title: Iron-sulfur N-1 clusters studied in NADH-ubiquinone oxidoreductase and in soluble NADH dehydrogenase.
abstract: Two N-1 type iron-sulfur clusters in NADH-ubiquinone oxidoreductase (Complex I, EC 1.6.5.3) were potentiometrically resolved: one was titrated as a component with a midpoint oxidation-reduction potential of -335 mV at pH 8.0, and with an n-value equal to one; the other as an extremely low midpoint potential component (Em 8.0 less than -500 mV). These two clusters are tentatively assigned to N-1b and N-1a, respectively. Cluster N-1b is completely reducible with NADH and has a spin concentration of about 0.8/FMN. Its EPR spectrum can be simulated as a single rhombic component with principal g values of 2.019, 1.937, and 1.922, which correspond to the Center 1 reported earlier by Orme-Johnson, N. R., Hansen, R. E., and Beinert, H. (1974) J. Biol. Chem. 249, 1922-1927. At extremely low oxidation-reduction potentials (less than -450 mV), additional EPR signals emerge with apparent g values of gz = 2.03, gy = 1.95, and gx = 1.91, which we assign to cluster N-1a. It is difficult, however, to simulate the detailed spectral line shape of this component as a single rhombic component, suggesting some degree of protein modification or interaction with a neighboring oxidation-reduction component. EPR spectra of soluble NADH dehydrogenase, containing 5-6 g atoms of non-heme iron and 5-6 mol of acid-labile sulfide/mol of FMN, were examined. Signals from at least two iron-sulfur species could be distinguished in the NADH-reduced form: one of an N-1b type spectrum; the other of a spectrum with g values of 2.045, 1.95, and 1.87 (total of about 0.5 spin equivalents/FMN). This is the first example of an N-1 type signal detected in isolated soluble NADH dehydrogenase.
mesh_terms: D001665:Binding Sites; D003579:Cytochrome Reductases / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D005486:Flavin Mononucleotide / Q000032:analysis; D007501:Iron / Q000032:analysis*; D007506:Iron-Sulfur Proteins / Q000378:metabolism*; D007700:Kinetics; D008667:Metalloproteins / Q000378:metabolism*; D016660:NAD(P)H Dehydrogenase (Quinone); D009245:NADH Dehydrogenase / Q000378:metabolism*; D009247:NADH, NADPH Oxidoreductases / Q000378:metabolism*; D010084:Oxidation-Reduction; D011199:Potentiometry; D011485:Protein Binding; D011487:Protein Conformation; D011808:Quinone Reductases / Q000378:metabolism*; D013455:Sulfur / Q000032:analysis*
chemical_list: D007506:Iron-Sulfur Proteins; D008667:Metalloproteins; D013455:Sulfur; D005486:Flavin Mononucleotide; D007501:Iron; D009247:NADH, NADPH Oxidoreductases; D003579:Cytochrome Reductases; D016660:NAD(P)H Dehydrogenase (Quinone); D011808:Quinone Reductases; D009245:NADH Dehydrogenase