Direct electrochemistry of two genetically distinct flavodoxins isolated from Azotobacter chroococcum grown under nitrogenfixing conditions

Abstract

Two genetically distinct flavodoxins, designated AcFldA and AcFldB, were isolated from Azotobacter chroococcum (MCD1155) grown under nitrogen-fixing conditions. AcFldA and AcFldB differ in their midpoint potentials for the semiquinone-hydroquinone couple (E(m)-305 mV and-520 mV respectively). Only AcFldB was competent to act as an electron donor to the Mo-containing nitrogenase of A. chroococcum. The N-terminal amino acid sequence (20 residues) of AcFldB was identical with that predicted from the nifF DNA sequence of A. vinelandii OP [Bennett, Jacobsen and Dean (1988)J.Biol.Chem.263,1364-1369],suggesting that AcFldB is the nifF gene product of A.chroococcum(MCD1155). Direct fast reversible electrochemistry of these flavodoxins has been achieved at a polished edge-plane graphite electrode using the aminoglycoside neomycin as a promoter. The heterogeneous rates of electron transfer between thc graphite electrode and AcFldA and AcFldB were determined to be 1.2xl0-3 cm.s(-l) and 2.0xl0-3 cm.s(-l) respectively. The natures of two minor species of flavodoxin designated AcFldC and AcFldD, which were resolved by f.p.l.c., are also discussed.