title: The role of the membrane-bound iron-sulphur centres A and B in the photosystem I reaction centre of spinach chloroplasts.
abstract: Photosystem I particles prepared from spinach chloroplast using Triton X-100 were frozen in the dark with the bound iron-sulphur Centre A reduced. Illumination at cryogenic temperatures of such samples demonstrated the photoreduction of the second bound iron-sulphur Centre B. Due to electron spin-electron spin interaction between these two bound iron-sulphur centres, it was not possible to quantify amounts of Centre B relative to the other components of the Photosystem I reaction centre by simulating the line-shape of its EPR spectrum. However, by deleting the free radical signal I from the EPR spectra of reduced Centre A alone or both Centres A plus B reduced, it was possible to double integrate these spectra to demonstrate that Centre B is present in the Photosystem I reaction centre in amounts comparable to those of Centre A and thus also signal I (P-700) and X. Oxidation-reduction potential titrations confirmed that Centre A had Em congruent to -550 mV, Centre B had Em congruent to -585 mV. These results, and those presented for the photoreduction of Centre B, place Centre B before Centre A in the sequence of electron transport in Photosystem I particles at cryogenic temperatures. When both A and B are reduced, P-700 photooxidation is reversible at low temperature and coupled to the reduction of the component X. The change from irreversible to reversible P-700 photooxidation and the photoreduction of X showed the same potential dependence as the reduction of Centre B with Em congruent to -585 mV, substantiating the identification of X as the primary electron acceptor of Photosystem I.
mesh_terms: D002736:Chloroplasts / Q000378:metabolism*; D003580:Cytochromes / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D007425:Intracellular Membranes / Q000378:metabolism; D007506:Iron-Sulfur Proteins / Q000378:metabolism*; D007700:Kinetics; D008667:Metalloproteins / Q000378:metabolism*; D010084:Oxidation-Reduction; D010788:Photosynthesis; D010944:Plants; D011199:Potentiometry
chemical_list: D003580:Cytochromes; D007506:Iron-Sulfur Proteins; D008667:Metalloproteins