flavin-adenine-dinucleotide and ferric-citrate

A novel NADH-dependent, soluble flavoreductase of 60 kDa, active toward ferric chelates and quinones, has been purified from maize seedlings. Two closely related isoforms were separated. The two isoforms are similar in several biochemical features, with the exception of the apparent molecular mass of their subunits (29 and 31 kDa, respectively). They are homodimers in the native state, they bind FAD as the prosthetic group and show strong preference for NADH over NADPH as the electron donor. Ferric chelates (chiefly ferric citrate, Km 3-5 x 10(-5) M; kcat/Km 3.4-3.7 x 10(5) M-1 s-1), and some quinones (benzoquinone, coenzyme Q-0, and juglone) are used as electron acceptors. Enzymatic reduction of benzoquinone occurs with formation of radical semiquinones. Both soluble ferric chelate reductase isoforms are strongly inhibited by p-hydroxymercuribenzoic acid (I50 5 nM) and by cibachron blue, the latter giving nonlinear inhibition. It is suggested that soluble ferric chelate reductase might be involved in the symplastic reduction of iron chelates which is required for the assembly of iron-containing macromolecules such as cytochromes and ferritin.

Topics: Chromatography, Affinity; Electrophoresis, Polyacrylamide Gel; Ferric Compounds; Flavin-Adenine Dinucleotide; FMN Reductase; Isoenzymes; Molecular Weight; NAD; NADH, NADPH Oxidoreductases; Oxidation-Reduction; Plants; Quinones; Zea mays

Iron is an essential element in all living cells. Solubilization, uptake and transport of iron by microorganisms is controlled by highly efficient and specific Fe(3+)-chelating agents named siderophores. However, mechanisms of mobilization of iron from ferrisiderophores are still enigmatic. Here, we demonstrate that Escherichia coli contains a powerful enzymatic system for the reduction of ferrisiderophores. Siderophores have a much lower affinity for ferrous iron, which then can be liberated. This system has been previously purified and characterized as a NAD(P)H:flavin oxidoreductase [Fontecave, M., Eliasson, R. and Reichard, P. (1987) J. Biol. Chem. 262, 12,325-12,331)]. It catalyzes the reduction of free flavins, FMN, FAD or riboflavin by NADH or NADPH. Reduced flavins, in turn, transfer their electrons to physiological ferric complexes: ferrisiderophores, ferric citrate and ferritins. The reaction is inhibited by molecular oxygen and greatly stimulated by Fe(2+)-acceptors such as ferrozine or the iron-free form of ribonucleotide reductase subunit R2. We suggest that the reduction and the mobilization of iron from ferrisiderophores in the cell might be regulated by the presence of physiological ferrous traps such as apoproteins.

Topics: Catalysis; Electron Transport; Escherichia coli; Ferric Compounds; Ferrichrome; Ferritins; Ferrous Compounds; Flavin Mononucleotide; Flavin-Adenine Dinucleotide; FMN Reductase; Hydrogen-Ion Concentration; Iron; Kinetics; NAD; NADH, NADPH Oxidoreductases; NADP; Oxidation-Reduction; Riboflavin