FA/FB Protein from the Spinach Photosystem I Complex: Isolation in a Native State and Some Properties of the Iron-Sulfur Clusters

The F_A/F_B protein of the photosystem I complex was isolatedfrom spinach leaves in a native state by use of anaerobic systems.The protein contained 8.5 non-heme iron atoms and 8.0 acid-labilesulfur atoms per molecule, consistent with the current conceptthat it has two [4Fe-4S] clusters. Its absorption spectrum wasvery similar to those of bacterial-type ferredoxins. The ratioof the absorbance at 390 nm to that at 280 nm was 0.6, and themolar extinction coefficient at 390 nm was 32,000 M^–.cm^–.Theoxidation-reduction properties of the iron-sulfur clusters wereexamined by redox potentiometry and EPR spectroscopy. The twoclusters were distinguishable in terms of their oxidation-reductionmidpoint potentials; their E_m values were determined to be about-470mV and-560 mV, respectively. (Received July 16, 1990; Accepted October 8, 1990)