and tetranuclear iron-sulfur centers, respectively; MV“ and MV’, oxidized methyl viologen and itsreduced cation radical, respectively; MeZSO, dimethyl sulfoxide
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