title: 57Fe and 1H electron-nuclear double resonance of three doubly reduced states Escherichia coli sulfite reductase.
abstract: We have employed electron-nuclear double resonance (ENDOR) spectroscopy to study the 57Fe hyperfine interactions in the bridged-siroheme [4Fe-4S] cluster that forms the catalytically active center of the two-electron-reduced hemoprotein subunit of Escherichia coli NADPH-sulfite reductase (SiR2-). Previous electron paramagnetic resonance (EPR) and Mössbauer studies have shown that this enzyme oxidation state can exist in three distinct spectroscopic forms: (1) a "g = 2.29" EPR species that predominates in unligated SiR2-, in which the siroheme Fe2+ is believed to be in an S = 1 state; (2) a "g = 4.88" type of EPR species that predominates in SiR2- in the presence of small amounts of guanidinium sulfate, in which the siroheme Fe2+ is in an S = 2 state; and (3) a classical "g = 1.94" type of EPR species that is seen in SiR2- ligated with CO, in which the siroheme Fe2+ is in an S = 0 state. In all three species, the cluster is in the [4Fe-4S]1+ state, and two distinct types of Fe site are seen in Mössbauer spectroscopy. ENDOR studies confirm the Mössbauer assignments for the cluster 57Fe in the g = 1.94 state, with A values of 37, 37, and 32 MHz for site I and ca. 19 MHz for site II. The hyperfine interactions are not too different on the g = 2.29 state, with site I Fe showing more anisotropic A values of 32, 24, and 20 MHz (site II was not detected).(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D004926:Escherichia coli / Q000201:enzymology*; D006859:Hydrogen; D007501:Iron; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism*; D050862:Oxidoreductases Acting on Sulfur Group Donors / Q000378:metabolism*; D050864:Sulfite Reductase (NADPH)
chemical_list: D006859:Hydrogen; D007501:Iron; D010088:Oxidoreductases; D050862:Oxidoreductases Acting on Sulfur Group Donors; D050864:Sulfite Reductase (NADPH); C494314:sulfite reductase (NADPH), E coli