Biophys J, October 2002, p. 1760-1773, Vol. 83, No. 4

Crystallization and Electron Paramagnetic Resonance Characterization of the Complex of Photosystem I with its Natural Electron Acceptor Ferredoxin

Petra Fromme,^* Hervé Bottin, Norbert Krauss,^§ and Pierre Sétif

 ^*Max Volmer Laboratorium, Institut für Chemie, Fakultät II, Technische Universität Berlin, 10623 Berlin, Germany;  Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604 USA;  Département de Biologie Joliot Curie, Commissariat à l'Energie Atomique, Service de Bioénergétique and CNRS URA 2096, Gif sur Yvette 91191, France; and  ^§Institut für Chemie/Kristallographie, Freie Universität Berlin, D-14195 Berlin, Germany

The formation of a transient complex between photosystem I and ferredoxin is involved in the process of ferredoxin photoreduction in oxygenic photosynthetic organisms. Reduced ferredoxin is an essential redox intermediate involved in many assimilatory processes and is necessary for the reduction of NADP⁺ to NADPH. Single crystals from a complex of photosystem I with ferredoxin were grown using PEG 400 and CaCl₂ as precipitation agents. The crystals diffract x-rays to a resolution of 7-8 Å. The space group was determined to be orthorhombic with the unit cell dimensions a = 194 Å, b = 208 Å, and c = 354 Å. The crystals contain photosystem I and ferredoxin in a 1:1 ratio. Electron paramagnetic resonance (EPR) measurements on these crystals are reported, where EPR signals of the three [4Fe-4S] clusters F_A, F_B, F_X, and the [2Fe-2S] cluster of ferredoxin were detected. From the EPR spectra observed at three particular orientations of the crystal in the magnetic field, the full orientation pattern of the F g-tensor was simulated. This simulation is consistent with the presence of 12 magnetically inequivalent F clusters per unit cell with the C₃ axis of the PSI trimers oriented at (23°, 72°, 77°) to the unit cell axes.

Biophys J, October 2002, p. 1760-1773, Vol. 83, No. 4
© 2002 by the Biophysical Society   0006-3495/02/10/1760/14  $2.00