Electron Transfer between Plastocyanin and P700 in Highly-Purified Photosystem I Reaction Center Complex. Effects of pH, Cations, and Subunit Peptide Composition

Abstract

Treatment of isolated spinach thylakoid fragments with Triton X-100 followed by repeated sucrose density gradient centrifugations and Sephacryl S-300 and DEAE-Sephacel chromatographies yielded a highly purified P700-chlorophyll a protein complex which consists of five polypeptides. The protein complex is virtually free of chlorophyll b (Chl a/Chl b>10) with approximately 30 chlorophylls per P700, and contains iron-sulfur centers A, B, and X. At pH values higher than 6, divalent cations, but not monovalent or trivalent cations, efficiently accelerated the electron transfer from reduced spinach plastocyanin to the photooxidized P700 in the P700-chlorophyll a protein complex. At pH values lower than 6, the reaction rate drastically increased with decreasing pH with a maximum at about pH 4.3 without cations. Divalent salts as well as monovalent or trivalent salts decreased the P700 reduction rate at low pH, indicating the involvement of electrostatic interaction in those pH regions. The rate of electron transfer from plastocyanin to the photooxidized P700 in the reaction center protein, which consists of only the largest peptide subunit and no iron-sulfur centers, was reduced only 50% at pH 7.0 in the presence of MgCl₂ as compared to the case of P700-chlorophyll a protein complex. Essentially similar effects of pH and metal ions on this electron transfer reaction were observed as in the case of P700-chlorophyll a protein complex. These results strongly suggest that plastocyanin donates electrons directly to the largest peptide of P700-chlorophyll a protein complex and the observed effects of pH and cations are mainly due to the interaction between the largest peptide of P700-chlorophyll a protein complex and plastocyanin. The four small subunits in the protein complex seemed to have only a minor role in the reaction with plastocyanin.