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

• Published in: Biochimica et biophysica acta Vol. 503; no. 1; pp. 120 - 134
• Main Authors: Sauer, K, Mathis, P, Acker, S, van Best, J A
• Format: Journal Article
• Language: English
• Published: Netherlands 06.07.1978
• Subjects: Chloroplasts - metabolism; Electron Transport; Lasers; Light; Photosynthesis; Pigments, Biological - metabolism; Plants; Polyethylene Glycols; Spectrophotometry
• ISSN: 0006-3002
• DOI: 10.1016/0005-2728(78)90166-4

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.