Ferric iron reductase of Rhodopseudomonas sphaeroides

Ferric iron reductase activity was examined in the facultative photosynthetic bacterium Rhodopseudomonas sphaeroides. The specific activities of extracts from cells grown under phototrophic and aerobic conditions were similar and not affected by the concentration of iron in the growth media. The activity was resolved by ion-exchange column chromatography into two fractions, designated iron reductase A and iron reductase B, with molecular weights of 41,000 and 32,000, respectively. Both of these soluble cytoplasmic enzymes required the presence of flavin mononucleotide for activity and utilized NADH to reduce iron supplied as ferric citrate. Iron reductase B was responsible for the majority of activity in crude extracts and was purified 556-fold by conventional protein purification techniques. The apparent Km values of iron reductase B for NADH, Fe3+, and flavin mononucleotide were determined to be 18.2, 8.3, and 3.2 microM, respectively.