Escherichia coli sulfite reductase hemoprotein subunit. Prosthetic groups, catalytic parameters, and ligand complexes.
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.
D003486:Cyanides / Q000494:pharmacology; D004578:Electron Spin Resonance Spectroscopy; D004926:Escherichia coli / Q000201:enzymology*; D006420:Hemeproteins / Q000302:isolation & purification / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D010088:Oxidoreductases / Q000378:metabolism*; D050862:Oxidoreductases Acting on Sulfur Group Donors / Q000302:isolation & purification / Q000378:metabolism*; D011485:Protein Binding
D003486:Cyanides; D006420:Hemeproteins; D008024:Ligands; D010088:Oxidoreductases; D050862:Oxidoreductases Acting on Sulfur Group Donors