Electron transfer reactions in both the oxidizing and reducing sites of photosystem I: A laser flash absorption spectroscopy study

Abstract

The rate constants of electron transfer from reduced plastocyanin and cytochrome c-552, obtained from the green alga Monoraphidium braunii, to the photo-oxidized P700 (P700 +) in photosystem I (PSI) particles from spinach were determined using the laser flash absorption spectroscopy technique. The redox properties of both proteins were found to be quite similar, which is consistent with their physiological role of substituting for one another as electron donors to PSI. In analogous experiments, the electron transfer reactions from the reducing site of PSI to oxidized ferredoxin, isolated from both spinach and Monoraphidium braunii, were studied. Second-order rate constants of PSI reduction by either cytochrome c-552 or plastocyanin and oxidation by ferredoxin are of the same order of magnitude (approximately 10 7 M −1 s −1), whereas the corresponding first-order rate constants are significantly greater (approximately 10 2) for PSI reduction than for its oxidation. In all cases, the protein concentration dependencies of the reaction rates are characteristic of a mechanism involving the formation of a transient complex preceding electron transfer, whereas the effects of salt ions and pH suggest the occurrence of attractive electrostatic interactions in complex formation between the redox metalloproteins and their respective binding sites in PSI. Divalent cations such as magnesium appear to play a specific role in the interaction between PSI and both the donor and acceptor proteins.