Electron acceptors associated with P-700 in Triton solubilized Photosystem I particles from spinach chloroplasts

Abstract

Flash-induced absorption changes of Triton-solubilized Photosystem I particles from spinach were studied under reducing and/or illumination conditions that serve to alter the state of bound electron acceptors. By monitoring the decay of P-700 following each of a train of flashes, we found that P-430 or components resembling it can hold 2 equivalents of electrons transferred upon successive illuminations. This requires the presence of a good electron donor, reduced phenazine methosulfate or neutral red, otherwise the back reaction of P-700+ with P-430 occurs in about 30 ms. If the two P-430 sites, designated Centers A and B, are first reduced by preilluminating flashes or chemically by dithionite under anaerobic conditions, then subsequent laser flashes generate a 250 microseconds back reaction of P-700+, which we associate with a more primary electron acceptor A2. In turn, when A2 is reduced by background (continuous) illumination in presence of neutral red and under strongly reducing conditions, laser flashes then produce a much faster (3 microseconds) back reaction at wavelengths characteristic of P-700. We associate this with another more primary electron acceptor, A1, which functions very close to P-700. The organization of these components probably corresponds to the sequence P-700-A1-A2-P-430[AB]. The relation of the optical components to acceptor species detected by EPR, by electron-spin polarization or in terms of peptide components of Photosystem I is discussed. Preliminary experiments with broken chloroplasts suggest that an analogous situation occurs there, as well.

Is phylloquinone an obligate electron carrier in photosystem I?

“…This assertion is supported by the observation that reduction of the PS I sample by dithionite results in the acceleration of P-700 + decay to 265/~s (not shown). Such a -250/~s decay is diagnostic at room temperature for the backreaction between P-700 ÷ and A~ [20]. Note that similar kinetics are observed in the presence of dithionite in both the control and UV-irradiated samples.…”

Identification of multiple components in the intermediary electron carrier complex of photosystem I

“…However, kinetic measurements of the re-reduction of the reaction centre chlorophyll P700 have indicated that, as in bacteria, other electron carriers are also involved in the electron-transfer process [4]. We have shown that if photosystem I samples are prepared under extreme reducing conditions and frozen under illumination, a component giving rise to an EPR signal in the g = 2.00 region is reduced [5,6].…”

A photosystem I mutant in barley (Hordeum vulgare L.)

“…Three polypeptides with molecular weights of 18,000, 16,000, and 8,000 were suggested to be iron-sulfur proteins mediating the transfer of electrons from the primary acceptor on the 110,000 molecular weight polypeptide to soluble ferredoxin (4). Recently, an additional ironsulfur protein has been shown involved in this electron transfer (35). Iron-sulfur proteins are likely candidates for the three small polypeptides present in minute amounts in viridis-n 34.…”