Location of the iron-sulfur clusters FA and FB in photosystem I:: An electron paramagnetic resonance study of spin relaxation enhancement of P700+

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 PS I 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 PS I and characterize them by progressive microwave power saturation measurements to determine the arrangement of the iron-sulfur clusters F-X/A/B relative to P700 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 PS I 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.