title: Electron transfer to nitrogenase. Characterization of flavodoxin from Azotobacter chroococcum and comparison of its redox potentials with those of flavodoxins from Azotobacter vinelandii and Klebsiella pneumoniae (nifF-gene product).
abstract: Flavodoxin in the hydroquinone state acts as an electron donor to nitrogenase in several nitrogen-fixing organisms. The mid-point potentials for the oxidized-semiquinone and semiquinone-hydroquinone couples of flavodoxins isolated from facultative anaerobe Klebsiella pneumoniae (nifF-gene product, KpFld) and the obligate aerobe Azotobacter chroococcum (AcFld) were determined as a function of pH. The mid-point potentials of the semiquinone-hydroquinone couples of KpFld and AcFld are essentially independent of pH over the range pH 7-9, being -422 mV and -522 mV (normal hydrogen electrode) at pH 7.5 respectively. The mid-point potentials of the quinone-semiquinone couples at pH 7.5 are -200 mV (KpFld) and -133 mV (AcFld) with delta Em/pH of -65 +/- 4 mV (KpFld) and -55 +/- 2 mV (AcFld) over the range pH 7.0-9.5. This indicates that reduction of the quinone is coupled to protonation to yield a neutral semiquinone. The significance of these values with respect to electron transport to nitrogenase is discussed. The amino acid compositions, the N- and C-terminal amino acid sequences and the u.v.-visible spectra of KpFld and AcFld were determined and are compared with published data for flavodoxins isolated from Azotobacter vinelandii.
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis; D001395:Azotobacter / Q000378:metabolism*; D004579:Electron Transport; D005418:Flavodoxin / Q000378:metabolism*; D005420:Flavoproteins / Q000378:metabolism*; D007711:Klebsiella pneumoniae / Q000378:metabolism*; D008564:Membrane Potentials; D009591:Nitrogenase / Q000378:metabolism*; D010084:Oxidation-Reduction; D013053:Spectrophotometry
chemical_list: D000596:Amino Acids; D005418:Flavodoxin; D005420:Flavoproteins; D009591:Nitrogenase