Electron spin resonance studies of the bound iron-sulfur centers in Photosystem I. Photoreduction of center A occurs in the absence of center B |
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Authors: | John H Golbeck Joseph T Warden |
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Institution: | 1. Martin Marietta Laboratories, 1450 South Rolling Road, Baltimore, MD 21227 U.S.A.;2. Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 72181 U.S.A. |
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Abstract: | The Photosystem I acceptor system of a subchloroplast particle from spinach was investigated by optical and electron spin resonance (ESR) spectroscopy following graduated inactivation of the bound iron-sulfur proteins by urea/ferricyanide solution. The chemical analysis of iron and sulfur and the ESR properties of centers A, B and X are consistent with the participation of three iron-sulfur centers in Photosystem I. A differential decrease in centers A, B and X is observed under conditions that induce S2? →S0 conversion in the bound iron-sulfur proteins. Center B is shown to be the most susceptible, while center ‘X’ is the least susceptible component to oxidative denaturation. Stepwise inactivation experiments suggest that electron transport in Photosystem I does not occur sequentially from X→B→A, since there is quantitative photoreduction of center A in the absence of center B. We propose that center A is directly reduced by X; thus, X may serve as a branch point for parallel electron flow through centers A and B. |
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Keywords: | Photosystem I Iron-sulfur protein Electron flow ESR (Spinach subchloroplast particle) DCIP 2 6-dichlorophenolindophenol Chl chlorophyll PS photosystem |
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