Abstract
Ferredoxin NADP+ oxidoreductase (Fpr) and oxygen-insensitive NAD(P)H nitroreductase (NfnB) are purified from Escherichia coli JM109 (E. coli JM109) as a predominant free flavin-independent ferric reductase. In the present study, we prepared natural iron storage proteins, E. coli ferritin A (FtnA) and bacterioferritin (Bfr), to show the effective ferrous iron release from these proteins by Fpr and NfnB in the presence of free flavins. Fpr and NfnB showed flavin reductase activity for flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN) and riboflavin, and their ferrous iron release activities were positively associated with the catalytic efficiencies (kcat/Km) for individual flavins. The ferrous iron release activity of E. coli cell-free extracts was affected by flavin reductase activity of the extracts. The Butyl TOYOPEARL column chromatography of the extracts, on the basis of NAD(P)H-dependent flavin reductase activity, resulted in the separation of six active fractions containing Fpr, NfnB, NAD(P)H-quinone oxidoreductase (QOR), flavin reductase (Fre) or alkyl hydroperoxide reductase subunit F (AhpF) as major components. Like Fpr and NfnB, recombinant QOR, Fre, and AhpF showed flavin reductase activity and ferrous iron release activity in the presence of free flavins, indicating an association of flavin reductase activity with ferrous iron releasing activity. Taken together, both free flavin-dependent and free flavin-independent ferric reductases in E. coli require free flavins to mediate an electron transfer from NAD(P)H to ferric iron in the iron storage proteins for the effective ferrous iron release.
