The Isolation and Characterization of a New Iron-Sulfur Protein from Photosynthetic Membranes

A new iron-sulfur protein, distinct from the soluble chloroplast ferredoxin, was isolated from chloroplast membranes. The isolated protein, purified to homogeneity, had a molecular weight of about 8000 and 4 atoms of iron and 4 inorganic sulfides per mole. Its absorption spectrum had a broad absorbance band in the 400 nm region, a shoulder at approximately 310 nm, and a peak around 280 nm. The absorbance ratio A₄₀₀ to A₂₈₀ was 0.55. The electron paramagnetic resonance spectrum (measured at 12°K) of the reduced protein was similar to that of other reduced iron-sulfur proteins, showing a major resonance line at g = 1.94. The isolated protein, when photoreduced by spinach chloroplasts, can in turn transfer electrons to mammalian cytochrome c. However, the photoreduced protein cannot replace soluble ferredoxin in NADP⁺ reduction because of its apparent inability to interact with the chloroplast enzyme, ferredoxin-NADP⁺ reductase. The relation of the isolated iron-sulfur protein to the bound ferredoxin that acts as the primary electron acceptor in Photosystem I is discussed.