Structural organization of the iron-sulfur centers in Synechocystis 6803 photosystem I. EPR study of oriented thylakoid membranes and analysis of the magnetic interactions.
The structural properties of the iron-sulfur centers of photosystem I (PSI) from the cyanobacterium Synechocystis 6803 have been investigated by EPR spectrometry. The stoichiometry of centers A, B, and X, determined by EPR intensity measurements, gives direct evidence for center X being a [4Fe-4S] center in the native system and for the core reaction center protein being a dimer. The directions of the magnetic axes of centers A, B, and X were accurately determined by EPR experiments carried out on membrane fragments oriented on thin Mylar films. These directions are very similar to those previously reported for plants and algae. To get a detailed description of the relative arrangement of A and B, the magnetic interactions between these centers have been analyzed through numerical simulations of X-band and Q-band EPR spectra. The relative orientation of the magnetic axes deduced from these simulations is fully consistent with that given by oriented multilayer experiments. Numerical simulations of X-band and Q-band EPR spectra given by spinach PSI lead to a very similar set of structural parameters, which demonstrates that the functional unit of PSI is highly conserved in all photosynthetic organisms. Moreover, the results of these studies indicate that the A-B direction is close to the membrane normal, which supports a sequential electron transfer mechanism between the iron-sulfur centers in PSI.
Laboratoire d'Electronique des Milieux Condensés, URA 784, Centre National de la Recherche Scientifique (CNRS), Université de Provence, Centre St Jérôme, Marseille, France.
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D000458:Cyanobacteria / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D007425:Intracellular Membranes / Q000737:chemistry / Q000378:metabolism; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D045331:Photosystem I Protein Complex; D011487:Protein Conformation
D007506:Iron-Sulfur Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex