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 (P700), a monomeric chlorophyll electron acceptor (A0), a phylloquinone (A1), and three [4Fe-4S] clusters (FX/A/B). The sequence of electron-transfer events between the iron-sulfur cluster, FX, 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 C2-axis of symmetry in PS I is uncertain as well. An unequivocal determination of the spatial arrangement of the iron-sulfur clusters FA and FB 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 (P700+-FredX/A/B) in PS I and characterize them by progressive microwave power saturation measurements to determine the arrangement of the iron-sulfur clusters FX/A/B relative to P700. The microwave power at half saturation (P1/2) of P700+ is greater when both FA and FB are reduced in untreated PS I than when only FA is reduced in mercury-treated PS I. The experimental P1/2 values are compared to values calculated by using P700-FA/B crystallographic distances and assuming that either FA or FB is closer to P700+. On the basis of this comparison of experimental and theoretical values of spin relaxation enhancement effects on P700+ in P700+ [4Fe-4S]- charge-separated pairs, we find that iron-sulfur cluster FA is in closer proximity to P700 than the FB cluster.
Lakshmi
K V
KV
Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, USA.
Jung
Y S
YS
Golbeck
J H
JH
Brudvig
G W
GW
D002734:Chlorophyll / Q000737:chemistry*; D018360:Crystallography, X-Ray; D000458:Cyanobacteria / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D004579:Electron Transport; D005609:Free Radicals / Q000737:chemistry; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D008958:Models, Molecular; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D019097:Spin Trapping
D005609:Free Radicals; D007506:Iron-Sulfur Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D002734:Chlorophyll; C001785:chlorophyll P 700