### Abstract

Photosystem I (PS I) mediates electron-transfer from plastocyanin to ferredoxin via a photochemically active chlorophyll dimer (P_{700}), a monomeric chlorophyll electron acceptor (A_{0}), a phylloquinone (A_{1}), and three [4Fe-4S] clusters (F(X/A/B)). The sequence of electron-transfer events between the iron-sulfur cluster, F(X), and ferredoxin is presently unclear. Owing to the presence of a 2-fold symmetry in the PsaC protein to which the iron-sulfur clusters F(A) and F(B) are bound, the spatial arrangement of these cofactors with respect to the C_{2}-axis of symmetry in PSI is uncertain as well. An unequivocal determination of the spatial arrangement of the iron- sulfur clusters F(A) and F(B) within the protein is necessary to unravel the complete electron-transport chain in PS I. In the present study, we generate EPR signals from charge-separated spin pairs (P_{700}/^{+}-F(X/A/B)/(red)) in PSI and characterize them by progressive microwave power saturation measurements to determine the arrangement of the iron-sulfur clusters F(X/A/B)relative to P_{700}. The microwave power at half saturation (P(1/2)) of P_{700}/^{+} is greater when both F(A) and F(B) are reduced in untreated PSI than when only F(A) is reduced in mercury-treated PS I. The experimental P(1/2) values are compared to values calculated by using P_{700}-F(A/B) crystallographic distances and assuming that either F(A) or F(B) is closer to P_{700}/^{+}. On the basis of this comparison of experimental and theoretical values of spin relaxation enhancement effects on P_{700}/^{+} in P_{700}/^{+} [4Fe-4S]^{-} charge-separated pairs, we find that iron-sulfur cluster F(A) is in closer proximity to P_{700} than the F(B) cluster.

Original language | English |
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Pages (from-to) | 13210-13215 |

Number of pages | 6 |

Journal | Biochemistry |

Volume | 38 |

Issue number | 40 |

DOIs | |

Publication status | Published - Oct 5 1999 |

### ASJC Scopus subject areas

- Biochemistry

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## Cite this

*Biochemistry*,

*38*(40), 13210-13215. https://doi.org/10.1021/bi9910777